- Email: [email protected]
Very short NMR relaxation times of anions in ionic liquids: New pulse sequence to eliminate the acoustic ringing
Accepted Manuscript Very short NMR relaxation times of anions in ionic liquids: new pulse sequence to eliminate the acoustic ringing Vytautas Klimavicius, Zofia Gdaniec, Vytautas Balevicius PII: DOI: Reference:
S1386-1425(14)00713-6 http://dx.doi.org/10.1016/j.saa.2014.04.140 SAA 12104
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 October 2013 10 March 2014 23 April 2014
Please cite this article as: V. Klimavicius, Z. Gdaniec, V. Balevicius, Very short NMR relaxation times of anions in ionic liquids: new pulse sequence to eliminate the acoustic ringing, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.140
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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* 1
Department of General Physics and Spectroscopy, Vilnius University, Sauletekio 9-3, LT-10222 Vilnius,
Lithuania 2
Institute 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: [email protected] (V. Klimavicius), [email protected] (Z.Gdaniec), [email protected] (V. Balevicius).
1
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 supramolecular 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.
2
NMR experiments were carried out on BRUKER AVANCEIII/500 NMR spectrometer operating at 49 and 135 MHz for
35
Cl and
81
Br, 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 10 3 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
3
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 81
35
Cl and
Br 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
4
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 (∆ν 1/2):
1 ≈ π∆ν 1 / 2 . T2
(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.
5
The
81
Br and
35
Cl 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:
τc =
4πηr 3 , 3kT
(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.
6
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]. 35 81 Cl Br Ionic radius (r), Å
1.81
1.96
Spin (I)
3/2
3/2
Resonance frequency at 11.75 T 48.95
135.03
(ω/2π), MHz Quadrupole moment (Q), fm2
-8.165
25.4
[C10mim][Cl]
[C10mim][Br]
47.6
77.0
57a
122a
Signal width (∆ν 1/2), 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
Chemical shift (δ), ppm
8.8b Rotational correlation time (τc), 17.9
15.2
ns Effective
quadrupole
constant (χeff), MHz
coupling 1.8 1.54
16.0 a
a
NMR data for [C4mim][Cl] and [C4mim][Br], Ref. 21;
b
Merck KGaA data, for comparison.
8.20 a
7
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 spinlattice relaxation data and comparing the strengths of quadrupolar interaction. The effective quadrupole coupling constants χeff , expressed as
χ eff = χ 1 +
ς2 3
,
(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 = q xx – q yy/q zz is the asymmetry parameter, have been determined for 81Br and
35
Cl using the measured T1 and τc
values and the well-known formula of the quadrupolar relaxation rate: 1 3π 2 2 2 I + 3 τ c 4τ c = χ eff 2 + . 2 2 T1 50 I (2 I − 1) 1 + ω τ c 1 + 4ω 2τ c2
(4)
The effective 35Cl quadrupole coupling constant was found to be almost one order lower than that for
81
Br (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
8
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 2 H 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.
9
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 imidazoliumbased 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.
11
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= ;$COMMENT= #include "p2=p1*2" "d11=30m" "d13=4u" "acqt0=-p1*2/3.1416" 1 ze 2 d1 p2 ph1 vd p1 ph2 d13
12
p1 ph3 d13 p1 ph4 go=2 ph31 d11 wr #0 if #0 ivd lo to 1 times td1 exit ph1=0 2 ph2=0 ph3=2 0 ph4=0 0 2 2 1 1 3 3 ph31=0 2 2 0 1 3 3 1 ;pl1 : f1 channel - power level for pulse (default) ;p1 : f1 channel - 90 degree high power pulse ;p2 : f1 channel - 180 degree high power pulse ;d1 : relaxation delay; 1-5 * T1 ;d11: delay for disk I/O [30 msec] ;d13: short delay [4usec] ;vd : variable delay, taken from vd-list ;NS: 8 * n ;DS: 4 ;td1: number of experiments = number of delays in vd-list ;FnMODE: undefined ;define VDLIST ;this pulse program produces a ser-file (PARMOD = 2D)
;$Id: t1ir,v 1.0 2012/01/13 12:43:00 VK Exp $
13
Supplement 2
The curve fitting results processing the exponential decay of integral intensities of 81Br and 35 Cl NMR signals shown in Fig. 3.
14
16
16 15
14
15
13
12
14
11
10
12
9
11
10
Cl–
8
7 N 4
9
Br–
8
1
13
+ 5
7
H
2
N N
6
2
4
H
2
+ 5
N
6
Graphical Abstract
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.
16
S1386-1425(14)00713-6 http://dx.doi.org/10.1016/j.saa.2014.04.140 SAA 12104
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
17 October 2013 10 March 2014 23 April 2014
Please cite this article as: V. Klimavicius, Z. Gdaniec, V. Balevicius, Very short NMR relaxation times of anions in ionic liquids: new pulse sequence to eliminate the acoustic ringing, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.140
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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* 1
Department of General Physics and Spectroscopy, Vilnius University, Sauletekio 9-3, LT-10222 Vilnius,
Lithuania 2
Institute 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: [email protected] (V. Klimavicius), [email protected] (Z.Gdaniec), [email protected] (V. Balevicius).
1
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 supramolecular 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.
2
NMR experiments were carried out on BRUKER AVANCEIII/500 NMR spectrometer operating at 49 and 135 MHz for
35
Cl and
81
Br, 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 10 3 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
3
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 81
35
Cl and
Br 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
4
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 (∆ν 1/2):
1 ≈ π∆ν 1 / 2 . T2
(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.
5
The
81
Br and
35
Cl 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:
τc =
4πηr 3 , 3kT
(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.
6
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]. 35 81 Cl Br Ionic radius (r), Å
1.81
1.96
Spin (I)
3/2
3/2
Resonance frequency at 11.75 T 48.95
135.03
(ω/2π), MHz Quadrupole moment (Q), fm2
-8.165
25.4
[C10mim][Cl]
[C10mim][Br]
47.6
77.0
57a
122a
Signal width (∆ν 1/2), 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
Chemical shift (δ), ppm
8.8b Rotational correlation time (τc), 17.9
15.2
ns Effective
quadrupole
constant (χeff), MHz
coupling 1.8 1.54
16.0 a
a
NMR data for [C4mim][Cl] and [C4mim][Br], Ref. 21;
b
Merck KGaA data, for comparison.
8.20 a
7
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 spinlattice relaxation data and comparing the strengths of quadrupolar interaction. The effective quadrupole coupling constants χeff , expressed as
χ eff = χ 1 +
ς2 3
,
(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 = q xx – q yy/q zz is the asymmetry parameter, have been determined for 81Br and
35
Cl using the measured T1 and τc
values and the well-known formula of the quadrupolar relaxation rate: 1 3π 2 2 2 I + 3 τ c 4τ c = χ eff 2 + . 2 2 T1 50 I (2 I − 1) 1 + ω τ c 1 + 4ω 2τ c2
(4)
The effective 35Cl quadrupole coupling constant was found to be almost one order lower than that for
81
Br (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
8
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 2 H 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.
9
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 imidazoliumbased 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.
11
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= ;$COMMENT= #include
12
p1 ph3 d13 p1 ph4 go=2 ph31 d11 wr #0 if #0 ivd lo to 1 times td1 exit ph1=0 2 ph2=0 ph3=2 0 ph4=0 0 2 2 1 1 3 3 ph31=0 2 2 0 1 3 3 1 ;pl1 : f1 channel - power level for pulse (default) ;p1 : f1 channel - 90 degree high power pulse ;p2 : f1 channel - 180 degree high power pulse ;d1 : relaxation delay; 1-5 * T1 ;d11: delay for disk I/O [30 msec] ;d13: short delay [4usec] ;vd : variable delay, taken from vd-list ;NS: 8 * n ;DS: 4 ;td1: number of experiments = number of delays in vd-list ;FnMODE: undefined ;define VDLIST ;this pulse program produces a ser-file (PARMOD = 2D)
;$Id: t1ir,v 1.0 2012/01/13 12:43:00 VK Exp $
13
Supplement 2
The curve fitting results processing the exponential decay of integral intensities of 81Br and 35 Cl NMR signals shown in Fig. 3.
14
16
16 15
14
15
13
12
14
11
10
12
9
11
10
Cl–
8
7 N 4
9
Br–
8
1
13
+ 5
7
H
2
N N
6
2
4
H
2
+ 5
N
6
Graphical Abstract
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.
16