Accepted Manuscript

Very short NMR relaxation times of anions in ionic liquids: new pulse sequenceto eliminate the acoustic ringing

Vytautas Klimavicius, Zofia Gdaniec, Vytautas Balevicius

PII: S1386-1425(14)00713-6DOI: http://dx.doi.org/10.1016/j.saa.2014.04.140Reference: SAA 12104

To appear in: Spectrochimica Acta Part A: Molecular and Biomo-lecular Spectroscopy

Received Date: 17 October 2013Revised Date: 10 March 2014Accepted Date: 23 April 2014

Please cite this article as: V. Klimavicius, Z. Gdaniec, V. Balevicius, Very short NMR relaxation times of anionsin ionic liquids: new pulse sequence to eliminate the acoustic ringing, Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.140

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1

Spectrochimica Acta Part A

Very short NMR relaxation times of anions in ionic liquids: new pulse

sequence to eliminate the acoustic ringing

Vytautas Klimavicius1, Zofia Gdaniec2, Vytautas Balevicius1*

1Department of General Physics and Spectroscopy, Vilnius University, Sauletekio 9-3, LT-10222 Vilnius,

Lithuania

2Institute of Bioorganic Chemistry, Polish Academy of Sciences, Z. Noskowskiego 12/14, PL-61704 Poznan,

Poland

ABSTRACT

NMR relaxation processes of anions were studied in two neat imidazolium-based room temperature ionic liquids (RTILs) 1-decyl-3-methyl-imidazolium bromide- and chloride. The spin-lattice and spin-spin relaxations of 81Br and 35Cl nuclei were found to be extremely fast due to very strong quadrupolar interactions. The determined relaxation rates are comparable with those observed in the solids or in some critical organic solute/water/salt systems. In order to eliminate the acoustic ringing of the probe-head during relaxation times measurements the novel pulse sequence has been devised. It is based on the conventional inversion recovery pulse sequence, however, instead of the last 90° pulse the subsequence of three 90° pulses applied along axes to fulfill the phase cycling condition is used. Using this pulse sequence it was possible to measure T1 for both studied nuclei. The viscosity measurements have been carried out and the rotational correlation times were calculated. The effective 35Cl quadrupolar coupling constant was found to be almost one order lower than that for 81Br, i.e. 1.8 MHz and

16.0 MHz, respectively. Taking into account the facts that the ratio of (Q(35Cl)/Q(81Br))2 ≈ 0.1 and EFG tensors on the anions are quite similar, analogous structural organizations are expected for both RTILs. The observed T1/T2 (1.27 - 1.44) ratios were found to be not sufficiently high to confirm the presence of long-living (on the time scale of ≥ 10–8 s) mesoscopic structures or heterogeneities in the studied neat ionic liquids. Keywords: NMR relaxation, ionic liquids, quadrupolar interactions, acoustic ringing.

*Corresponding author. Tel.: +370 5 2366001; fax: +370 5 2366003. E-mail addresses: vytautas.klim@gmail.com (V. Klimavicius), Zofia.Gdaniec@ibch.poznan.pl (Z.Gdaniec), vytautas.balevicius@ff.vu.lt (V. Balevicius).

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Introduction

Ionic liquids (ILs) and room temperature ionic liquids (RTILs) are one of the most

successful breakthroughs creating various smart materials and multifunctional compositions

possessing many appealing features important for the applications in high technologies [1-3].

The physical understanding of processes in ionic liquids on a molecular level how the certain

peculiar properties may arise from the long-range interionic interactions coupled with their

structural and dynamic features remains to be one of very attractive challenges for

fundamental research [3].

Dielectric constant measurements classify ionic liquids as only moderately polar systems.

Static dielectric constants of RTILs are usually spread over ∼ 9 - 15 [4], though their values

for some protic ionic liquids can reach up to ∼ 22 - 57 [5]. Therefore in order to rationalize the

differences between ‘classical’ solvents and RTILs it is necessary to understand supra-

molecular structuring, short-range (in the nano-, or mesoscopic scales) effects, phase

equilibrium and dynamical processes in these systems [3, 6]. Indeed, numerous experimental

and theoretical works confirm a presence in RTIL systems of distinct degree of mesoscopic

order, structural heterogeneities that occur over a spatial scale of nanometers, the segregation

of the non-polar (alkyl) tails into mesoscopic domains, etc. [7 - 10, and the Refs. therein].

The anions often play extremely significant role determining many important

physicochemical properties of ionic liquids, such like diffusion [10, 11], conductivity [10 -

12], hydrogen bonding and charge transfer [13], structural organization, self-aggregation

(micellization) and the rates of proton/deuteron (H/D) exchange [10, 14]. Therefore, the

purpose of the present work was to study the NMR relaxation processes of anions in two neat

imidazolium-based RTILs, namely, in 1-decyl-3-methyl-imidazolium bromide- and chloride

([C10mim][Br] and [C10mim][Cl]). In order to eliminate the acoustic ringing during relaxation

measurements the novel pulse sequence has been devised.

Experimental

Samples. The ionic liquids 1-decyl-3-methyl-imidazolium bromide- and chloride from Merck

KGaA Darmstadt and from Ionic Liquids Technologies GmbH (Fig. 1) were dried under

vacuum at 80 oC for one day.

3

NMR experiments were carried out on BRUKER AVANCEIII/500 NMR spectrometer

operating at 49 and 135 MHz for 35Cl and 81Br, respectively, using 5 mm BBO probe-head.

The temperature in a probe of 298 K was controlled with an accuracy of ± 0.5 K. The signal

of DSS in D2O solution and DMSO-d6 in capillary insert were used as the reference and then

converted to δ-scale in respect TMS. The D2O and DMSO-d6 in the same capillary insert were

used for locking. In order to eliminate the acoustic ringing the pulse sequence has been

devised and applied (for details see Supplement 1 and discussion below).

Fig. 1. Molecular structures of the ionic liquids 1-decyl-3-methyl-imidazolium bromide (1)

and chloride (2).

Viscosity measurements were made using a Brookfield DV-2 rotational viscometer. Upper

detection limit of this viscometer is 103 Pa⋅s. The temperature of the sample was maintained

at 298 ± 1 K during the measurement via an external temperature control unit.

Results and discussion

The simplified 2D model of the structural organization of the neat imidazolium-based

ionic liquids has been proposed taking into account the data of X-ray studies reported in the

4

last years on the structure of 1,3-dialkylimidazolium salts [15]. According to this model, the

neat imidazolium-based RTILs can be described as polymeric hydrogen-bonded

supramolecules (highly ordered hydrogen bonded materials). The number of anions that

surround the cation (and vice-versa) can vary depending on the type of the n-alkyl substituent

and anion size. However, the structural motif of one imidazolium ring hydrogen bonded to at

least three anions and one anion hydrogen bonded to at least three cations is a general trend in

imidazolium salts [15]. Such structural organization of the crystalline phase is maintained to a

great extent in the liquid phase despite higher disorder. This statement was based on the fact

that in most cases there is only 10-15% volume expansion when going from the crystalline to

the liquid phase and the ion-ion or atom-atom distances are similar in both, solid and liquid

states [15].

NMR studies of chosen RTILs, viz. 1-decyl-3-methyl-imidazolium bromide- and

chloride ([C10mim][Br] and [C10mim][Cl]) can provide new information on (i) the effect of

anion size on structural organization and (ii) the short-range ordering induced by the

formation of long-living aggregates or heterogeneities in imidazolium-based ionic liquids

with long hydrocarbon chains. In this work we focused our attention mainly on the 35Cl and 81Br NMR relaxation experiments because NMR parameters of quadrupolar nuclei are

extremely sensitive to the micro-surrounding effects [16, 17]. However, the nuclear

quadrupole interactions in the ionic liquids with halogens (Cl, Br, I) as anions were found to

be very strong [18 - 21] and in consequence NMR relaxation processes are extremely fast.

These rates can be comparable with those observed in the solid-like (e. g. clathrate [16] or

coherent dipoles [17]) structures or in some highly fluctuating organic solute/water/salt

systems close to the critical point [22, 23].

The problem of ‘acoustic ringing’ can appear when measuring very short relaxation

times (tens to hundreds of microseconds) [24]. Namely, when the pulse is applied, the

oscillating rf current in the circuit can induce mechanical (acoustic) oscillations in metal parts

of the probe. These mechanical oscillations usually decay also within several tens to hundreds

of microseconds after the pulse, depending on resonance frequency and probe construction.

Acoustic ringing occurs more often at high fields or low frequencies and particularly when

wide spectra widths are employed because the initial part of acquired FID is affected by this

effect. This impedes the observation of very broad lines and causes baseline and phasing

5

problems. In order to reduce acoustic ringing artifacts the Hahn Echo [20, 21] or 90°-90°-90°

ARING pulse train [25, 26] sequences were used.

The NMR signal distortions due to the acoustic ringing were observed for 35Cl nuclei

which have lower resonance frequency in comparison with 81Br (Table 1). Therefore, in order

to eliminate the acoustic ringing during the measurements of the spin-lattice relaxation time,

the novel pulse sequence has been devised. It is based on the conventional inversion recovery

sequence, however, instead of the last 90° pulse the subsequence of three 90° pulses applied

along axes to fulfill the phase cycling condition is used (see Supplement 1). Using this pulse

sequence it was possible to measure T1 for both studied nuclei.

The perfectly Lorentz-shaped 81Br and 35Cl NMR signals have been observed for both

neat [C10mim][Br] and [C10mim][Cl] at 77.0 and 47.6 ppm, respectively (Fig. 2). It makes the

determination of the spin-spin relaxation times (T2) possible using the well-known relation to

the width of the NMR signal (∆ν1/2):

2/1

2

π1 ν∆≈

T. (1)

Fig. 2. 81Br and 35Cl NMR signals of the neat ionic liquids [C10mim][Br] and [C10mim][Cl] (parts A and B, respectively) at 298 K; black points – experimental data, red lines – nonlinear curve fitting by a Lorentzian function. The correlation coefficients R2 ≥ 0.99 were achieved for both fitted contours.

6

The 81Br and 35Cl spin-lattice relaxation time (T1) measurements were carried out

using the designed NMR pulse sequence. Moreover, the values of T1 were obtained by the

analysis of exponential decay of the integral intensities of the signals (Fig. 3). All

experimental data and the parameters of the studied nuclei were compiled in Table 1.

The viscosity (η) measurements have been carried out and the rotational correlation

times (τc) calculated using a general expression that follows Debye–Stokes–Einstein (DSE)

theory for a spherical particle undergoing isotropic rotation:

,3

π4 3

kT

rc

ητ = (2)

where r is the hydrodynamic radius, k is the Boltzmann constant and T is the temperature. The

equation (2) was applied without any correction for the size and shape using the

corresponding radii of Cl and Br anions (Table 1). The measured η and the calculated τc

values are presented in Table 1.

7

Table 1 The parameters of 35Cl and 81Br nuclei used in formulas (1 – 4) and the experimental data for the neat [C10mim][Br] and [C10mim][Cl]. 35Cl 81Br

Ionic radius (r), Å 1.81 1.96

Spin (I) 3/2 3/2

Resonance frequency at 11.75 T

(ω/2π), MHz

48.95 135.03

Quadrupole moment (Q), fm2 -8.165 25.4

[C10mim][Cl] [C10mim][Br]

Chemical shift (δ), ppm 47.6

57a

77.0

122a

Signal width (∆ν1/2), Hz 1960 19700

T2, µs 162.5 16.2

T1, µs 205.6 23.3

T1/T2 1.27 1.44

Viscosity (η), Pa⋅s 10.0 8.5

8.8b

Rotational correlation time (τc),

ns

17.9 15.2

Effective quadrupole coupling

constant (χeff), MHz

1.8

1.54a

16.0

8.20a

a NMR data for [C4mim][Cl] and [C4mim][Br], Ref. 21; b Merck KGaA data, for comparison.

8

Fig. 3. Processing of the exponential decay of integral intensities of 81Br and 35Cl NMR signals in the neat [C10mim][Br] and [C10mim][Cl] (parts A and B, respectively) at 298 K. The 3 parameter curve fitting results are given in Supplement2. The structural organization of both RTILs can be studied using the 81Br and 35Cl spin-

lattice relaxation data and comparing the strengths of quadrupolar interaction. The effective

quadrupole coupling constants χeff , expressed as

,3

12ςχχ +=eff (3)

where χ = e2Qqzz/h is the quadrupole coupling constant, qzz is the largest principal component

of the electric-field-gradient (EFG) tensor at the nuclear position and ζ = qxx – qyy/qzz is the

asymmetry parameter, have been determined for 81Br and 35Cl using the measured T1 and τc

values and the well-known formula of the quadrupolar relaxation rate:

( )

++

+−+

=22222

22

1 41

4

112

32

50

π31

c

c

c

ceff

II

I

T τωτ

τωτχ . (4)

The effective 35Cl quadrupole coupling constant was found to be almost one order lower than

that for 81Br (Table 1). These values are comparable to those obtained for the neat

[C4mim][Cl], [C4mim][Br] and other closely related RTILs in the solid state [21]. Taking into

9

account facts that the ratio of (Q(35Cl)/Q(81Br))2 ≈ 0.1 and EFG tensors on the anions are quite

similar, analogous structural organizations are expected for both RTILs.

When mixed with other molecules, RTILs should be regarded as nanostructured

materials with polar and non-polar regions rather than homogeneous solvents [15]. The role of

ions is very significant, despite small difference in their sizes. Indeed, Cl– and Br– ions

significantly influence the structure and dynamic properties of RTIL/water solutions [14, 27].

The 2H NMR experiments carried out using deuterated species of [Cnmim][X], n = 8 and 10,

X = Br– and Cl– in aqueous solutions have revealed that Cl– anion is more strongly solvated

than Br–, the larger anions (Br–) are less tightly bound to the micelle surface and enhanced

repulsive interactions destabilize the mesophases [27]. The results obtained during present

study show that the role of anions in the structuring effects in the neat imidazolium-based

RTILs is much less significant than in solutions.

The ratio T1/T2 can be used as a criterion in probing the presence of the local order in

the molecular systems. Namely, a higher ratio indicates the presence of local structures while

the values T1/T2 ≈ 1 are common for isotropic viscous liquids [21]. The T1/T2 values of 1.07 -

2.63 were determined for the neat [C4mim][Cl] and [C4mim][Br] close to their melting points

[21]. The observed ratios, according to the opinion of the authors, are not sufficiently high to

confirm the presence of sustained local order on a time scale of ∼ 1/ω, i.e. ∼ 10–8 s in the

liquid state of these RTILs.

It is well known that some of imidazolium-based RTILs [Cnmim][X] with sufficiently

long alkyl chains (n = 6 - 18) demonstrate a broad variety of phenomena in the phase behavior

and the self-aggregation (micellization) processes [14 and Refs cited therein]. For example,

SANS (small angle neutron scattering) experiments on aqueous solutions of [C8mim][Cl]

suggest the presence of some structures with micellar rods, sheets of bilayers, etc. [26].

Moreover, [C8mim][Cl] appears to form disk-like rather than spherical aggregates as in the

case of [Cnmim][X] with X= Br– and I–. However, the T1/T2 values of 1.27 – 1.44 obtained in

the present work for the neat [C10mim][Cl] and [C10mim][Br] show that the formation of the

long-living (≥ 10–8 s) ordered mesoscopic structures or heterogeneities cannot be confirmed

also for long-chained (decyl-) imidazolium-based RTILs, though in aqueous solutions these

compounds reveal strong tendency to self-aggregate by forming various micelles and

mesophases.

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Conclusions

1. The novel pulse sequence has been devised. It is based on the conventional

inversion recovery sequence, however, instead of the last 90° pulse the

subsequence of three 90° pulses applied along axes to fulfill the phase

cycling condition was used. Using this pulse sequence it was possible to

eliminate the baseline and phasing problems originating from acoustic

ringing of the probe-head. This allowed us to determine spin-lattice

relaxation time of order of tens microseconds.

2. The role of anions in the structuring effects in the neat imidazolium-based

RTILs is considerably less significant than in solutions, i.e. when they are

mixed with other molecules.

3. The presence of long-living (on the time scale of ≥ 10–8 s) ordered

mesoscopic structures or heterogeneities was not confirmed in imidazolium-

based RTILs with relatively long (decyl-) hydrocarbon chains.

Acknowledgments

Funding from the European Community’s social foundation under Grant Agreement

No. VP1-3.1-ŠMM-08-K-01-004/KS-120000-1756 is acknowledged. We thank Professor R.

Makuška (Dept. of Polymer Chemistry, Vilnius University) for the help during viscosity

measurements.

References

[1] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, Wiley-Interscience, New Jersey, 2005. [2] P. Wasserscheid, T. Welton (Eds.), Ionic liquids in synthesis, 2nd ed., Wiley-VCH, Weinheim, 2008. [3] H. Weingärtner, Angew. Chem. Int. Ed. 47 (2008) 654 – 670. [4] C. Wakai, A. Oleinikova, M. Ott, H. Weingärtner, J. Phys. Chem. B 109 (2005) 17028 – 17030. [5] M. M. Huang, H. Weingärtner, ChemPhysChem 9 (2008) 2172 – 2173. [6] D. Bankmann and R. Giernoth, Progress in Nuclear Magn. Resonance Spectroscopy 51 (2007) 63 – 90. [7] O. Russina, A. Triolo, L. Gontrani, R. Caminiti, J. Phys. Chem. Lett. 3 (2012) 27-33. [8] Y. Wang, G. A. Voth, J. Am. Chem. Soc. 127 (2005) 12192-12193. [9] Y. Ji, R. Shi, Y. Wang, G. Saielli, J. Phys. Chem. B 117 (2013) 1104 – 1109.

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[10] F. Castiglione, M. Moreno, G. Raos, A. Famulari, A. Mele, G. B. Appetecchi, S. Passerini, J. Phys. Chem. B 113 (2009) 10750 – 10759. [11] H. Tokuda, K. Hayamizu, K. Ishii, Md. Abu Bin Hasan Susan, M. Watanabe, J. Phys. Chem. B 108 (2004) 16593 – 16600. [12] I. Jerman, V. Jovanovski, A. Šurca Vuk, S. B. Hočevar, M. Gaberšček, A. Jesih, B. Orel, Electrochimica Acta 53 (2008) 2281 – 2288. [13] T. Cremer, C. Kolbeck, K. R. J. Lovelock, N. Paape, R. Woelfel, P. S. Schulz, P. Wasserscheid, H. Weber, J. Thar, B. Kirchner, F. Maier, H. P. Steinrueck, Chem. Eur. J. 16 (2010), 9018 – 9033. [14] V. Klimavicius, Z. Gdaniec, J. Kausteklis, V. Aleksa, K. Aidas, V. Balevicius, J. Phys. Chem. B 117 (2013) 10211 – 10220. [15] J. Dupont, J. Braz. Chem. Soc. 15 (2004) 341 – 350. [16] B.Lindman, S. Forsen, E. Forslind, J. Phys. Chem. 72 (1968) 2805–2813. [17] H. G. Hertz, M. Holz, J. Phys. Chem. 78 (1974) 1002 – 1013. [18] V. Balevicius, Z. Gdaniec, K. Aidas, J. Tamuliene, J. Phys. Chem. A 114 (2010) 5365 – 5371. [19] H. A. Every, A. G. Bishop, D. R. MacFarlane, G. Orädd, M. Forsyth, J. Mater. Chem. 11 (2001) 3031 – 3036. [20] P. G. Gordon, D. H. Brouwer, J. A. Ripmeester, J. Phys. Chem. A 112 (2008) 12527 – 12529. [21] P. G. Gordon, D. H. Brouwer, J. A. Ripmeester, ChemPhysChem 11 (2010) 260 – 268. [22] V.Balevicius, Z. Gdaniec, H. Fuess, J. Chem. Phys. 123 (2005) 224503. [23] V. Balevicius, Z. Gdaniec, J. Tamuliene, H. Fuess, Phase Trans. 81 (2008) 293 – 301. [24] http://u-of-o-nmr-facility.blogspot.com/2008/05/acoustic-ringing.html [25] R. C. Remsing, J. L. Wildin, A. L. Rapp, G. Moyna, J. Phys. Chem. B 111 (2007) 11619 – 11621. [26] R. C. Remsing, Z. Liu, I. Sergeyev, G. Moyna, J. Phys. Chem. B 112 (2008) 7363 – 7369. [27] I. Goodchild, L. Collier, S. L. Millar, I. Prokeš, J. C. D. Lord, C. P. Butts, J. Bowers, J. R. P. Webster, R. K. Heenan, J. Colloid Interface Sci. 307 (2007) 455 – 468.

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Supplement 1

NMR pulse sequence and pulse program to eliminate the acoustic ringing during the measurements of the spin-lattice relaxation time.

;t1iraring ;avance-version (07/04/03) ;T1 measurement using inversion recovery with antiring 90 pulse; ;$CLASS=HighRes ;$DIM=2D ;$TYPE= ;$SUBTYPE= ;$COMMENT= #include <Avance.incl> "p2=p1*2" "d11=30m" "d13=4u" "acqt0=-p1*2/3.1416" 1 ze 2 d1 p2 ph1 vd p1 ph2 d13

13

p1 ph3 d13 p1 ph4 go=2 ph31 d11 wr #0 if #0 ivd lo to 1 times td1 exit ph1=0 2 ph2=0 ph3=2 0 ph4=0 0 2 2 1 1 3 3 ph31=0 2 2 0 1 3 3 1 ;pl1 : f1 channel - power level for pulse (default) ;p1 : f1 channel - 90 degree high power pulse ;p2 : f1 channel - 180 degree high power pulse ;d1 : relaxation delay; 1-5 * T1 ;d11: delay for disk I/O [30 msec] ;d13: short delay [4usec] ;vd : variable delay, taken from vd-list ;NS: 8 * n ;DS: 4 ;td1: number of experiments = number of delays in vd-list ;FnMODE: undefined ;define VDLIST ;this pulse program produces a ser-file (PARMOD = 2D) ;$Id: t1ir,v 1.0 2012/01/13 12:43:00 VK Exp $

14

Supplement 2Supplement 2Supplement 2Supplement 2 The curve fitting results processing the exponential decay of integral intensities of 81Br and 35Cl NMR signals shown in Fig. 3.

+ 2 H

4 5

6

7

16 15

14 13

12 11

10 9

8

N

N

Cl–

2 + 2 H

4 5

6

7

16 15

14 13

12 11

10 9

8

N

N

Br–

1

Graphical Abstract

16

Highlights

1. The novel pulse sequence has been devised to eliminate the acoustic ringing during relaxation studies. 2. The role of anions in the structuring effects in the neat RTILs is less significant than in solutions. 3. The presence of long-living ordered mesoscopic structures or heterogeneities was not confirmed.