-
A comparative ion chemistry study of acetone, diacetone alcohol,
and mesityl oxide
AFAF KAMAR,' ALEXANDER BALDWIN YOUNG, AND RAYMOND EVANS MARCH^
Department of Chemistry, Trent University, Peterborough, Ont.,
Canada K9J 7B8
Received June 17. 1 9 8 5 ~
AFAF KAMAR, ALEXANDER BALDWIN YOUNG, and RAYMOND EVANS MARCH.
Can. J. Chem. 64, 1979 (1986). The evolution of ion species by
unimolecular and bimolecular reactions, both concurrent and
sequential, haa been
investigated for each of 2-propanone, d6-2-propanone,
4-hydroxy-4-methyl-2-pentanone, and 4-methyl-3-penten-2-one.
Infrared multiphoton dissociation (IRMPD) has been used in order to
differentiate between gaseous ionic isomers. It is concluded that
the isomeric species, protonated 2-propanone dimer and protonated
4-hydroxy-4-methyl-2-pentanone, both of m / z 117, are of different
structures. The ion species C6Hl10+ of m / z 99, and its
perdeuterated analogue, which is observed in all three systems, may
exist in two forms, one of which is unique to 2-propanone while an
alternative form appears to be common to
4-hydroxy-4-methyl-2-pentanone and 4-methyl-3-penten-2-one. The ion
species of m / z 83 (C5H70+) which is observed only in the latter
two systems only could not be differentiated and may have a common
structure. In the protonated dimers of 2-propanone and
4-hydroxy-4-methyl-2-pentanone, evidence obtained by IRMPD
indicates that the activation energy for dedimerization (134 kJ
molpl) is less than that for the dehydration process.
AFAF KAMAR, ALEXANDER BALDWIN YOUNG et RAYMOND EVANS MARCH. Can.
J . Chem. 64, 1979 (1986) On a CtudiC 1'Cvolution des especes
ioniques provenant de reactions unimolCculaires ainsi que
bimolCculaires, tant des
rCactions qui se produisent d'une faqon concurente que
~Cquentielle, de la propanone-2, de la propanone-2-d6, de
I'hydroxy-4 mCthyl-4 pentanone et de la mCthyl-4 pentene-3 one-2.
Dans le but de distinguer les divers isomeres ioniques gazeux, on a
fait appel la dissociation multiphotonique infrarouge (DMPIR). On
en conclut que les especes isomeres dimeres protonCs de la
propanone-2 et de l'hydroxy-4 mbthyl-4 pentanone-2 protonCe, de m /
z = 117, possbdent deux structures diffkrentes. L'espece ionique
C6H1,0+, de m / z = 99, et celle de son analogue perdeutCrC qui est
observee dans les trois systemes peut exister sous deux formes;
l'une est unique a la propanone-2 alors qu'une autre forme semble
&tre commune a I'hydroxy-4 mCthyl-4 pentanone-2 et 2 la
mCthyl-4 penthe-3 one-2. Les especes ioniques de m / s = 83
(C5H70+), qui ne sont observCes que dans les deux derniers
systemes, ne presentent pas de diffkrences et possedent peut Ctre
la m&me structure. Dans le cas des dimeres protonCs de la
propanone-2 et de l'hydroxy-4 methyl-4 pentanone-2, on a obtenu des
donnCes a l'aide de la DMPIR a l'effet que 1'Cnergie d'activation
pour la dCdimCrisation (1 34 kJ mol-' ) est plus faible que celle
requise pour le processus de deshydratation.
[Traduit par la revue]
Introduction Previous studies employing slow Infrared
Multiphoton Dis-
sociation (IRMPD), as a probe of activation energy hierarchies
in the gas phase photolysis of proton-bound dimers, have been
carried out with each of 2-propanol (1-4), ethanol (3,
1-butanol(6), and deuterated propanols (7). With proton-bound
alcohol dimers it is possible to determine in some degree the
hierarchy of activation energies among the reaction channels
leading to loss of alkene, water, and monomer or parent molecule.
With the proton-bound dimers of 2-propanone and
4-hydroxy-4-methyl-2-pentanone studied here, it is possible to
assess the activation energy hierarchy for loss of water and
monomer. IRMPD may be used also to differentiate between isomeric
ion species in the gas phase. The distinguishing criteria for
photochemical differentiation are relative absorptivity at the
wavelength available for laser irradiation and, of greater
importance, the photoproduct distribution, i.e. the ionic species
produced and their relative intensities. In some cases it is
informative to photolyze the species of interest in both the ground
state and in its nascent state; nascent ion internal excitation is
enhanced by multiple photon absorption to access reaction channels
of higher activation energy, limited in practice, by competition
between laser fluence and collision frequency.
The ion-chemistry for each of 2-propanone, 4-hydroxy-4-
methyl-2-pentanone, and 4-methyl-3-penten-2-one is presented
'Registered in the Ph.D. programme in Chemistry Department,
Queen's University.
2~djunc t Professor, Department of Chemistry. Queen's
University, Kingston.
'~evis ion received February 21, 1986.
here. These three compounds are related in the following ways:
the proton-bound dimer of 2-propanone is isomeric with protonated
4-hydroxy-4-methyl-2-pentanone and the ion struc- tures could be
identical if aldol condensation occurs in the gas phase; an ion
species of m / z 99 (C6Hl10f ) is produced in both of the above
systems and is isomeric with protonated 4-methyl- 3-penten-2-one;
and lastly an ion species of m l z 83 (C5H70+) is observed only
with 4-hydroxy-4-methyl-2-pentanone and 4- methyl-3-penten-2-one
and is conspicuously absent in 2-pro- panone. Thus these three
compounds present an opportunity for isomer differentiation using
the techniques of ion storage and IRMPD.
Experimental Although the basic apparatus has been described
previously (1-3), a
brief description of the technique used in this work is
presented here. A three-dimensional quadrupole ion store (QUISTOR)
mounted in place of the ion source of a conventional quadrupole
mass filter (Vacuum Generators QXK 400), serves as the reactor in
which ion/molecule reactions take place over the period 0-200 ms.
The ring electrode of the QUISTOR has two central perforations
diametrically opposed. A low power CW C 0 2 laser beam is directed
through a sodium chloride window and through the first ring
perforation, of diameter 3 mm. The beam passes radially through the
centre of the QUISTOR and totally illuminates the ion cloud which
rapidly becomes tightly focused at the centre of the device. A
portion of the beam passes through the perforation at the opposite
side of the ring electrode and is monitored externally through a
second sodium chloride window. The remainder of the beam is thought
to undergo multiple reflections within the QUISTOR. The laser beam
is chopped mechanically and phase-locked with the pulsing sequence
as shown in Fig. 1.
The repetition rate of the pulse sequence is controlled by a
square- wave generator (Heathkit SG 18.4). The iaser beam is
interrupted by a PAR 222 chopper phase-locked to the square-wave
generator. A range
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1980 CAN. J. CHEM. VOL. 64, 1986
1pZS-b7 I+- Laser On -L Heathkit SG18A
Axial Ejection m i l
J JL
&--Reaction-+I fl Time . .. . .-
I- Irradiarion +I u Time
'dz,"r I n-e----- Storage Time ------+I FIG. 1. Pulse sequence
used in IRMPD with axial ejection employed
to isolate an ionic species of interest by removal of all lower
m/z ions prior to laser irradiation.
of total storage times was effected using chopper wheels with a
variety of wheel apertures, while variation of the laser
irradiation period within
erent a fixed total storage time was achieved with chopper
wheels of diff light-to-dark ratio. The mechanical chopper triggers
the creation pulse (Hewlett Packard pulse generator 214B, PGA)
which triggers in turn the extraction pulse (PGB) and a Brookdeal
scan delay generator (9425) which provides a detection pulse to
drive a linear gate (Brookdeal 9415). The square-wave generator
also drives a delay pulse generator which triggers in turn a Sweep
Ciznerator (Wavetek 134) which is then used to remove, by axial
ejection (8), ions of mass-to- chxge ratio less than that of
protonated dimer prior to laser irradiation.
The ion abundances are obtained by recording mass spectra at
zero storage (electron impact) and at a variety of storage times;
the relative ion intensities are calculated for each species at
each storage time to yield the data points shown in the figures.
The temporal variation of the intensity of each ion species was
checked by single ion monitoring over the storage period
employed.
2-Propanone (acetone) lon/irolecule chemistry The ion chemistry
of 2-propanone has been srudied previously by
Munson (9) and by Blair and Harrison (lo), and is re-examined
here both to resolve the minor differences between the findings of
these two groups and to provide a basis for explanation of our
photodissociation results obtained with proton-bound dimers of
2-propanone and 4- hydroxy-4-methyl-2-pentanone. Munson employea a
variable pressure source and reported on secondary ion intensities
as a function of pressure only. Blair and Harrison, in an electron
beam trapping study with a trapping period of 3 ms, used an
ionizing electron energy sufficient to produce only (CH3)2CO+' and
cH3COt as primary ions, with the former con~prising 70% of the
primary ion abundance. Protonated parent molecules, m/z 59, are
produced by reactions of the primary ions
The proton-bound dimer m/z 117 is produced continuously with
either increasing pressure or increasing storage time by reaction
with parent neutrals
[3] (CH3)2COH+ + (CH3)2C0 S [(CH3)2C0]2Ht* m/z 59 m/z 117
where * denotes a degree of internal excitation. The nascent
proton- bound dimer may then be either collisionally
deactivated
or may dissociate
[5I [(CH3)zC012Ht* CH3CO(CH3COCH3)+ + CH4 mlz 117 rnlz 101
L---, caI10+ + H?O mlz 99
An alternative and preferred mechanism for the formation of m/z
101, is the clustering reaction
[7] CH3CO+ + (CH3)2C0 -+ (cH~)~co---6---OCCH~* m/z 43 m/z
101
which is then collisionally stabilized. This species may be
regarded as the product of a mixed associative ion (or a
heterogeneous proton- bound dimer) of ketene uroton-bridged to
2-urouanone. Mixed associa-
- . A
tive ions of ketene with methanol, ethanol, and 2-propanol have
been reported previously ( I I). The stabilization of this mixed
associative ion, m/z 101, may require a reactant CM3COf of modest
internal excitation only, which would account for the fact that it
was observed by Blair and Harrison under their conditions of low
ionizing electron energy, but not by Munson who employed ionizing
electrons of 70 eV and a high pressure source. The C H 3 C ~ ( C ~
3 C O ~ ~ 3 ) + product reacts slowly at longer reaction times to
produce the proton-bound dimer ion in a ligand replacement
reaction
The species of m/z 99 formed by loss of water in the
dissociation of the protonated dimer, reaction [6], was tentatively
identified by Munson as protonated 4-methyl-3-penten-2-one. Since
the proton- bound dimer of 2-propanon: is isomeric with protonated
4-hydroxy-4- methyl-2-pentanone, it was plausible to propose that
gaseous ion reactions may be observed which correspond with
solution ionic reactions in which the acid-catalyzed formation of
4-hydroxy-4- methyl-2-pentanone and 4-methyl-3-penten-2-one is
observed from 2-propanone. The requirement of reactant excitation
in order for reaction [6] to proceed in a manner which is
endothermic for ground state reactants would explain the
observation of m/z 99 by Munson but not by Blair and Harrison.
In the work reported here and shown in Fig. 2, both species of
m/z 99 and m/z 101 were observed as the ions were created xith
ionizing electrons of 70 eV and the reactions of both excited and
ground state (collisionally cooled) CH3CO+ ions were monitored.
Perdeuterated 2-propanone was investigated also as there was
some interest in the infrared relative absorptivity of
deuteron-bound deuter- ated dimer ions [(CD3)2C0]2Df, m/z 130.
The ion profiles obtained with 2-propanone and d6-2-propanone
are similar in general form as shown in Figs. 2 and 3.
In 2-propanone, the ions of m/z 99 may be formed by the loss of
water, reaction [6], while ions of m/z 101 may be formed by
reaction [ 5 ] and/or reaction [7] to yield C&II1O+ and
CsHsOzf, respectively. The corresponding product ions in
d6-2-propanone will be C6D110' and C5D9O2+, respectively, formed in
reactions [9], and I101 and/or [ I I] to produce isobaric species
of m/z 110.
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KAMAR ET AL. 1981
1 100 , I
Storage Time lms) FIG. 2. Variation in the logarithm of
normalized ion abundances in
2-propanone with storage time 0-200 ms, 0.8 x Ton.
Thus the ion profile for m / z 110 in Fig. 3 is a composite of
the two isobaric ions.
While the ions of m / z 80 and m/z 82 observed in d6-2-propanone
were of low relative intensity, they can be reconciled with the
observation of ions of m / z 72,73, and 74 in 2-propanone as shown
in Fig. 2.
(i) Let us examine the possible origin for m/z 82 in
d6-2-propanone. The CSD902+ species may lose CO or C2D2 to produce
C4D90+ and C3D7O2+, respectively. The corresponding ions in
2-propanone would be C4H90+, m / z 73 and C3H7O2+, m / z 75. The
latter species was not observed therefore ~ ~ ~ 9 0 2 ' does not
lose C2D2.
The isobaric C 6 ~ 1 1 0 + ion may lose CO to produce CsDll+ or
lose C2D2 to produce C4D90+. The corresponding ions in 2-propanone
would be CSHI I + , m / z 71, which was not observed, and C4H90+, m
/ z 73, which was observed. Thus we conclude that either reaction
[12] [I21 CsD902+* + C4D90+ + CO AH1* or reaction [13]
05 / / ' I I 0 I 103 m
Storage T~me (ms) FIG. 3. Variation in the logarithm of
normalized ion abundances in
perdeutero-2-propanone with s t~rage time 0-200 ms, 1. l X lop4
Torr.
occurs or both occur, where AHl2 and A H l 3 are the enthalpy
changes for reaction [I21 and [13], respectively. As either
reaction [12] or reaction [13] may account for the observations, it
is appropriate to consider the energetics of the processes bearing
in mind that a small fraction of stored ions may acquire
translational energy from the three-dimensional quadrupole field
and produce minor amounts of fragment ions in super-thermal
collisions. As reactions 1121 and [13] produce a common daughter
ion with an assumed common structure, and the heats of formation of
CO and C2H2 are - 110 and 227 kJ mol-', respectively, then for
reaction [13] to occur the heat of formation of C6D110+ plus
internal excitation must exceed the heat of formation of C5D9O2+
plus iniernal excitation of 337 kJ mol-', provided that AH12 = AHl3
. Thus the occurrence of reaction [13] is less probable than
reaction [12].
( i i ) In a similar examination of the origin of m / z 80 in
d6-2- propanone, the CsD9o2+ may lose C2D3' or CDO' to produce
C3D602+' or C4D80+', respectively. The corresponding product ions
in 2-propanone are C3H6O2+', m / z 74, and c 4 H 8 0 " , m / z 72.
both of which were observed.
Thus as m / z 74 was observed in 2-propanone, we conclude that
reaction [14] occurs
The observation of m/z 72 in 2-propanone may be explained by the
analogous reactions to either
[I51 CsD902+* J C4D80+' + CDO'
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1982 CAN. J. CHEM. 1 JOL. 64, 1986
or both. The heats of formation of HCO' and C2H3' are reported
as 33.9 and
332 kJ m o l l , respectively (12). The observation of m/z 74
infers that the heat of formation of C3D6O2+ is some 332 kJ molp'
less than the heat of formation of C5D902+ plus internal excitation
when reaction [I41 is thermoneutral. By an argument similar to that
presented above for reactions [I21 and (131, it can be shown that
for reaction [16] to occur the heat of formation of C6D110+* must
exceed that of C5D901; * by some 298 kJ mol-' again provided that
the enthalpy changes for reactions [15] and [16] are similar. Thus
the occurrence of reaction [16] is less probable than reaction
[IS].
The observation of C3H3+ as a persistent ion species in each of
the systems studied here is in keeping with previous observations
of this species wherever the parent molecule has a 2-propyl
grouping. The reactivity of C3H3+ is discussed elsewhere (13).
Photochemical studies
The IRMPD of the collisionally relaxed proton-bound dimer of
2-propanone yielded but a single photodissociative channel, as
is
nhv [I71 [(CH3)2COlzH+ - ( c H ~ ) ~ ~ o H + (CH3)2C0
m/z 117 m/z 59 shown in Fig. 4, that of protonated
2-propanone.
In this figure, two sets of experiments a& shown; the first
experiment employing an initial ionization period followed by 45 ms
of ion- molecule reaction time, wherein the proton-bound dimer ion
is produced. Axial ejection removes all ionic species of m/z <
117, isolating the dimer in the trap. These isolated dimers possess
a range of internal energies and some undergo thermal unimolecular
dissociation to produce the protonated parent m/z 59 during the 80
ms following isolation and prior to ejection and mass analysis. In
the second experiment shown as solid lines in Fig. 4, the dimer is
irradiated at 944 cmpl for the 80 ms period following its isolation
in the QUISTOR, then the trap population is ejected and mass
analyzed.
The period of laser irradiation was varied from 50 to 100 ms,
that is one half the total storage time, and in each case greater
than 90% of the proton-bound dimers were dissociated. As
proton-bound dimers are being produced continually, the ions
irradiated will have a range of ages and internal energies. As but
a single photoproduct was obtained we conclude that the energies of
activation for reactions [5] and [6] exceed that of reaction [17]
which is reported to be 134 kJ mol-I ( 1 4). While precise
measurements of the cross-section for photodissociation (ao) were
not made. the high absorptivity at 944 cmp' of proton- bound dimers
of 2-propanone is comparable to that of 2-dl -2-propanol (7). This
high absorptivity is remarkable as neutral 2-propanone has
negligible absorption in the vicinity of 944 c m ' .
In contrast the deuteron-bound dimer of d6-2-propanone exhibited
somewhat lower absorptivity in that approximately 40% only of this
species was photodissociated under similar conditions; reduced
absorptivity in perdeutero-2-propanol has been observed (7) also.
As expected, a single photoproduct, deuteron-bound d6-2-propanone,
was observed upon IRMPD. 4-Hydroxy-4-methyl-2-pentanone (diacetone
alcohol)
Ion/molecule chemistry The ion chemistry of
4-hydroxy-4-methyl-2-pentanone obtained at a
pressure of 6 X lop5 Torr is depicted in Fig. 5 where it is seen
that the predominant second order product ion at long skorage times
is the protonated parent molecule, (CH3)2C(OH)CH2COHCH3, m/z 1 17.
Of the two possible structures for the m/z 117 ion structure 11
is
recommended by Parker et al. (15) on the basis of collisionally
activated dissociation (CAD) studies. The base peak in the
electron
P=lxlO-"orr
Laser 1-1 Nn l a w r (---I
FIG. 4. Experimental sequence and results from IRMPD of 2-pro-
panone protonated dimers at 1 . O x lop4 Torr. Dashed lines
correspond to the ion intensities in an experiment wherein the
laser was blocked, solid lines correspond to ion intensities
obtained with laser irradiation at 944 cmpl for the latter 80 ms of
the experimental period.
impact mass spectrum, i.e. 38% relative ion intensity at zero
storage time, is due to CH3CO+, m/z 43; this primary ion reacts
rapidly along with other minor primary ions during the first 20 ms
principally by proton transfer reactions such as
[18] CH3CO+ * + (CH3)2C(OH)CH2COCH3 m/z 43
+ ( C H ~ ) ~ C ( O H ) C H ~ C O H C H ~ + CH2C0 m/z 117
where * denotes a degree of internal excitation. At this
juncture, it is appropriate to consider the nature of the
C2H30+ species which is observed in the electron impact
fragmenta- tions of simple oxygen-containing molecules. In the
2-propanone system discussed above, it was suggested that the
stabilization and subsequent observation of the ion of m/z 101 may
require a reactant CH3COt of but modest internal excitation, while
in the 4-hydroxy-4- methyl-2-pentanone system it is evident from
Fig. 5 that two forms of CzH30+ exist which differ in reactivity.
The temporal variation of the ion abundance of m/z 43 in Fig. 5
clearly shows two linear components in the decay curve; the
bimolecular rate constant derived from the steep component is
almost an order of magnitude greater than that derived from the
shallow component. While one form of C2H30+ may derive its greater
reactivity by virtue of internal energy, it is improbable that the
internal energy of some 90% of the initial C2H30+ abundance would
survive the many collisions by which deactivation could occur. The
co-existence of C2H30+ in three structures from a single precursor
has been reported (16). While as many as eleven possible isomeric
structures for C2H30+ have been examined recently, only five
structures will be considered here.
VI VII
Of the potential C2H30- above. the acetyl cation (111).
l-hydroxy- vinyl cation (IV) and oxiranyl cation (VI) have been
well characterized as being stable, observable species (16-18): the
acetyl cation is the lowest energy isomer. The 1-hydroxyvinyl
cation (IV) was found to lie 181 kJ mol-I above 111, with a barrier
to rearrangement by
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100
C H ~ C ~ H C H ~ C ( @ H ~ ( C ~ ! & . * *
*
I P=6 x Torr
I I 1 0 60 120
Storage Time (rnsl FIG. 5. Variation in the logarithm of
normalized ion abundances
in 4-hydroxy-4-methyl-2-pentanone with storage time 0-120 ms, 6
x 10-~Torr. successive 1,2-hydrogen shifts of 287 kJ mol-' (19).
CH30C' (V) was found to be the third lowest isomer in energy of the
C2H30+ ions examined, lying 214 kJ mol-' above CH3CO+ (111) (19).
There appears to be no experimental study of CH30C+. The results of
a recent study (16) of metastable peak intensities and shapes and
collision induced processes over a range of pressures provide
unequivocal evidence for identification of the acetyl (111),
I-hydroxyvinyl (IV) and oxiranyl (VI) cations. The oxiranyl cation
was predicted (19) to be 244 kJ mol-' above 111 with a barrier to
rearrangement of 85 kJ m o l ' , while VII was ~redicted to
collapse without activation energy to either VI or 111.
The acetyl cation 111 is known to react rapidly with oxygen-
containing molecules such as 2-propanone (20). In previous studies
we have noted the rapid reaction of primary C2H30+ cations in 1-
and 2-propanol and in tetrahydrofuran (unpublished work by the
authors) and conclude that this ion must be of structure 111.
However, the ion chemistry of the 1- and 2-propanol and
tetrahydrofuran systems. following the initial decay of the acetyl
cation, is characterized by a steady accumulation of a C2H30+
cation which is presumed to be of a different but unspecified
structure.
While it is proposed that (CH~)~C(OH)CH~COHCH~+, m / z 117, is
formed with excess internal excitation in reaction [18] from an
excited acetyl ion in order to explain the subsequent unimolecular
dissociation of m/z 117, some discussion of the proton affinities
of 4-hydroxy-4- methyl-2-pentanone and ketene would be useful here.
The proton affinity of the former does not appear in the literature
(22) but has been determined in this laboratory (23) to be 831 2
0.8 kJ mol-' (198.7 + 0.2 kcal mol-I), while the proton affinity
for ketene is 828 kJ mol-' (22). Thus for ground state acetyl ions
from which a proton is transferred and ketene remains, the reaction
is virtually thermoneutral.
The excited protonated parent molecules formed may then be
collisionally deactivated.
[19] (CH3 j2C(0H)CH2COHCH3+* mlz 117
+ (CH3j2C(OH)CH2COHCH2 + M rnlz 117
I1 where M represents a collision partner, or may dissociate
rnlz 117 mlz 99 VIII
to give m/z 99 of structure VIII which is that of protonated
mesityl oxide (4-methyl-3-penten-2-one). The excited VIII may then
either be collisionally deactivated or eject CH4 to produce an ion
of m / z 83
miz 99 I miz 99 L 6=c-"1, + CH',
rnlz 83 IX
The possibility of CH4 loss occurring initially from m / z 117
followed by loss of H 2 0 to produce m / z 83 is rejected as no
species of m/z 101 was detected. Although the species of m / z 99
and m / z 83 have been observed previously (15), neither the
structures of these ions nor the identities of the neutral
fragments ejected have been investigated further except for an
attempt to photodissociate m / z 99 by multiphoton absorption as
discussed below.
The principal minor primary ions, m / z 57, 59, 98, and 101
reacted rapidly during the first 10 ms and linear plots of the
logarithm of ion intensities with time have been omitted from Fig.
5 so that the ion profiles of the more stable ions may be portrayed
clearly. After the first 20 ms of reaction time, the remaining
unreacted m/z 43 ions isomeric with CH3CO+ reacted slowly along
with ions of m / z 83 and 99 in proton transfer reactions to
produce the protonated parent molecule, m/z 117. At pressures
higher than 6 X lo-' Torr, proton-bound dimers of
4-hydroxy-4-methyl-2-pentanone of m / z 233 were observed.
Photochemical studies + IRMPD of (CH3)2C(OH)CH2COHCH3, m/z 117,
was carried out
as described earlier. Two photoproducts as depicted in Fig. 6
were observed.
nhv 10% [23] ( C H ~ ) ~ C ( O H ) C H ~ C O H C H ~ 7' CzH30' +
C4H100
rnlz 117 mlz 43
I241 C6H1 )0+ + H 2 0 mlz 99
where n and n' are the minimum number of photons (hv) necessary
to overcome the activation energy barriers for reactions [23] and
[24], respectively. IRMPD has been shown to be a useful probe of
activation energy hierarchies when ground state reactants are
irradiated. It is concluded from the observed ratio of
photoproducts obtained with 33 ms of laser irradiation that the
activation energy for reaction [24] is less than that for reaction
[23]; this conclusion is supported by the observation that with
prolonged laser irradiation, the fractional yield of reaction [23]
can be increased.
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P ; S X ~ O - ~ Tori
Laser (-1 No Laser C--1
FIG. 6. Experimental sequences and results from IRMPD of pro-
tonated 4-hydroxy-4-methyl-2-pentanone at 6 X lop5 Torr. Dashed
lines correspond to the ion intensities obtained in an experiment
wherein the laser was blocked, solid lines correspond to ion
intensities obtained with laser irradiation at 944 cm-' for the
latter 50 ms of the experimental period.
In the discussion above of the ion chemistry of
4-hydroxy-4-methyl- 2-pentanone, it was assumed that the unreactive
form of C2H30+ had been produced as a fragment ion upon electron
impact. However, during experiments using axial ejection in which
all ion species of mlz < 117 are ejected from the QUISTOR, it
was observed that a relatively unreactive form of C2H30+ was
generated by dissociation of protonated parent molecules, as shown
in reaction [25] which is the thermal analogue of the photochemical
process, reaction [23].
It is of interest to speculate on the possibility that the
products of reactions [23] and [25] are derived from the protonated
keto form via a 4-centred elimination
whereas the products of reaction [24] are derived from the
protonated en01 form via a 6-centred elimination as depicted in
reaction [20]. As activation energies for 6-centred rearrangements
are generally lower than those for 4-centred, then the above
speculative mechanistic argument is consistent with the IRMPD
results.
Let us consider now the structures of protonated 4-hydroxy-4-
methyl-2-pentanone and the isomeric cation formed by the addition
of 2-propanone to protonated 2-propanone which has been identified
in reaction [3] as a proton-bound dimer. Parker et al. (15) found a
qualitative similarity between the relative ion abundances in the
self- chemical ionization mass spectrometry (self-CIMS) of
2-propanone and that of 4-hydroxy-4-methyl-2-pentanone; the
relative ion abun- dances are tabulated in ref. 15 and reproduced
here as stick form mass spectra in Fig. 7. Furthermore, the same
workers found that the CAD spectrum of the m/z 117 ion, obtained
with the magnetic ( B ) and electric ( E ) sectors linked so as to
maintain a constant B / E value, gave the same daughter ions for
both 2-propanone and 4-hydroxy-4-methyl- 2-pentanone although no
spectra were given. On the basis of the self-CIMS spectra and the
CAD B / E linked-scan spectra, Parker et al. concluded that the two
species of m/z 1 17 had a common structure and that protonated
2-propane must undergo an aldol condensation with 2-propanone in
the gas phase identical wlth that of the acid-catalyzed
condensation of 2-propanone in the solution phase.
Relative ion intensities obtained by self-CIMS should be
comparable with those of a particular storage time as obtained in
this work.
m i z FIG. 7. Relative abundances for self-CIMS of (a)
2-propanone and
( b ) 4-hydroxy-4-methyl-2-pentanone.
Although the pressures and residence times in various sources
may differ, the dependence of ion intensities on the product of
pressure (raised to the power of the reaction order) and time
permit a ready comparison. Thus the relative ion intensities shown
in Fig. 7 a and b correspond to a storage time of -5 ms in each of
Figs. 2 and 5, respectively. There is, however, a disparity in the
ion abundances of mlz 58 and m l z 59 in Fig. 7 b; only negligible
abundances of these ions were observed in our QUISTOR studies.
We are not convinced that an aldol condensation occurs in the
gas phase with 2-propanone on the basis of the evidence shown in
Fig. 7. On the contrary it has been shown that IRMPD of protonated
4-hydroxy-4-methyl-2-pentanone, m/z 117, isolated in the QUISTOR
yielded two photoproducts m l z 43 and m l z 99 (reactions [23] and
[24]), whereas IRMPD of proton-bound 2-propanone dimers, mlz 117,
carried out under identical conditions, yielded but the single
photoproduct, m l z 59 (reaction [17]).
Thus we conclude that the structures of the ion species of mlz
117 obtained in these two systems are different, and that an aldol
condensation does not occur with 2-propanone in the gas phase.
The species of m/z 99 was irradiated by the laser for periods of
33 ms, 50 ms, and 100 ms which is the entire period of its genesis
yet no photodissociation products were observed. Thus it is
concluded that the mlz 99 species is virtually transparent to the
laser irradiation at the wavelength employed. It had been
anticipated that photodissociation of the mlz 99 species would lead
to the formation of the mlz 83 species by the photo>hemical
reaction similar to reaction 1221. As no such species as m / z 83
was observed even from nascent m / z 99 ions it was concluded that
the m / z 99 species is transparent at the laser wavelength.
At higher pressure, protonated 4-hydroxy-4-methyl-2-pentanone
reacts with parent neutrals to form the proton-bound dimer, m / z
233.
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KAMAR ET AL. 1985
P = 6 x Torr
m/z
0 I 50 100 Storage Time (ms)
FIG. 8. Variation in the logarithm of normalized ion abundances
in 4-methyl-3-penten-2-one with storage time 0- 100 ms, 6 x
Torr.
In addition, ions were observed at m / z 215 [2M + 1 - H20]+,
m/z 175 [2M + 1 - 58]+, and m / z 157 [2M + 1 - H 2 0 - 58]+.
Isolation of the proton-bound dimer was beyond the limit of the
ramped rf sweep for axial ejection, thus it was not possible to
isolate m/z 233 for irradiation. The intensities of all of the
above species were diminished by IRMPD. As the fractional
dissociation of the [2M + 1 - H20]+ species exceeded that of the
[2M + 11' species, we conclude that the former is a mixed
associative dimer and the activation energy for dehydration of [2M
+ 1]+ exceeds that for the dedimeriza- tion process.
nhv I261 [(CH3)2C(OH)CH2COCH3]x -
(CH~)~C(OH)CH~COHCH~ + (CH3)2C(OH)CH2COCH3 The thermal and
photochemical reactions of [2M + 11' will be compared with those in
the a,w-hydroxy thiols in a forthcoming publication in this
journal.
4-Methyl-3-penten-2-one (rnesityl oxide) Ion/molecule reactions
It was decided to investigate the ion chemistry of 4-methyl-3-
penten-2-one (mesityl oxide) as the structure of protonated
4-methyl- 3-penten-2-one was proposed for the major unimolecular
product of protonated 4-hydroxy-4-methyl-2-pentanone and for the
ion formed by loss of water from the proton-bound dimer of
2-propanone [9]. Furthermore the ion chemistry of this compound is
not evident in the literature. The temporal variation of ion
intensities of 4-methyl-3- penten-2-one at a pressure of 6 x Torr
is shown in Fig. 8.
The base peak in the electron impact mass spectrum is due to m /
z 83 while the other major ions were observed at mlz 39, 43, and 55
in order of decreasing relative intensity. Minor ions (< 10%)
which were observed were the molecular ion of m / z 98, and m/z
53.
The molecular ion reacted rapidly and completely within 10 ms
presumably by proton transfer to form protonated 4-methyl-3-penten-
2-one
Within 40 ms of storage all primary ions save mlz 83 reacted
completely also presumably to form m / z 99. Only the reactive form
of C2H30+, CH3CO+, was observed in this system. The species of m/z
53, C4H5+, rose slightly in intensity during the rapid decay of the
molecular ion then reacted slowly and completely.
The intensity due to the protonated 4-methyl-3-penten-2-one rose
rapidly during the first 20 ms of storage (or reaction) time to
become the dominant ion in the system. It reached a maximum
intensity after 40 ms and proved unreactive for the duration of the
storage time. The intensity of another ion, m/z 83, increased
initially, then after 40 ms of storage, remained constant. The
behaviour of m / z 99 and m / z 83 suggests that the proton
affinity of the neutral species C5H60 (82 mu) must be greater than
that of 4-methyl-3-penten-2-one. The proton affinity of the latter
compound also is not in the literature, but has been determined in
this laboratory (23). Proton-bound dimer ions of m / z 197 were not
observed. Photochemical studies
The protonated molecule, m l z 99, was isolated and subjected to
laser irradiation but no photoproducts were observed; this species
is thought also to be transparent at the laser wavelength.
Discussion The structure of mlz 99 in 2-propanone constitutes
a
problem. The loss of H20 from a proton-bridged 2-propanone dimer
of mlz 117 must be explained. It has been demonstrated earlier by
IRMPD that the structure of the proton-bound dimer of %-propanone
must differ from that of protonated 4-hydroxy-
4-methyl-2-pentanone, mlz 117. It is significant here that the
dehydration channel for the proton-bound 2-propanone dimer is not
accessed by laser irradiation. In an attempt to explain the
dehydration channel in the 2-propanone dimer which is observed
thermally only, we have considered the keto and en01 forms of
protonated 2-propanone. MIND013 calculations show the latter to be
less stable by 110 W mol-' (15), hence relaxed protonated
2-propanone is expected to exist as the keto form, structure X , as
opposed to the isomeric enol, structure XI.
It is important to recognize that nascent proton-bound 2-
propanone dimers contain a minimum of 134 W mol-' (14) excess
internal energy if formed from fully relaxed neutrals and
protonated monomers.
As the protonated monomers, m l z 59, are formed via exothermic
proton transfer reactions, the nascent dimers may contain more than
134 W mol-' and may isomerize to form a mixed keto-en01 species of
structure XI1 as opposed to the anticipated fully relaxed structure
XIII.
While either isomer may dedimerize to produce m/z 59, the
elimination of water is more easily rationalized from the high
energy structure, XIH, as shown in reaction 1281.
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CAN. J . CHEM. VOL. 64, 1986
XIV XV
I FIG. 9. Schematic energy profile for rearrangements involving
the
proton-bound dimer of 2-propanone.
In Fig. 9 , the relative enthalpies (AH) of the major ionic
species derived from 2-propanone are shown. The height of the
barrier leading to the formation of the dehydration product, mlz
99, is unknown, as is the activational energy requirement to form
the enol.
The apparent difference between the observed products of thermal
and photolysis reactions, as shown in Figs. 2 and 4, respectively,
may be explained by noting that in the photolysis experiment, all
ions of mlz < 117 were removed, leaving ''agedW or partially
relaxed dimer ions at 80 ms of storage time which were subsequently
photolyzed. In Fig. 2 it is apparent that thermal genesis of m/z 99
reaches a maximum at -70 ms storage time, hence the "aged dimers
photolyzed at the onset of laser irradiation in the lRMPD study
contain insufficient internal energy for the dehydration process.
It has been demonstrated that in slow IRMPD, laser pumping of
vibrational states is slower than the rate of unimolecular
dissociation (7), hence solely the reaction channel of lowest
activation energy will be accessed if relaxed species are
irradiated and the activation energy requirements for competing
channels differ by at least the energy of the photon employed. The
IRMPD results demonstrate that the dedimerization reaction is of
lower E, than dehydration in proton-bound 2-propanone dimers.
The structure of mlz 99 in 4-methyl-3-penten-2-one may be either
that of the keto. XIV or the en01 XV. Structure XIV is the
preferred form as it is presumably the more stable of the two
and it lacks a 6-hydrogen. The presumption of greater stability of
XIV relative to XV is based on a comparison of the energies of
protonated acetaldehyde, XVI and protonated vinyl alcohol, XVII
XVI XVII The heat of formation of protonated acetaldehyde is
given by [29] AHf (XVI) = AHf(CH3CHO)
+ AHf (H+) - PA(CH3CHO) where PA is the proton affinity.
Similarly
[30] AHf (XVII) = AHf (CH2=CHOH) + AHf(H+) - PA(CH2=CHOH)
Thus for XVI to be thermodynamically more stable than XVII and,
by analogy, XIV to be more stable than XV, AHf(XVII) - AHf(XVI)
should be greater than zero, that is
4Hf(CH2=CHOH) lies in the range - (1 11 - 125) kJ mol-' (24)
The PA(CH2=CHOH) is reported (26) as 47-54 W mol-' less than
PA(C2H50H) which is given (22) as 788 kJ mol-'; hence PA(CH2=CHOH)
lies in the range 734-741 kJ mol-'. As the sum of the thermodynamic
quantities in eq. [31] lies in the range 81-102 kJ mol-', structure
XIV is probably more stable than XV. The presence of a 6-hydrogen
is required for the elimination of water from protonated ketones
(27) and, since neither structure XIV nor XV has a @-hydrogen,
dehydration is not expected and no dehydration product (m/z 81) was
observed. Apart from the energy consideration discussed above, no
distinction may be made at this time between the keto and en01
tautomers for the identity of m/z 99.
The identity of m/z 99 derived via dehydration of protonated
4-hydroxy-4-methyl-2-pentanone, m/z 117, was proposed ear- lier as
XIV above, and as such further dehydration was neither expected,
nor observed.
The structure of mlz 83 produced from protonated 4-methyl-
3-penten-2-one is proposed as IX as shown in the alkane elimination
reaction [22].
XIV IX
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TABLE 1. Activation energies for a-cleavage reactions of ionized
methyl ketones estimated from the appearance energy of the
a-cleavage product and the ionization energy of the
methyl ketone
a-Cleavage reaction AE (eV) 1E" (eV) AE - 1E- E,
"Reference 28. 'Reference 21.
The protonated ketone, XIV, may also eliminate isobutene, as
4-methyl-2-pentanone is also proposed as IX above. Although shown
in reaction [32]; m / z 43 was observed both thermally and
photolytically from the
protonated hydroxy ketone, m / z 117, IRMPD of the isolated +/H
m / z 117 demonstrated that m / z 43 was derived directly from
O.'
- A + CH3CO+ AH = 136 LJ mol-l the protonated parent, rather
than from its dehydration product, ~321
mlz 43 m / z 99. XIV Acknowledgements
however, no trace of m / z 43 was observed, thus reaction [32]
is of higher E, than reaction [22]. The enthalpy change for
reaction [22] is not known as the heat of formation of m / z 83 of
structure IX is not available. However, an estimate of the
activation energy may be obtained by examination of the ionization
energy (IE) of a number of ketones and the appearance energy (AE)
for the a-cleavage reaction
As the energy of activation may be approximated by the
expression
it is apparent from Table 1 that as the alkyl (or alkenyl)
group, R, is extended the activation energy of the reaction is
dimi- nished. Thus it seems probable that the heat of formation of
m / z 83 is less than that for CH3CO+ in reaction [32]; however, it
has been brought to our attention that such may not be the case as
AH?(CH~CO+) in some 240 kJ molpl less than AN?(CH~=CHCO+) although
the AE - IE values in Table 1 suggest otherwise. Therefore, despite
a lower AH': for methane than for isobutene in reaction [32], the
enthalpy hierarchy between reactions [22] and [32] may not be
satisfactorily established.
The structure of m/z 83 formed from protonated 4-hydroxy-
The authors acknowledge with thanks the financial support of the
Natural Sciences and Engineering Research Council of Canada, Trent
University, and Queen's University for a Graduate Student
Assistantship to A. Kamar. We acknowledge also the technical
assistance of W. King and J. A. Tomlinson. We are greatly
appreciative of the co-operation of Drs. C . W. Willis and D.
Rayner of the National Research Council of Canada for the loan of
the C 0 2 laser. The constructive comments of the referees are much
appreciated.
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