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ANALYSIS OF THE PHYSICAL PROPERTIES
OF IONIC LIQUIDS
by
NICHOLE M. JACKSON, B.S.
A THESIS
IN
CHEMISTRY
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Accepted
December, 2001
ACKNOWLEDGEMENTS
I would like to thank Dr. Allan D. Headley for all his support and input on
my research project. The many emails that were sent back and forth with his
ideas and suggestions were a constant help. I would also like to thank Dr.
Richard A. Bartsch for his support and willingness to be on my committee. The
experience I received under his mentorship was invaluable. Dr. Jaewook Nam
was helpful in the theoretical analyses. For the NMR spectroscopic analysis, I
would like to thank Mr. David Purkiss. The financial support came from the
Welch Foundation without which none of this would have been feasible.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
ABSTRACT v
LIST OF TABLES vi
LIST OF FIGURES vii
CHAPTER
I. INTRODUCTION 1
II. RESULTS AND DISCUSSION 12
Synthesis of 1-Alkyl-3-methylimidazollum Salts 13
NMR Studies of 1-Alkyl-3-methylimidazolium Salts 17
Nature of the Solvent Interactions 42
Use of SolvatochromIe Parameters to Analyze the Solvation Interactions of BMrBF4' 46
Analysis of the Solvent Interactions of BMrPFe' 49
Analysis with Gaussian 94 for Imidazollum Cations 51
Analysis of Solvation Properties of Ionic Liquids by Theoretical Descriptors 56
III. CONCLUSIONS 62
IV. EXPERIMENTAL 63
Preparation of 1-Methyl-3-propyllimidazolium Chloride ... 63
Preparation of 1-Butyl-3-methylimldazolium Chloride 63
Preparation of 1-Butyl-3-methylimldazolium Hexafluorophosphate 64
Preparation of 1-Butyl-3-methylimidazolium Tetrafluoroborate 64
Preparation of 1-Methyl-3-pentyllimidazolium Chloride.... 65
Preparation of 1-Oetyl-3-methylimidazolium Chloride 65
Preparation of 1-Hexadeeyl-3-methylimidazolium Bromide 66
Preparation of 1-Hexadecyl-3-methylimidazolium Tetrafluoroborate 66
Preparation of 1-Hexadecyl-3-methylimidazolium Hexafluorophosphate 66
Preparation of NMR Solvent Study Samples 67
Procedure for Calculating the Density 67
REFERENCES 69
IV
ABSTRACT
The recognition of ionic liquids as possible green solvents has boosted the
interest in this field. Ionic liquids are simply, liquids that are comprised entirely of
ions. The aspect of ionic liquids as solvents and catalysts has gained notable
attention but little knowledge in the physical properties of these liquids has been
achieved. It is important to gain a quantitative understanding of how these new
class of solvents affect reaction rates and the outcome of reactions. As a result of
a better understanding, the ability to better predict the outcome of these reactions
performed in these solvents should enhance their usage. In this study, the
solute/solvent interactions of a group of ionic liquids, which have different alkyl
side chains and anions, were investigated. By changing the length of the side
chain or the anion, the physical properties of the ionic liquids vary. Ionic liquids
with hydrophobic side chains or bulky polarizable anions are not as good a
hydrogen bond donor as those with a less hydrophobic side chain or a smaller,
more basic anion.
LIST OF TABLES
1 H NMR chemical shifts in ppm for the aromatic hydrogens of BMrBF4' in the deuterated solvents 18
2 H NMR chemical shifts in ppm for the aromatic hydrogens of BMrPFe' in the deuterated solvents 23
3 H NMR chemical shifts in ppm for the aromatic hydrogens of HDMrBF4'in deuterated solvents 30
4 H NMR chemical shifts in ppm for the aromatic hydrogens of HDMrPFe" in the deuterated solvents 35
5 SolvatochromIe Parameters 47
6 Coefficients and statistics using equation 14 without polarizability (71*) for BMrBF4" 49
7 Coefficients and statistics using equation 14 without polarizability (71*) for BMrPFe" 51
8 Z-Matrix for imidazolium cation of Figure 31 53
9 Charge distribution of atoms on the imidazolium cation 54
10 Charge distribution of atoms of 1-fluoroimidazolium cation 55
11 Theoretical descriptors for different ionic liquids and molecular solvents 59
VI
LIST OF FIGURES
1 A, 1-ethyl-3-methylimidazolium, EMI""; B, 1-butyl-3-methylimidazolium, BMT; or C, A/-butylpyridinium cations, respectively 2
2 Organic cation with R side-arm groups and X" anion 3
3 A low-melting-point ionic liquid, 1-ethyl-3-methylimidazolium tetrachloroaluminate 3
4 Diels-Alder reaction in an ionic liquid system 5
5 Arene exchange of ferrocenes using a 1-butyl-3-methylimidazolium BMI*- based ionic liquid 6
6 Hydrogenation of 1-pentene to form pentane and 2-pentene 7
7 1-Butyl-3-methylimidazolium tetrafluoroborate (X" = BF4"), 1-butyl-3-methylimidazolium hexafluorophosphate (X' = PFe) 8
8 Reaction mechanism for synthesis of BMrci" 13
9 Reaction scheme for BMrBF4" and BMrPFe' 14
10 Reaction scheme for HDMrBF4' and HDMrPFe" 16
11 Aromatic hydrogen numbering for BMI* BF4" 17
12 Chemical shifts of the aromatic hydrogens Hi and H2 of BMrBF4' in the deuterated solvents 19
13 Chemical shifts of the aromatic hydrogens Hi and H3 of BMrBF4" in the deuterated solvents 20
14 Chemical shifts of the aromatic hydrogens H2 and H3 of BMrBF4' in the deuterated solvents 21
15 Chemical shifts of the aromatic hydrogens Hi and H2 of BMrPFe" in deuterated solvents 25
16 Chemical shifts of the aromatic hydrogens Hi and H3 of BMrPFe" in the deuterated solvents 26
VII
17 Chemical shifts of the aromatic hydrogens H2 and H3
of BMrPFe" in the deuterated solvents 27
18 Slope comparison of BMrBF4' and BMrPFe' 29
19 Chemical shifts of the aromatic hydrogens Hi and H2 of HDMrBF4" in the deuterated solvents 31
20 Chemical shifts of the aromatic hydrogens Hi and H3 of HDMrBF4' in the deuterated solvents 32
21 Chemical shifts of the aromatic hydrogens H2 and H3 of HDMrBF4" in the deuterated solvents 33
22 Chemical shifts of the aromatic hydrogens Hi and H2 of HDMrPFe" in the deuterated solvents 36
23 Chemical shifts for the aromatic hydrogens Hi and H3 of HDMrPFe" in the deuterated solvents 37
24 Chemical shifts for the aromatic hydrogens H2 and H3 of HDMrPFe' in the deuterated solvents 38
25 Slope comparison of HDMrBF4" and HDMrPFe" 40
26 Slope comparison of BMrBF4", BMrPFg", HDMrBF4", andHDMrPFe" 41
27 Proposed interaction between the BMI* cation and the acidic solvent 42
28 Proposed interaction between the BMI* cation and the basic solvent 43
29 Proposed interaction between the HDMI* cation and the acidic solvent 44
30 Proposed interaction between the HDMI* cation and
the basic solvents 45
31 Aromatic hydrogen designation for BMrPFe" 50
32 Numbering system for imidazolium cation 52
VIII
33 1-Fluoroimidazolium cation 55
34 Structure of 1,3-disubstituted imidazolium cation for which theoretical descriptors have been developed 57
iX
CHAPTER I
INTRODUCTION
Organic reactions are mainly performed in nonaqueous media. Chemists
face a major challenge in the separation of the organic products, or catalysts,
from the reaction mixtures. Polar solvents, such as dimethyl sulfoxide (DMSO)
or dimethylformamide (DMF) are conventionally used to create the polar
environment for most organic reactions, present tremendous difficulty in the
separation of the products from the reaction mixtures. Added to this difficulty is
the cost to dispose of these solvents, which is estimated at approximately 5
billion dollars per year. Ionic liquids, a new field of chemistry, has extremely
interesting properties from a fundamental chemist's point of view and could have
a major impact on society. Industry is eagerly exploring ways in which ionic
liquids can deliver some benefits. The environmental impact of replacing
conventional solvents with ionic liquids could result in less waste formation,
reduction of cost, and the reduction in energy consumption.
Ionic liquids are liquids at room temperature that are comprised entirely of
ions. It is important to note that the term "ionic liquid" In this context represents a
whole class of liquids with a wide range of stabilities and properties, which
importantly can be readily tailored. It has been stated that there are in excess of
10^ readily accessible potential room temperature ionic liquid systems. Ionic
liquids have some unique properties that make them very important solvents for
a large array of organic reactions. These ionic liquids may have a liquid range
from temperatures as low as -96 °C and as high as 300 C."
The organic cation is usually a heterocyclic cation such as 1-ethyl-3-
methylimidazolium, EMr, 1-butyl-3-methylimidazolium, BMr, or
A/-butylpyridinium shown in Figure 1. These large organic cations account for the
low melting points of the salts. The size of the cation is responsible for the
magnitude of the melting point, i.e., the bigger the cation, the lower the melting
point
H3C ^ ^ CH2CH3 N 1 CH2CH2CH2CH3
/r^ c .Nki>;N
|_| Q^ ^^"^ "CH2CH2CH2CH3 '3^
B
Figure 1. A, 1-ethyl-3-methylimidazolium, EMr; B, 1-butyl-3-methylimidazolium, BMI*; or C, A/-butylpyridinium cations, respectively.
By varying the organic cation, including its R group(s) and the identity of
the cation, as shown in Figure 2, ionic liquids can be fine-tuned to meet specific
properties. For example, the melting points differ as a function of the length of
the 1-alkyl group of 1-alkyl-3-methylimidazolium tetrafluoroborates^ and 1-alkyl-3-
methylimidazolium hexafluorophosphates.
A general trend develops from hydrophilic to hydrophobic side chains, on
increasing the chain length due to increasing hydrocarbon character.
N3V ; N I
/ ^ \
Ro ^2 R,
Figure 2. Organic cation with R side-arm groups and X" anion.
The numbering system used in naming ionic liquids is also shown in
Figure 2. Anions, such as tetrachloroaluminate, [AICU]", on the other hand,
determine to a large extent the chemical properties of the system. Varying the
molar ratio of tetrachloroaluminate influences the Lewis acidity of Figure 3.
+ )\ AICI4"
Figure 3. A low-melting-point Ionic liquid, 1-ethyl-3-methylimidazolium tetrachloroaluminate.
As the molar ratio of aluminum trichloride to dialkylimidazolium chloride ranges
from <1 to >1, the ionic liquid transitions from a highly complexing mixture to
what can be regarded as a Lewis acid-noncoordinating media. 1-Ethyl-3-
methylimidazolium chloride-aluminum chloride, EMrcrAICl3 is a liquid and
thermally stable from almost -100 °C to approximately 200 °C. For
chloroaluminate ionic liquid systems, such as EMrci'AICb, CI" is the main anion,
which is a Lewis base; the anion [AICU]" is neither acidic or basic and [AI2CI7]' is a
Lewis acid." The concentration of these anions and also the Lewis acidity of the
system vary depending on the ratio of AICI3 and EMrci" which determines the
acidity of the system and ultimately the chemical properties.
The role of the solvent in organic reactions can influence the course of
reactions in a variety of ways, such as higher reaction rates and better reaction
control. The Diels-Alder reaction is one of the most powerful carbon-carbon bond
forming reactions.' It has been shown that the polarity of the solvent has a
dramatic influence upon the endo-exo ratio in some Diels-Alder reactions. The
more polar (endo) transition state is stabilized by a more polar solvent.
For the Diels-Alder reaction shown in Figure 4, the use of an acidic ionic liquid
increased the selectivity of the endo product in comparison to several other
solvents.""
w // * H3C CH2CH3
CH2=CHC02CH3 endo
CO2CH3 exo
Figure 4. Diels-Alder reaction in an ionic liquid system.
An observed endo selectivity enhancement of four fold from the 48% AICI3 basic
mixture to the 51% AICI3 acidic mixture was a direct result of the increase in
Lewls/Bronsted acidity of the medium, while the observed 5:1 endo/exo product
ratio is a reflection of the polarity of the medium.
"Probably the most Important advantage of using ionic liquids is that they
have no measurable vapor pressure," says chemistry professor Kenneth R.
Seddon, who leads a team at Queen's University of Belfast, Northern Ireland,**
making ionic liquids strong candidates for recyclable "green" solvents
The use of ionic liquids as solvents has taken research into a direction of
dual functioning of the Ionic liquid wherein the ionic liquid is used as the solvent
and as the catalyst. Use of BMrCfAICb in an arene exchange reactions of
ferrocenes has been reported (Figure 5). ^
^
Arene
BMrcrAici3 Fe • Fe I +[BMI]HCl2 +arene I
Figure 5. Arene exchange of ferrocenes using a 1-butyl-3-methylimidazolium BMI*- based ionic liquid.
Ionic liquids that use the chloroaluminate ions are important because AICI3 Is
used industrially to catalyze a large number of reactions. For such biphasic
systems, the AICI3 is dissolved in the ionic liquid and, after the reaction Is
complete, the organic products separate into an organic layer and the catalyst
remains In the ionic liquid phase.
1-Butylpyridinium chloride-aluminum(lll) chloride, NbuPy' CfAICb, and 1-
ethyl-3-methylimidazolium chloride-aluminum(IH) chloride, EMrCPAICb, are Ionic
liquids used as solvents for different reactions and exhibit Bronsted, Lewis, and
Franklin acidity. ^ These properties make it possible to dissolve a wide range of
organic molecules to an appreciable extent, meaning much lower volumes of
solvent are required for a given process. ^
By varying the anion, ionic liquids can be fine-tuned to provide desired
properties. Ionic liquids have been found to dissolve charged species so the
hydrogenation of 1-pentene using the Osborn [Rh(NBD)(PPh3)2]' PFe'complex
as the cationic catalyst precursor was tested (Figure 6). ^ With BMrSbFe" as the
solvent, hydrogenation rates of 1-pentene are nearly five times higher compared
to the homogeneous reaction in acetone, in spite of the expected limited
solubilities of reactants in the polar phase."* With BMrci" and CuCI (1:1.5), only
isomerization to 2-pentene with 100% selectivity was observed. This
demonstrates that selectivity can be strongly influenced by the nature of the
anion of an ionic liquid.
H3C [Rh(NBD)(PPh3)2]'^PF6"
H 2 C ^ ^ ^ ^CH3
"" H3C H2
Figure 6. Hydrogenation of 1-pentene to form pentane and 2-pentene.
In traditional solvent extraction, ^ the two immiscible phases employ an
organic solvent (diluent) and an aqueous solution. The diluents, which are
volatile, are a danger to the environment, expensive to purchase, and have a
high cost to dispose of the waste. Studies of BMrPFe" as a medium for liquid-
liquid extraction prove that the distribution ratios of aromatic solutes in an ionic
liquid versus distribution ratios in 1-octanol-water system are similar in
relations. ^ This evidence proves that ionic liquids may be suitable media for the
design of novel liquid-liquid extraction systems.
1-Butyl-3-methylimidazolium tetrafluoroborate, BMrBF4" (Figure 7), has
been investigated as a solvent in two-phase catalytic hydrogenation reactions. 1-
Butyl-3-methylimidazollum hexafluorophosphate BMrPFe" has also been studied
in two-phase catalytic hydrogenation and in liquid-liquid extraction systems. ' '
HoC CH2CH2CH2CH3
Figure 7. 1-Butyl-3-methyllmidazolium tetrafluoroborate (X' = BF4'), 1-butyl-3-methylimidazolium hexafluorophosphate (X" = PFe").
Most of the research carried out thus far on these solvents has involved
their use in electrochemical systems and in organic syntheses. The ability to
predict property variations of molecules in different environments is important.
With the growing interest in ionic liquids, research on physlochemical properties
of these ionic liquids should be conducted. One way to probe specific properties
8
of ionic liquids is to analyze their ability to be involved in the solute-solvent
interactions. The nature and extent of solute-solvent interactions are able to alter
various properties of solutes.'' Solvent effects on the electronic spectrum of a
molecule are referred as "solvatochromism". The SolvatochromIe shifts
associated with electronic transitions in solution result from different solvation
energies of the initial and final electronic states. Thus, study of SolvatochromIe
shifts gives relevant information concerning the kind of solute-medium
interactions and allows for the understanding of electronic distribution changes
between the states involved in the electronic transition. °
Various solvent parameters have been proposed to quantify the interactive
abilities of solvents with solutes. Over the years, a number of descriptors have
been developed, ^ but those developed by Taft and co-workers have been widely
used to analyze solvent effects on rates and equilibria of many organic reactions
and are used in this research project. ' ^
Kamlet, Taft, and co-workers have examined the nature and importance of
solute/solvent interactions and described them with Equation 1.^^
Property = bulk/cavity term + dipolarity/polarizability term(s)
+ hydrogen bonding term(s) + constant (1)
The bulk/cavity term measures the energy that is needed to overcome the
cohesive solvent/solvent interactions to form a cavity for the solute molecule.
The dipolarity/polarizability terms measure the energies of solute/solvent dipole
and induced dipole interactions that contribute to solvation. The hydrogen
bonding term is a measure of specific interactions between the solvent and
solute. This reflects the ability of the solvent to accept hydrogen bonds from the
solute, which is designated as the hydrogen bond acceptor basicity (HBAB), and
the ability of the solvent to donate hydrogen bonds to the solute, which is
designated as the hydrogen bond donor acidity (HBDA). The first term in
Equation 1 is endoergic and the last two terms are exoergic. A linear statistical
fitting among the variables is used for Equation 1.
For an ionic liquid, the important medium contributions should be
dipolarity/polarizabiiity (owing to the charge separation), the hydrogen bond
donor abilities (owing to the presence of the acidic C-H), and the hydrogen bond
acceptor abilities (owing to the presence of an anion). The product of the
dipolarity/polarizability term TI* is a measure of the energies of the solvent dipole
and induced dipole interactions that contribute to solvation. "
Another way to evaluate the solvation properties of ionic liquids is the use
of theoretical descriptors. The relationships that exist between the chemical
properties of molecules and their molecular structural features have been used to
correlate the molecular structural properties of compounds with known biological,
chemical, and physical properties. ^ These relationships are often referred to as
10
quantitative structure-activity relationships (QSAR). In cases where a specific
property is examined, the term that Is used is quantitative structure-property
relationships (QSPR). The success of QSAR and QSPR depends on the
assumption that quantitative relationships exist between microscopic features
and macroscopic properties of molecules. QSAR and QSPR have been used
successfully to predict different properties of compounds that have similar
molecular features to other compounds in a particular series. ® The ability of
QSAR and QSPR to make successful predictions of macroscopic properties
depends strongly on the accurate quantification of microscopic features of the
molecule, which are often refen-ed to as descriptors.
Molecular orbital calculations have been used for the development of
molecular descriptors. ^ The descriptors that are obtained by computational
methods are often reliable and may be obtained quickly. ® Statistically based
Interaction indices derived fi'om molecular surface electrostatic potentials have
been used also to predict the properties of molecules. ® A set of six theoretical
linear solvation energy relationship (TLSER) descriptors has been developed by
Famini and Wilson for a wide variety of compounds and used successfully to
correlate properties of the compounds,^ including five nonspecific toxins, the
activity of some local anesthetics and their molecular transform; opiate activity of
some fentanyl-like compounds; and the six physicochemical properties of
absorption on charcoal, HPLC retention index, octanol-water partition
coefficients, phosphononthiolate hydrolyslsrate constants, aqueous equilibrium
11
constants and electronic absorption of some yields. TLSER descriptors have
been used successfully also to describe the effects of structural variations of
carboxylic acids, alcohols, silanols, anilines, hydrocarbons, and oximes on their
acid/base properties in the gas phase. ^ The TLSER descriptors were developed
to correlate with linear solvation energy relationships (LSER) descriptors. ^ The
descriptors give multi-linear regression (MLR) equations with correlation
coefficients, R, and standard deviations (SD) close to those for LSER and are
widely applicable to solute/solvent interactions as the LSER set. The TLSER
descriptors represent electronic properties of molecules in the gas phase. These
descriptors depict the interactions between the molecules - a solute and a
solvent.
Deciding on the most appropriate ionic liquid for a particular reaction
environment depends on a greater understanding of the solvents themselves. In
particular, the features of the ionic liquids which can be tailored, include: acid-
base characteristics, solubility, hydrophilicity, viscosity, and thermal range. ^
Changing the anion or the side chain will have an infiuence on the properties of
ionic liquids. With this in mind, my research focuses on the physical properties of
ionic liquids using NMR chemical shifts and theoretical calculations to study
solute/solvent interactions.
12
CHAPTER II
RESULTS AND DISCUSSION
Svnthesis of 1-Alkvl-3-methvlimidazolium Salts
In the experimental section several ionic salts were synthesized but due to
time constraints only the BMI* and the HDMr cations were used to perform the
property studies. Figure 9 shows the reaction scheme for the synthesis of the
specific Ionic liquids, BMrBF4' and BMrPFe". The neat reaction of 1-
methylimidazole and 1-chlorobutane at refiux provided a high yield of the desired
salt.
The reaction of the 1-methylimldazole with the alkyl halide is likely an SN2
reaction as shown in Figure 8. The nucleophilic 1-methyllmidazole attacks the
V/" \ /" y V ^^ ^ CI CH2CH2CH2 CI • NAjy N / ^ \ / ^ ^
I \«^ _ - / CI "2 H H
Figure 8. Reaction mechanism for synthesis of BMrci".
primary alkyl halide to create the transition state which goes on to produce the
desired product. The yields for the reactions of BMrBF4' and BMfPFe" are
essentially the same, ranging from 94-98%.
13
+ CH3CH2CH2CH2CI
Reflux 48 hours
' '
NaBF4
BF,
Figure 9. Reaction scheme for the synthesis of BMrBF4' and BMrPFe"
14
As shown in Figure 9, equal moles of sodium tetrafluoroborate and 1-
butyl-3-methylimldazolium chloride, BMrci", are stired at room temperature in
acetone to produce BMrBF4" and equal moles of hexafluorophophoric acid and
BMrci" are stirred at room temperature In water to produce BMrPFe".
For the synthesis of 1-hexadecyl-3-methylimidazolium salts, the first
reaction of 1-methylimldazole and 1-bromohexadecane took place in essentially
quantitative yield. The reactions for producing the HDMrBF4" and HOMfPFe" are
the same as for BMrBF4" and BMrPFe' as seen In Figure 10. The alkyl bromide
was used on the saturated carbon chain because bromide is a better leaving
group.
Yields of 1-hexadecy-3-methyllmldazolium tetrafluoroborate and 1-
hexadecy-3-methylimidazolium hexafluorophophorate were essentially
quantitative.
15
CH3(CH2)i5Br
heat 30 minutes
1 '
Br" (CH2)i5CH3
NaBF4
48 hours 48 hours
BF. (CH2)i5CH3
N \ >/N. pp - ^(CH2)i5CH3
Figure 10. Reaction scheme for the synthesis of HDMrBF4' and HDMrPFe"
16
NMR Studies of 1-Alkvi-3-methvlimldazolium Salts
Figure 11 gives the numbering for the aromatic hydrogens of the ionic
liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, BMI* BF4".
HoC CH2CH2CH2CH3
Figure 11. Aromatic hydrogen numbering for BMf BF4".
Solutions of the ionic liquid were prepared In the deuterated solvents
shown in Table 1 at a concentration of 0.45 M. Table 1 also shows the chemical
shift values in ppm for the aromatic hydrogens of BMI* BF4"in the different
deuterated solvents.
17
Table 1. H NMR chemical shifts in ppm for the aromatic hydrogens of BMrBF4' in the deuterated solvents.
Solvent
Acetone
Acetonitrile
Acetic Acid
DMF
DMSO
Ethand
Methanol
Trifluoroacetic Acid
Water
H i
8.89
8.46
8.67
9.15
9.07
8.77
8.78
8.59
8.54
H2
7.70
7.37
7.51
7.87
7.77
7.57
7.58
7.47
7.34
H3
7.65
7.34
7.47
7.80
7.69
7.50
7.51
7.45
7.30
As can be seen from Table 1, the different solvents have little influence on the
chemical shifts for H2 and H3compared to Hi. The chemical shifts of H2 and H3
show minor changes in the different solvents, while Hi has a larger chemical shift
range and appears be more sensitive to the change In solvent. Hi is shifted
farther downfield in a more polar solvent compared to H2 or H3. The chemical
shift for Hi in the polar DMF is 9.15 ppm whereas, H2 and H3 have chemical
shifts of 7.87 and 7.80, respectively.
Figures 12,13, and 14, are the plots of the chemical shift data for the
aromatic hydrogens of BMrBF4".
18
^.£. -
9.1 -
9.0-
liftf
or H
ipp
m
00
CO
^ 8.8-co o "E (D
O 8.7-
8.6-
8.5-
8.4-
H20 •
• /
8HI R2 =
\t\
T •
= 1.306H2-1.06 = 0.984, SD = 0.04
y/ktOH
HOAc
r m
TFA
r — ., J
DMSO •
/ "Acetone
»
DMF
7.3 7.4 7.5 7.6 7.7 Chemical shift for H2 ppm
7.8 7.9
Figure 12. Chemical shifts of the aromatic hydrogens Hi and H2 of BMrBF4" in deuterated solvents. (DMF, dimethyl formamide; DMSO, dimethyl sulfoxide; acetone; MeOH, methanol; EtOH, ethanol; HOAc, acetic acid; TFA, trifluoroacetic acid; H2O, water; and AN, acetonitrile)
19
9.2
9.1 -
9.0-
ppm
X
Shift
le
mic
al
8.9
8.8
8.7 o
8.6-
8.5-
8.4
DMSO
8HI = 1 . 4 0 5 H 3 - 1 . 7 3 R = 0.970, SD = 0.06
MeOH
HOAc
H20
Acetone
DMF
7.2 7.3 7.4 7.5 7.6 7.7
Chemical shift for H3 ppm 7.8 7.9
Figure 13. Chemical shifts of the aromatic hydrogens Hi and H3 of BMrBF4' in deuterated solvents.
20
7.9
7.8-
7.7-
E Q.
x7 .6 a </)
o 17.5 0 O
7.4-
7.3
6H2 = 1 .096H3 -R = 0.996, SD
-
-
-
^ A N
yH20
1 T"
0.60 = 0.02
MeOH /
/Eton
y/noAc
\ F A
• ' 1 " " T
yoMso
A Acetone
I ' —
>DMF
i
1
7.2 7.3 7.4 7.5 7.6 7.7
Chemical shift for H3 ppm
7.8 7.9
Figure 14. Chemical shifts of aromatic hydrogens H2 and H3 of BMrBF4" in the deuterated solvents.
21
Since the concentrations of the solutions are the same, the chemical shift
variations are deemed to be solvent dependent. If the hydrogens of the ionic
liquid have different solvent dependencies, a plot of the chemical shifts of one
hydrogen versus another in different solvents should have a non-unity slope. If
these hydrogens are affected by solvation similarly, a plot of the chemical shifts
of the hydrogens should have a plot of unity.
Equation 2 shows the relationship that exists between the chemical shifts
of H2 and H3 in BMrBF4' from Figure 14. The goodness of the fit and the
> 2 _ 6H2 = 1 .098H3 - 0.60; R = 0.996, SD = 0.02 (2)
closeness of the slope to unity between these two hydrogens, reveals that the
aromatic hydrogens H2 and H3 are similarly affected by solvation. This
observation is reasonable because of the similar chemical environments for
these two hydrogens.
Similar relationships for the chemical shifts of Hi vs. H2 and Hi versus H3
give different slopes as shown in Equations 3 and 4 and Figures 12 and 13,
respectively. These results are not surprising since Hi is in a different
environment than that of H2 and H3. Hi is located between two electronegative
nitrogen atoms and the expectation is that Hi should be more acidic, and hence,
more sensitive to solvation effects than H2 and H3. The difference in slopes
22
between Equations 3 and 4 reflects the slighfly different environments of H2 and
H3 owing to the different substltuents bonded to the nitrogen atoms.
8HI = 1 .306H2 - 1.06; R = 0.984, SD = 0.04 (3)
8HI = 1 .408H3 - 1.73; R = 0.970, SD = 0.06 (4)
Table 2 shows the chemical shift values in ppm of the NMR solvent study
conducted with BMrPFa' to evaluate the effect of changing the counterion.
Table 2. H NMR chemical shifts in ppm for the aromatic hydrogens of BMrPFe' in the deuterated solvents.
Solvent
Acetone
Acetonitrile
Acetic Acid
DMF
DMSO
Methanol
Trifluoroacetic Acid
H i
8.88
8.37
8.60
9.20
9.07
8.72
8.63
H2
7.69
7.35
7.48
7.88
7.73
7.54
7.56
H3
7.64
7.32
7.44
7.81
7.66
7.48
7.53
Ethanol and water were not used in these correlations because BMfPFe" is only
partially soluble In these solvents. Once again, diffierent solvents have little
influence on H2 and H3 compared to Hi. The chemical shifts of H2 and Hsshow
minor changes in the different solvents. Hi is more sensitive with a larger
23
chemical shift range and shifts farther downfield in the more polar deuterated
solvents. The chemical shift of Hi in the polar DMF is 9.20 ppm; whereas H2 and
H3 are shifted to 7.88 and 7.81, respectively.
Figures 15,16, and 17, are plots of the chemical shift data for BMI* PFe".
Equation 5 shows the relationship between H2 and H3 for BMI* PFe".
)2^ 8H2 = 1 .088H3 - 0.56; R = 0.998, SD = 0.01 (5)
Since the slope is approximately unity, the indication is that the different solvents
affect H2 and H3 similarly. With the goodness of the fit and the closeness of the
slope to unity between the two hydrogens, reveals that the aromatic hydrogens,
H2 and H3 have similar solvation effects. Since the slope shown in Equation 5 is
similar to that for the BMrBF4" (Equation 3), these hydrogens for both salts
exhibit similar sensitivity to solvation effects.
24
9.3 T
9.2 -
9.1 -
9 . 0 -
E Q.
^ 8.9 H
1 8.8-1 (/}
"TO o
I 8.7 o
8.6-
8.5-
8.4-
8.3 7.3
DMF
8HI = 1 . 6 1 8 H 2 - 3 . 4 4 R = 0.982, SD = 0.06 DMSO
MeOH
HOAc
• Acetone
7.4 7.5 7.6 7.7 Chemical shift for H2 ppm
7.8 7.9
Figure 15. Chemical shifts of the aromatic hydrogens Hi and H2 of BMrPFe" in deuterated solvents.
25
9.25
9.15-
9.05-
8.95 E Q. Q.
^ 8.85 o
sz </)
S 8.75 E
JCZ
O
8HI = 1 . 7 2 8 H 3 - 4 . 2 0 R2 = 0.969, SD = 0.08
8.65-
8.55-
8.45-
8.35
DMSO
Acetone
MeOH
HOAc
7.3 7.4 7.5 7.6 7.7 Chemical shift for H3 ppm
DMF
7.8 7.9
Figure 16. Chemical shifts of the aromatic hydrogens Hi and H3 of BMrPFe' In the deuterated solvents.
26
7.9 T
7.8-
8H2= 1.088H3-0.56 R2 = 0.998, SD = 0.01
DMF
7.7-1 E Q.
CN
X a I 7.6 15 o E 0)
O 7.5
7.4-
7.3 7.3
DMSO
Acetone
MeOH^/ TFA
HOAc
— I 1 1 1 —
7.4 7.5 7.6 7.7 Chemical shift for H3 ppm
— I —
7.8 7.9
Figure 17. Chemical shifts of the aromatic hydrogens H2 and H3 of BMfPFe' in the deuterated solvents.
27
For BMr PFe", Equations 6 and 7 and Figures 16 and 17 show the
relationships for Hi vs. H2 and Hi vs. H3, respectively. Hi is in a different
environment than that of H2 and H3, being located between two electronegative
8HI = 1 .618H2-3 .44 ; R^ = 0.982, SD = 0.06 (6)
8HI = 1 .728H3 - 4.20; R = 0.969, SD = 0.08 (7)
nitrogen atoms in the ring. Based on the magnitude of the slopes, the implication
is that the chemical shift of Hi is approximately 1.6 times more sensitive to
changes in solvent properties, compared to H2 and 1.7 times more sensitive than
H3.
28
18.
The comparison of slopes for BMrPFe" and BMrBF4" is shown in Figure
1.30
BF, PF,
Figure 18. Slope comparison of BMrBF4' and BMrPFe'.
The slopes of H2 vs. HI and H3 vs. HI for BMrBF4', are different from those for
BMrPFe", the anion is noted to play a role in the chemical shift variation of Hi for
both compounds. The slopes of BMrPFe" are larger than the slopes of BMrBF4".
The implication is that Hi of BMrPFe' is more sensitive to a change in the nature
of the solvent compared to BMrBF4'. Typically, the formation of hydrogen bonds
from basic solvents to acidic hydrogens,^such as Hi, Influences the chemical
shifts of the hydrogen; hydrogen bond to an acidic hydrogen from the solvent
shifts the hydrogen downfield.^ The size of the anion may play a role in the
sensitivity difference between BF4" and PFe". PFe" is larger than BF4". The larger
size makes the distance of interaction of PFe" with Hi greater, allowing for more
interaction with the solvent.
29
Table 3 shows the chemical shifts of the aromatic hydrogens of 1-
hexadecyl-3-methylimidazolium tetrafluoroborate, HDMrBF4". The data in Table
Table 3. H NMR chemical shifts in ppm for the aromatic hydrogens of HDMrBF4"in deuterated solvents.
Solvent Acetic acid Acetone Acetonitrile DMF DMSO Chloroform Methanol Trifluoroacetic Acid
Hi 9.02 9.07 8.39 9.29 9.07 8.52 8.76 8.42
H2 7.74 7.78 7.35 7.94 7.74 7.30 7.52 7.28
H3 7.72 7.72 7.32 7.86 7.68 7.27 7.45 7.26
3 show that the different solvents have little influence on H2 and H3 in comparison
to Hi. The chemical shifts of H2 and H3are smaller in the different solvents;
whereas, Hi is more sensitive. The more polar the deuterated solvent, the more
downfield Hi shifts. Hi shifts to 9.29 ppm in polar DMF solvent, while H2 and H3
are shifted to 7.94 and 7.86, respectively.
Figures 19, 20, and 21 are plots of the chemical shift data for the aromatic
hydrogens of HDMrBF4".
30
9.4
9.2
9.0 ]
E QL Q.
X a ^ 8.8 sz <t) 15 o E 0) sz O 8.6 -
8.4
8.2
-
-
TFA ......J
-,...,_
—
6HI = 1 . 3 9 6 H 2 - 1 . 7 4 R2 = 0.98; SD = 0.04
/ M B O H
• / / C D C I 3
• AN
- 1 1 1
DMF A
DMSO / • y^ Acetone
/ HOAc
J i
I I • • • ! - • • i
7.2 7.3 7.4 7.5 7.6 7.7 7.8
Chemical shift for H2 ppm
7.9 8.0
Figure 19. Chemical shifts of the aromatic hydrogens Hi and H2 of HDMrBF4" in the deuterated solvents.
31
9.4
9.3
9.2
9.1 -
E9.0 Q. Q.
^8.9
sz <n 158.8 o "E (D
08.7
8.6
8.5
8.4
8.3
DMF
6HI = 1 .436H3 ~ 1.97 R = 0.97; SD = 0.06
MeOH
CDC 13
DMSO
7.2 7.3 7.4 7.5 7.6 7.7
Chemical shift for H3 ppm
7.8 7.9
Figure 20. Chemical shifts of the aromatic hydrogens Hi and H3 of HDMrBF4" in the deuterated solvents.
32
8.0
7.9
7.8
& 77 CM
X
1 7.6 </)
15 o
7.4
7.3
7.2
-
6H2= 1 . 0 6 6 H 3 - 0 . 4 1 R = 0.994; SD = 0.02
DMSO
MeOH /
y/AN
/CDCI3 •
TFA
1 1
• DMF
X Acetone
HOAc
1
7.2 7.4 7.6 7.8
Chemical shift for H3 ppm
8.0
Figure 21. Chemical shifts of the aromatic hydrogens H2 and H3 of HDMrBF4' in the deuterated solvents.
33
HDMrBF4" was soluble in ethanol only at elevated temperatures;
therefore, ethanol was not used in the correlations. From the data in Figures 18,
19, and 20, H2 and H3 appear to be affected similarly by the different solvents,
while Hi seems to have a larger chemical shift variation.
Equation 8 shows the relationship that exists between the chemical shifts
of H2 and H3 for HDMrBF4" from Figure 20.
. 2 _ 8H2 = 1 .068H3 - 0.41; R = 0.994, SD = 0.02 (8)
The closeness to unity of the slope and the goodness of the fit between
the two hydrogens indicates that the aromatic hydrogens, H2 and H3 are similarly
affected by solvation as HDMrBF4"and HDMI PFe". This observation is
reasonable because of the chemical environments that these two hydrogens
occupy. Relationships for the chemical shifts of Hi versus H2 and Hi versus H3
give non-unity slopes as shown in Equations 9 and 10 and Figures 18 and 19.
8HI = 1 .398H2 - 1.74; R = 0.98, SD = 0.04 (9)
8HI = 1-438H3 - 1.97; R = 0.97, SD = 0.06 (10)
34
Chemical shift data for the aromatic hydrogens of HDMrPFe' in various
deuterated solvents are given in Table 4.
Table 4. H NMR chemical shifts in ppm for the aromatic hydrogens of HDMrPFe" in the deuterated solvents.
Solvent Acetic Acid Acetone Acetonitrile Chloroform DMSO DMF Methanol Trifluoroacetic Acid
Hi
8.81 9.00 8.43 8.55 9.07 9.28 8.76 8.14
H2
7,48 7.73 7.36 7.25 7.73 7.93 7.50 7.05
H3
7-46 7.67 7.33 7,24 7.67 7.86 7.44 7.02
HDMrPFe" was only soluble In ethanol at elevated temperatures so this solvent
was not used in the correlations. According to the data, the different solvents
have little influence on H2 and H3 in comparison to Hi. From Table 4, it is
obvious that the chemical shifts of the aromatic hydrogens of HDMrPFe' are
solvent dependent, with Hi showing to have greater dependency in comparison
to H2 and H3. The deuterated solvents that are more polar, such as DMF, shift H-
further downfield in comparison to a less polar solvent such as acetonitrile.
Figures 22, 23, and 24 are the plots of the chemical shift data for
HDMrPFe".
35
9.4
9.2
9.0 -
E Q. CL
5 8.8
x : CO
15 .y 8.6 E x: O
8.4 -
8.2
8.0
-5HI = 1 .256H2 - 0.63 R = 0.96; SD = 0.06
HOAc / • /
y^MeOH
CDCI3 /
/ •AN
• TFA
1 1 1
DMF
DMSO / ^ /
/ • / Acetone
1
1
7.0 7.2 7.4 7.6
Chemical Shift for H2 ppm
7.8 8.0
Figure 22. Chemical shifts of the aromatic hydrogens Hi and H2 of HDMrPFe' in the deuterated solvents.
36
9.4
9.2
9.0
E Q. Q.
^ 8.8 o
CO
"co 8.6 o "E <D
O 8.4
8.2
8.0
- 6HI = 1 .346H3 - 1.25 R = 0.97; SD = 0.04
"
HOAc • y
# /
DMSO X
/ Acetone
XMeOH
CDCI3 / • X X •
X AN
• TFA
1 1 1 I
DMF
1 1 1
7.0 7.2 7.4 7.6 7.8
Chemical Shift for H3 ppm
8.0
Figure 23. Chemical shifts for the aromatic hydrogens Hi and H3 of HDMrPFe" in the deuterated solvents.
37
8.0
7.9
7.8 -
7.7
§: 7.6 H Csl
X a 7.5
CO
(Q
o E 0)
6
7.4 -
7.3
7.2
7.1
7.0
6.9 6.9
6H2 = 1 .066H3 - 0.43 R = 0.997; SD = 0.02
TFA
7.1
Acetone
DMSO
MeOH
DMF
HOAc
7.3 7.5 7.7
Chemical Shift for H3 ppm
7.9 8.1
Figure 24. Chemical shifts for the aromatic hydrogens H2 and H3 of HDMrPFe" in the deuterated solvents.
38
Equation 11 shows the unity that exists between H2 and H3. The similar
environments of H2 and H3 are influenced In the same way by the change in
solvent.
6H2 = 1 .066H3 - 0.43; R = 0.997; SD = 0.02 (11)
The relationship between Hi and H2 gives a different slope than the relationship
between Hi and H3, as seen in Equations 12 and 13.
6HI = 1 .256H2 - 0.63; R = 0.96; SD = 0.06 (12)
6HI = 1 .346H3 - 1.25; R = 0.97; SD = 0.04 (13)
Equations 12 and 13 show that Hi is affected slightly differently from H2 and H3.
Based on the magnitude of the slopes, the implication is that the chemical shift of
Hi is approximately 1.2 times more sensitive to changes in solvent properties,
compared to H2 and 1.3 times more sensitive than H3. The small difference in
slope can be a result of the large size of PFe" and the bent side arm decreasing
the solvent interactions.
39
The comparison of the slopes of HDMrPFe" and HDMrBF4"is shown in
Figure 25. The smaller slopes of HDMrPFe" compared to HDMrBF4" happens
due to the large size of the PFe" and the orientation of the bulky side chain in
which both are hindering the interaction of the solvent with Hi.
1.39
Figure 25. Slope comparison of HDMrBF4" and HDMrPFe'
The use of the different anions and side arms plays a role in the
solute/solvent interaction of the ionic liquids. Figure 26 illustrates the difference
in slope for BMrBF4", BMrPFe", HDMrBF4', and HDMrPFe". The slopes of
BMrBF4", HDMrBF4", and HDMrPFe" range from 1.2 to 1.4, revealing that the
solute/solvent Interactions for these ionic liquids behave similariy in the same
environments. The slopes of 1.6 and 1.7 for BMrPFe" reveal that this ionic liquid
has the greatest solute/solvent interaction.
40
BF, PF,
1.39
Figure 26. Slope comparison of BMrBF4", BMrPFe", HDMrBF4", and HDMrPFe".
41
Nature of the Solvent Interactions
The H NMR spectroscopic investigations give insight into possible
interactions of the anion with the cation and the solvent. The relationships
between two anions, PFe" and BF4" were studied along with altering the side
chain from a butyl to a hexadecyl. Figure 27 shows possible interaction of the
solvent with the anion and/or the cation. The proposed interaction between
CM 41^9
+ 'i">—Hl""'X""""H-sol
sol-H»»«'»>X"»«»»H3
H-C.H
/ 4^9
By-sol-H»»'»»»X"»»»"»H2
C4H9
N
CH-
Figure 27. Proposed interaction between the BMI* cation and the acidic solvent. X" = BF4" or PFe".
the anion and the solvent via hydrogen bond from the acidic solvents to the basic
anion could account for the slight chemical shift change of the protons as noted
for H2 and H3.
42
Figure 28 shows the possible solvent interacfions between the solvent and
the acidic hydrogens of the cation incorporated with the interaction of the anion.
C Hc
Figure 28. Proposed interaction between the BMr cation and the basic solvent. X" = BF4' or PFe".
The interaction between the solvent and the acidic hydrogens of the cation could
account for the difference in the slopes of the BF4" versus the PFe" anions. The
slope for PFe" is 0.30 greater than for BF4" of Hi indicating that the interaction of
PFe" with the cation may be a greater distance, thus allowing for more solvent
interaction with Hi. The smaller slope of Hi for BF4' indicates a closer interaction
of the anion with the cation giving less interaction for the solvent.
43
The role of the side arm may be more significant with the HDMI* cation
with regard to the orientation of the side arm. Solvent interaction with the cation
and anion is shown in Figure 29.
P16H33
))—HV"'X'"""H-sol
CH
N u N sol-H"""X"i""»H2 \ • 2 V u CH.
^ CifiHo-a sol-H'»«'»X"»«»«»H3 / ^^ ' '
CH3
Figure 29. Proposed interaction between the HDMI* cation and the acidic solvent. X" = BF4" or PFe".
H2 and H3 have only slight chemical shifts in the different solvents and may be
attributed to the close interaction of the anion with the cation not allowing for
interaction with the solvent. The slope of unity for H2 and H3 Is consistent with
their similar solvent effects.
44
Interaction of the solvent with the cation is illustrated in Figure 30. The
interaction of Hi with the anion and the different solvents is influenced by the
H3 ..^'^"^\. H3 .P'^"''
Hf ^ ' °l CH3
\
sol:
'''u P 16 33
2 CH3
Figure 30. Proposed interaction between the HDMr cation and the basic solvents. X" = BF4" or PFe".
saturated alkyl chain. With the BF4" anion of HDMI*, the slopes are essentially
the same as with the BMrBF4" but HDMrPFe" is different from BMfPFe" based
on the slope being closer to unity. The long side chain of HDMrPFeis in a
dynamic equilibrium where the chain does not stay in one position, but flops back
and forth. From the results, the bulky alkyl side chain is oriented toward Hi more
than H3. The lower slope of HDMTPFe" gives evidence that the side chain is
blocking solvent interaction as the interaction of PFe" with Hi blocks the other
side preventing a greater solvent dependence.
45
Use of SolvatochromIe Parameters to Analvze the Solvation Interactions of BMrBFz"
Another way to consider how the properties of ionic liquids are affected by
a change of the anion or the side chain is the use of the Kamlet-Taft equation.
Equation 14 is used to analyze the solute-solvent Interactions that affect the
chemical shifts (8AH) of BMIC in different solvents. 6AH represents changes in
chemical shift and n*, a, and p are the solvent's dipolarity-polarizability, hydrogen
bond donor ability, and hydrogen bond acceptor ability, respectively, and 8AHois
the intercept.
8AH = 571* + aa + /)P + 8AHO (14)
The extent and importance of the different solute-solvent interactions are
obtained fi'om the sign and magnitude of the coefficients s, a, and b, wherein a
negative value implies a favorable interaction between the solute and solvent and
a positive value implies the opposite. For the solvation property, the magnitude
and sign of the coefficients reflect the relative importance of the solvent-solute
interaction.
DMF>DMSO>CH3C02H>CDCl3>MeOH>D20>CH3CN is the polarity trend
of the solvents used. Solvent acidity is a product of the coefficient a in Equation
14. Protic solvents interact with solutes that have basic sites. This interaction is
created by the formation of hydrogen bonds between a solvent molecule and a
46
solute molecule. The effectiveness of such hydrogen bonds depends on the
ability of both solvent and solute molecules to approach close enough to allow
interaction of the hydrogen.
Table 5 presents the solvent parameters for the solvents used for the
regression equations.^ The contribution of TT* is typically much less than a
and p parameters because the polarizability effect is highly attenuated in the
solution phase. Therefore, a two-parameter equation was considered in this
research.
Table 5. SolvatochromIe Parameters.
Solvent rr 0 g DMSO
DMF
Acetonitrile
Acetone
Acetic Acid
Methanol
Ethanol
Water
1.00
0.88
0.75
0.72
0.62
0.60
0.54
1.09
0.00
0.00
0.15
0.07
1.09
0.98
0.86
1.13
0.76
0.69
0.37
0.48
0.00
0.62
0.77
0.18
Although the numbers look promising, the standard deviations show that
the regression equations are not reliable. To get reliable relationships, the rule of
thumb is, the number of solvents should be three times the number of
parameters being calculated. With this in mind, the equations are considered not
reliable due to an insufficient number of solvents analyzed. None the less, they
can give an indication of the nature of any solute/solvent interactions.
47
The regression analysis was performed first with all three coefficients and
without a significant contribution from TT*, the regression analysis was performed
without the TT* resulting in Equations 15, 16, and 17.
Regression equations without the TT* variable for BMrBF4" are:
8HI = 8.63-0.17a+ 0.47P (15)
8H2 = 7.51-0.17a+ 0.32P (16)
8H3 = 7.48 - 0.17a + 0.26p. (17)
Solvent basicity is a product of the coefficient b in Equation 14. Basic solvents
form hydrogen bonds to acidic sites of solutes. From the equations, the
magnitude of the coefficient b for the basicity property of the salt is similar for H2
and H3, but different for Hi. Since Hi is bonded to a carbon, which is bonded to
two nitrogen atoms, it is expected that Hi should be more sensitive to solvent
effects. The similarity of the coefficients for H2 and H3 indicates a similar
interaction between the solvent and these hydrogens. The different
environments of H2 versus H3 result in the slightiy different coefficients.
48
Table 6 shows the standard deviations and n represents the number of
solvents used.
Table 6. Coefficients and statistics using equation 14 without polarizability (TT*) for BMrBF4".
Constant a b R n SD
8H1 8.63 + 0.12 -0.17 + 0.11 0.47 + 0.20 0.580 7 0.17
8H2 7.51+0.10 -0.17 + 0..08 0.32 + 0.15 0.590 7 0.13
8H3 7.48 + 0.09 -0.17 + 0.08 0.26 + 0.14 0.580 7 0.12
Analysis of the Solvent Interactions of BMrPFe"
To see the significance of the anion contribution on the properties,
BMrPFe". Figure 31 shows the aromatic hydrogens of BMrPFe" and shows the
interaction between the solvent and the acidic hydrogens of the cation, along with
the interaction with the anion.
49
Again the equations using all three coefficients resulted in polarizability effect not
being significant. Because the polarizability values showed no significant
dependability or statistical significance, these values were not retained.
Figure 31. Aromatic hydrogen designation for BMrPFe"
This observation is not surprising since BMrPFe" is a polar compound and
solvation by this mode should not play a major role in the solvation of these
compounds since the polar interaction between the anion and the cation of the
salt should be much stronger than the interaction with the solvent.
The regression analysis was performed without the TT* values resulting in
Equations 18, 19, and 20.
Regression equations computed for Figure 15 without n* values:
8HI=8 .48 -0 .06a+ 0.73P (18)
8H2=7.45 - 0.07a + 0.40p (19)
8H3=7.42 - 0.08a + 0.34p. (20)
50
Solvent basicity is a product of the coefficient b in Equation 14. Basic solvents
form hydrogen bonds to acidic sites of solutes. From the equations, the
magnitude of the coefficient b for the basicity property of the salt are similar for
H2 and H3, but different for Hi. Since Hi is bonded to a carbon that is between
two electronegative nitrogen atoms, it is expected that Hi should be more
sensitive to solvent effects. The similarity of the coefficients for H2 and H3
indicates similar Interactions between the solvent and these hydrogens. Hi has a
higher b coefficient than that of BMrBF4'. This is because of the larger size of
the PFe" anion creating a greater distance for the interaction with Hi, allowing for
a greater interaction witfi the solvent.
Table 7 shows the standard deviations and n represents the number of
solvents used for the calculations.
Table 7. Coefficients and statistics using equation 14 without polarizability (Tt*) for BMrPFe".
Constant a b R n SD
8H1 8.48 + 0.34 -0.06 + 0.28 0.73 + 0.51 0.541 6 0.27
8H2 7.45 + 0.22 -0.07 + 0.19 0.40 + 0.33 0.498 6 0.18
8H3 7.42 + 0.21 -0.08 + 0.17 0.34 + 0.32 0.477 6 0.17
51
Analysis with Gaussian 94 for Imidazolium Cations
Charge distribution on the atoms of the imidazolium cation contributes to
the physical properties of the ionic liquid. Varying the side arm is one way to
compare the charge distribution, by using the Gaussian 94 program. Gaussian
94 using standard basis sets with no modification^^ was executed to prepare the
ab initio calculations. Ail calculations were performed on a Silicon Graphics
Indigo computer. Conformations were optimized at each level of theory.
Convergence was to the limits imposed internally by Gaussian 94. Vibrational
frequencies were calculated at each level of theory and the results were used to
determine the nature of the conformations
Figure 32 shows the numbering system used for the imidazolium cation
calculations.
Ci C9
Hs 94 He
^10
Figure 32. Numbering system for imidazolium cation.
52
Table 8 shows the z-matrix placed in the Gaussian equations to produce
the charge distribution.
Table 8. Z-Matrix for imidazolium cation of Figure 31.
Atom Bond BondLenqth Anale C(1) C(2) C(1) N(3) C(2) C(4) N(3) N(5) C(1) H(6) C(1) H(7) C(2) H(9) N(3) H(10) C(4) H(8) N(5)
1.341 1.382 1.314 1.382 1.068 1.068 1.000 1.070 1.000
C(1) C(2) C(2) C(2) C(1) C(2) N(3) qi)
First Angle Third Atom Second Angle
106.478 109.552 106.477 131.288 131.286 125.754 126.029 125.751
C(1) N(3) N(3) N(5) C(1) 0(2) C{2)
0.000 0.000
179.684 -180.000 180.000 180.000 180.000
Angle T
Dihedral Dihedral Dihedral Dihedral Dihedral Dihedral Dihedral
Bond lengths, bond angles, and dihedral angles are the descriptions given for the
molecular geometry that is collectively known as the Z-matrix. The identity and
position of a general atom are specified by the Z-matrix.
53
Table 9 shows the relative charge distribution for the imidazolium cation.
Table 9. Charge distribution of atoms on the imidazolium cation.
t
1
2
3
4
5
6
7
8
9
10
Atom
C
C
N
C
N
H (H2)
H (H3)
H
H
H (Hi)
Sto-3g
0.067613
0.067604
-0.257026
0.260872
-0.257013
0.155994
0.156031
0.309131
0.309134
0.187660
RHF/3-21g
0.069705
0.068701
-0.884550
0.616498
-0.884552
0.360563
0.360563
0.450179
0.450179
0.391713
RHF/6-31g*
0.027307
0.027307
-0.684162
0.407426
-0.684163
0.315867
0.315867
0.470660
0.470660
0.333230
RHF/6-31+g*
0.074369
0.074369
-0.595444
0.279707
-0.595444
0.326072
0.326072
0.536161
0.536161
0.335453
The Gaussian numbering system for imidazolium cation is different from
the other calculations done previously In the research. H10, H6, and H7 are
equivalent to the representation of HI, H2 and H3, respectively and are the
atoms that are most important to this study. The level of theory increases as the
columns go fi-om Sto-3g to RHF/6-31+g*. According to tfie data, H6 and H7 have
the same charge throughout the levels of theory which contributes to the
argument that these two hydrogens are essentially the same. H10 has the
greatest postive charge compared to H6 and H7 showing that it Is more sensitive
to changes in the environment.
54
Figure 33 shows the numbering of 1-fluoroimidazolium cation.
H
/ \
H 10
Figure 33. 1-Fluoroimidazolium cation.
Table 10 shows the charge distribution for 1-fluoroimidazolium cation.
Table 10. Charge distribution of atoms of 1-fluoroimidazolium cation.
1 2 3 4 5 6 7 8 9 10
Atom C C N C N H H H F H
(H2) (H3)
(HI)
Sto-3g 0.07450 0.06310 -0.06642 0.25100 -0.25461 0.16270 0.16880 0.31503 0.08578
0.200134
RHF/3-21g 0.06857 0.09182 -0.42569 0.63681 -0.89223 0.37514 0.39110 0.46112 -0.12916 0.422514
RHF/6-31g 0.10171 0.14635 -0.30834 0.60051 -0.87987 0.34281 0.35898 0.47416 -0.21715 0.380857
RHF/6-31g* 0.03440 0.01618 -0.01635 0.38843 -0.70141 0.32852 0.34212 0.47832 -0.22783 0.357625
RHF/6-31+g* -0.11335 -0.01629 0.05691 0.22214 -0.56730 0.33671 0.34699 0.54834 -0.17003 0.355873
55
The use of 1-fluoroimidazolium cation Is to determine the effect of
electi'onegative groups bonded to the nitrogen, to see how the positive charge of
HI versus H2 and H3 is affected. Similar to imidazolium cation, H10 has the
greater charge over H6 and H7. The charge of H10 for 1-fluorolmdazolium
cation is 0.355873 and H10 for imidazolium cation is 0.335453, which Illustrates
that the fluorine allows for a more positive charge on H10. The fluorine atom
also has an affect on H7 creating different charges for H6 and H7. Varying the
side arm on the cation can have an impact on the properties of the ionic liquid.
Analysis of Solvation Properties of Ionic Liquids bv Theoretical Descriptors
The generalized TLSER equation for solutes in a given medium is shown
in Equation 21:
SSP = aVmc +b7ti + csB +dq- +eeA + fq+ +SSPo. (21)
SSP represents the solute/solvent interactions that cause property variations.
Vmc describes the molecular van der Waals volume (in units of 100 cubic
angstroms, A" ). TIJ describes the dipolarity/polarizability contribution and is
obtained firom the division of the polarizability volume by the molecular volume to
produce a unitiess, size Independent quantity which indicates the ease with
which the electron cloud of a solute may be moved or polarized, SB is part of the
56
hydrogen bond acceptor basicity (HBAB) contribution and is the energy
difference between the highest occupied molecular orbital (HOMO) of the solute
and the lowest unoccupied molecular orbital of (LUMO) water. Water was
chosen as the reference because it is the most common solvent. The
electrostatic term (q.) Is the largest negative formal charge on an atom of the
solute; the units are atomic charge units (acu). 8A describes the covalent acidity,
which reflects the ability of a solute to act as a Lewis acid. These descriptors are
obtained fi-om the difference between the energies of he LUMO of the solute and
the HOMO of water. q+ is the electrostatic acidity term and is the largest positive
formal charge on an atom of the solute; the units are in acu. SSPo is the
intercept. The coefficients of the MLR equations, a, b, c, d, e, and f, indicate the
significance of the different solute/solvent interactions to the property being
analyzed.
With only four salts examined in this research, a complete TLSER analysis
could not be carried out. These TLSER parameters are very useful in predicting
the abilities of these and other ionic liquids to act as solvents for reactions.
57
Figure 34 is the structure used to analyze different substituents of the
imidazolium cation.
Figure 34. Structure of 1,3-disubstituted imidazolium cation for which theoretical descriptors have been developed. X and Y are shown in Table 11
For many ionic liquids, the sti-ucture of the cations are different while the
anion remains the same. Thus characterization of the cation can be useful in
designing the proper ionic liquid. The TLSER descriptors computed^ for various
1,3-disubstituted imidazolium cations in Table 11 were accomplished using the
MNDO algorithm contained In MacSpartan. ®
58
Table 11. Theoretical descriptors for different ionic liquids and molecular solvents.
Substituted Cation
X=H; Y=H X=H; Y=F X=H; Y=CH3 X=H; Y = C F 3
X=F; Y=CF3 X=F; Y=CH3 X=F; X=F X=CH3; Y = C F 3
X=CH3; Y=CH3 X = C F 3 ; Y=CF3 X^CHs; Y=C4H7 X=CH3;Y=Ci6H3i X=H; Y = 0 C H 3
X=0CH3; Y=0CH3 X=H; Y=SiMe3 X=SiMe3; Y=SiMe3
Molecular solvents
Benzene CH2CI2 CHCI3 THF Acetone water MeOH EtOH EG DMSO AN
Vmc
70.417 76.317 88.432
104.228 110.126
94.341 82.210
122.255 106.434 138.085 158.926 368.808
97.845 125.276 152.686 234.837
Vmc
86.893 58.898 73.455 80.609 67.969 20.478 38.558 56.073 65.172 75.295 49.110
7Ci
0.1094 0.1045 0.1111 0.0948 0.0925 0.1071 0.1004 0.0985 0.1126 0.0879 0.1121 0.1101 0.1103 0.1190 0.1127 0.1142
TCi
0.1173 0.1106 0.1210 0.0999 0.0922 0.0549 0.0814 0.0893 0.0869 0.0980 0.1182
es
0.0959 0.0908 0.0981 0.0921 0.0874 0.0931 0.0784 0.0942 0.1000 0.0889 0.1013 0.1166 0.0962 0.0966 0.1021 0.1069
SB
0.1517 0.1344 0.1310 0.1378 0.1380 0.1237 0.1314 0.1324 0.1372 0.1540 0.1425
q-
0.1749 0.1688 0.2084 0.3124 0.3077 0.2027 0.0289 0.3145 0.2111 0.3039 0.2121 0.2116 0.1764 0.1461 0.3378 0.3474
q-
0.0593 0.1183 0.0723 0.3277 0.2839 0.3256 0.3292 0.3234 0.3276 0.7146 0.3422
EA
0.2305 0.2367 0.2285 0.2360 0.2415 0.2342 0.2427 0.2339 0.2267 0.2405 0.2254 0.2252 0.2312 0.2316 0.2242 0.2196
EA
0.1744 0.1702 0.1783 0.1471 0.1714 0.1237 0.1402 0.1442 0.1455 0.1615 0.1903
q+
0.2738 0.2826 0.2707 0.2800 0.2229 0.2075 0.2320 0.1989 0.1831 0.2133 0.1817 0.1817 0.2742 0.2043 0.2642 0.1686
q+
0.0593 0.0642 0.1030 0.0209 0.0233 0.1628 0.1803 0.1784 0.1875 0.0564 0.0951
59
From the Table 11, it is obvious that as groups are added to the imidazolium
cation, the size of the cation increases. For effective solvation of some solutes,
especially dipolar molecules, solvents must gain access between the lines of
forces of these molecules and large solvents cannot effectively solvate such
solutes. Depending on the nature of the group added to the imidazolium cation,
the polarizability is affected differentiy. The addition of electronegative groups,
such as fluorine or the trifluoromethyl groups, decreases the polarizability,
relative to the imidazolium cation. Polarizbility effect is typically highly attenuate
in the condensed phase, compared to the gas phase and as a result, this factor
may not play a major role in solute/solvent interactions involving ionic liquids. By
changing the groups on the imidazolium cation, the ability to accept electrons
Increases slightiy only by adding electron donating groups. These numbers
indicate that the electron donating groups can only slightiy influence the
imidazolium cation's acceptability. An important solvation property for solvents in
the ability to donate electrons to the solute. It is obvious that the cation will not
be good at participating in this type of solute/solvent interaction. From Table 11,
EB only ranges as high as 0.1 and as low as 0.07 compared to molecular solvents
that range from 0.15 to 0.12. A similar observation is made for the most
negative atom on the imidazolium cation, and groups such as the trifluoromethyl
group, the negative charge is apparentiy pulled out to the fluorine. Of all the
solvent properties, the ability to accept a pair of electrons, changes of the groups
of the imidazolium cations have a tremendous effect. From Table 11, the
60
substituted cations range from 0.24 to 0.21 and the molecular solvents range
from only 0.19 to 0.12. A solvent that comes close to this is nitrobenzene. On
the other hand, the most positive atom of the imidazolium cations is increased by
adding the electronegative groups to the imidazolium cations. Therefore, by
taking a look at the influence of the side arm on the cation is useful in predicting
the cation's properties.
61
CHAPTER ill
CONCLUSIONS
The physical properties and solute/solvent interactions of ionic liquids can
be varied by changing the anion and/or the length of the side chain to fit specific
requirements. The solute/solvent interactions of BMrBF4" versus HDMrBF4" are
not affected significantiy by the different side chains. On the other hand, the
solute/solvent interactions of BMrPFe" versus HDMrPFe'are affected
significantiy by the difference in the side chain. By allowing the anion to remain
the same, theorectical properties of tiie cation can be predicted by varying the
character of the side chain. By studying the properties of the Ionic liquids, the
use of ionic liquids can be tuned to meet specific requirements as a solvent for
reactions.
62
CHAPTER IV
EXPERIMENTAL SECTION
Preparation of 1-Methvl-3-propvllimidazolium Chloride
In an oven-dried round bottom flask, 1-chloropropane (2.50 g, 0.0318 mol)
was added to 1-methylimidazole (2.10 g, 0.0256 mol). The mixture was attached
to a reflux condenser and placed in a silicone oil bath heated to approximately
50 °C. The reaction was allowed to stir and heat at approximately 50 °C for 48
hours. The resulting light yellow solution was cooled to room temperature and
washed 3 times with 10 mL portions of ethyl acetate. The excess ethyl acetate
was removed under vacuum using the ft-eeze/thaw method; H NMR (CDCI3, 200
MHz) 6 10.42 (1H, s), 7.55 (1H, s), 7.43 (1H, s), 4.21 (2H, t), 4.03 (3H, s), 1.85
(2H, sex), 0.89 (3H, t);
Preparation of 1-Butvl-3-methvlimidazolium Chloride
In an oven-dried round bottom flask with a stirring bar, 1-chlorobutane
(5.00 g, 0.0540 mol) was added to 1-methylimidazole (4.00 g, 0.0487 mol). A
reflux condenser topped with a drying tube of calcium chloride was attached and
the assembled apparatus was placed in a silicone oil bath. The clear mixture
was stirred and heated at 70 °C for 48 hours. The resulting, tan color viscous
liquid was allowed to cool to room temperature and then was washed three times
with 20 mL portions of etfiyl acetate. After the last washing, the remaining ethyl
acetate was removed under vacuum at approximately 70 °C; IR (NaCI) v 3139-
63
2872, 1570, 1171 cm-1; H NMR (CDCI3, 200 MHz) 6 10.04 (1H, s), 7.57 (1H, s),
7.42 (1H, s), 4.18 (2H, t). 3.95 (3H, s), 1.73 (2H, q), 1.19 (2H, sex), 0.78 (3H, t);
Preparation of 1-Butvl-3-methvlimidazolium Hexafluorophosphate
In an oven-dried round bottom flask with a stirring bar,
hexafluorophosphoric acid (2.00 g, 0.0164 mol) was added dropwise, to prevent
a significant temperature increase, to a mixture of 1-butyl-3-methylimidazolium
chloride (2.00 g, 0.0115 mol) in 10 mL of distilled water. The light tan mixture
was allowed to stir at room temperature for 48 hours. The upper acidic layer was
decanted and the lower ionic liquid was washed ten times with 20 mL of distilled
water or until the washings were no loner acidic. Acidity was tested with pH
paper. The resulting brown liquid was placed under vacuum at approximately
70°C to remove excess water. To ensure that the water was removed, a freeze-
thaw method was used. Under vacuum, the solution was quickly frozen with
liquid nitrogen and then thawed using a heat gun; H NMR (CDCI3, 200MHz) 6
8.35 (1H, s), 7.30 (2H, d). 4.08 (2H, t), 3.82 (3H, s), 1.82 (2H, q), 1.25 (2H, sex)
0.84 (3H,t); IR: 3169 and 3125 [vC—H) aromatic], 2966, 2939 and 2876 [v(C—
H) aliphatic]; 1573 and 1468 [v(C==C)l; 841 [v(PF)] cm"''. d= 1.36 g/cm^
Preparation of 1-Butvl-3-methvlimidazolium Tetrafluoroborate
In an oven-dried round bottom flask with a stirring bar, sodium
tetrafluoroborate (2.0o g, 0.0182 mol) was added to a mixture of 1-butyl-3-
methylimidazolium chloride (2.0o g, 0.0115 mol) in 10 mL of acetone. The light
64
brown mixture was allowed to stir at room temperature for 48 hours. The
resulting gold color liquid was filtered through a plug of Celite. To remove the
excess acetone, the liquid under vacuum was heated to approximately 50 °C; H
NMR (CDCI3, 200 MHz) 6 8.65 (1H, s), 7.41 (2H, d), 4.13 (2H, t), 3.85 (3H, s),
1.79 (2H, q), 1.28 (2H, sex), 0.89 (3H, t); IR; 3166 and 3121 MC—H) aromatic];
2967, 2941, and 2883 [v(C—H) aliphatic]; 1575 and 1472 MC==C)]; 1061
MBF)]cm"V c/=1.21 g/cm^
Preparation of 1-Methvl-3-pentvllimidazolium Chloride
In an oven-dried round bottom flask, 1-chloropentane (2.90 g, 0.0272 mol)
was added to 1-methylimidazole (2.20 g, 0.0268 mol). The mixture was attached
to a reflux condenser and placed in a silicone oil bath. The reaction was allowed
to stir and heat at approximately 70 °C for 48 hours. The resulting light green
solution was allowed to cool to room temperature and then washed with 10 mL
portions of ethyl acetate three times. After the last washing, the excess ethyl
acetate was removed under vacuum by heating to approximately 70 °C.
Preparation of 1-Octvl-3-methvlimidazolium Chloride
In an oven-dried round bottom flask, 1-chlorooctane (4.00 g, 0.0269 mol)
was added to 1-methylimidazole (2.00 g, 0.0244 mol). The mixture was
assembled to a reflux condenser and placed in a silicone oil bath. The reaction
was allowed to stir and heat to approximately 70 °C for 48 hours. The resulting
light gold solution was allowed to cool and then washed three times with 10 mL
65
portions of ethyl acetate. After the final wash, the excess ethyl acetate was
removed under vacuum by heating to approximately 70 °C. H NMR (CDCI3, 200
MHz) 6 10.16 (1H, s), 7.56 (1H, s), 7.35 (1H, s), 4.19 (2H, t), 3.99 (3H, s), 1.77
(2H, m), 1.12 (10H,m), 0.70 (3H,t);
Preparation of 1-Hexadecvl-3-methvlimidazolium Bromide
In an oven-dried round bottom flask, 1-bromohexadecane (22.40 g,
0.0734 mol) was added to 1-methylimidazole (6.00 g, 0.0731 mol). The mixture
was attached to a reflux condenser, placed in a silicone oil bath and heated to
140°C during a period of 10 minutes. An exothermic reaction in the latter stages
of heating produces an emulsion that disappears after a few minutes resulting in
a brown viscous liquid. Upon cooling the reaction a brown solid formed.
Preparation of 1-Hexadecvl-3-methvlimidazolium Tetrafluoroborate
In an oven-dried round bottom flask, sodium tetrafluoroborate (2.00 g,
0.0182 mol) was added to 1-hexadecyl-3-methylimldazolium bromide (4.00 g,
0.0103 mol) dissolved in 20 mL of acetone and allowed to stir for 48 hours. The
resulting mixture was filtered through a plug of celite. To remove the excess
acetone, the liquid under vacuum was heated to approximately 50 °C. The
resulting yellowish solid was dried under vacuum overnight. H NMR (CDCI3, 200
MHz) 5 8.81 (1H, s), 7.34 (1H, s), 7.27 (1H. s), 4.14 (2H, t), 3.89 (3H, s), 1.84
(2H, m), 1.22 (26H, m), 0.85 (3H, t); m. p. 155 °C
66
Preparation of 1-Hexadecvl-3-methvlimidazolium Hexafluorophosphate
In an oven-dried round bottom flask, hexafluorphosphoric acid (2.40 g,
0.0164 mol) was added to 1-hexadecyl-3-methylimidazolium bromide (4.00 g,
0.0103 mol) dissolved in 20 mL of acetone and allowed to stir for 48 hours. The
resulting mixture was filtered through a plug of celite. To remove the excess
acetone, the mixture under vacuum was heated to approximately 50 °C. The
resulting yellowish solid was dried under vacuum overnight. H NMR (CDCI3, 200
MHz) 6 8.56 (1H, s), 7.25 (1H, s), 7.24 (1H, s), 4.02 (2H, t), 3.79 (3H, s), 1.73
(2H, m), 1.12 (26H, m), 0.74 (3H, t); m. p. 163°C
Preparation of NMR Solvent Studv Samples
Each NMR sample was prepared in a glove box that was flushed out with
nitrogen gas. The ionic liquid was measured out and the appropriate solvent was
added to make consistent concentration of 0.45 mol/L. The samples were
analyzed on either the IBM/Bruker AF 200 MHz, IBM/Bruker AF 300 MHz, or the
Varian UnitylNOVA 500 MHz NMR machine.
Procedure for Calculating the Density
The Paar DMA 602 external measuring cell, the Paar 60 processing unit,
and the Neslab EX-210/FTC-350A Cooler/Circulator are the components that
make up the density measuring system. The measurement of densities with the
Paar instrument is based on the change of the natural frequency of a hollow
oscillator when filled with a liquid in the DMA 602 unit.
67
The instrument was calibrated by performing measurements on air and
delonized water. The accuracy of the calibration was checked by measuring the
density of acetone and comparing to the literature values of CRC Handbook.
Each Ionic liquid was measured three times.
68
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Agree (Permission is granted.)
Student Signatui'e Date
Disagree (Permission is not granted.)
Student Signature Date
