Rotational dynamics of benzene and water in an ionic liquid explored via molecular dynamics simulations and NMR T1 measurements

Abstract

The rotational dynamics of benzene and water in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride are studied using molecular dynamics (MD) simulation and NMR T(1) measurements. MD trajectories based on an effective potential are used to calculate the (2)H NMR relaxation time, T(1) via Fourier transform of the relevant rotational time correlation function, C(2R)(t). To compensate for the lack of polarization in the standard fixed-charge modeling of the IL, an effective ionic charge, which is smaller than the elementary charge is employed. The simulation results are in closest agreement with NMR experiments with respect to the temperature and Larmor frequency dependencies of T(1) when an effective charge of ±0.5e is used for the anion and the cation, respectively. The computed C(2R)(t) of both solutes shows a bi-modal nature, comprised of an initial non-diffusive ps relaxation plus a long-time ns tail extending to the diffusive regime. Due to the latter component, the solute dynamics is not under the motional narrowing condition with respect to the prevalent Larmor frequency. It is shown that the diffusive tail of the C(2R)(t) is most important to understand frequency and temperature dependencies of T(1) in ILs. On the other hand, the effect of the initial ps relaxation is an increase of T(1) by a constant factor. This is equivalent to an "effective" reduction of the quadrupolar coupling constant (QCC). Thus, in the NMR T(1) analysis, the rotational time correlation function can be modeled analytically in the form of aexp (-t/τ) (Lipari-Szabo model), where the constant a, the Lipari-Szabo factor, contains the integrated contribution of the short-time relaxation and τ represents the relaxation time of the exponential (diffusive) tail. The Debye model is a special case of the Lipari-Szabo model with a = 1, and turns out to be inappropriate to represent benzene and water dynamics in ILs since a is as small as 0.1. The use of the Debye model would result in an underestimation of the QCC by a factor of 2-3 as a compensation for the neglect of the Lipari-Szabo factor.

Nuclear spin relaxation in liquids and gases

“…Thus, Hayamizu and co-workers 180 studied lithium salt solutions in RTILs, using among others 7 Li T 1 measurements, Lingscheid et al 183 studied ionic liquids dissolved in dichloroethane and dimethylsulfoxide, while Marincola et al 184 studied mixtures of RTILs with water. In addition to these works, Yasaka et al 203 reported MD simulations of RTIL solutions of water and benzene, aiming at improved interpretation of earlier experimental data from the same group. 204 The simulated reorientational TCF were characterized by an initial rapid decay, leading to Lipari-Szabo-like spectral density functions.…”

Nuclear magnetic resonance study on rotational dynamics of water and benzene in a series of ionic liquids: Anion and cation effects

“…26,[39][40][41][42][43] However, a significant nonexponentiality has been disclosed for C 2R (t) in ILs by spectroscopic and MD studies. [30][31][32][33][34][35][36] In fact, a long-time MD simulation study 36 showed that C 2R (t) is bimodal for water and benzene in a representative IL, [bmim + ][Cl − ]; nearly 90% of the correlation is quickly lost within a few ps due to the local dynamics of solute, such as vibration and libration, and then the remaining 10% of the correlation is lost orders-ofmagnitude more slowly with relaxation of the solvent structure. The long-time (diffusive) part of C 2R (t) was shown to be the time domain relevant to the NMR T 1 , with nearly exponential decay in the form of…”

“…This assumption is problematic in ILs, however. Recent spectroscopic [30][31][32][33][34] and theoretical 35,36 studies have shown that the rotational dynamics in ILs obeys non-exponential functional form. In this study, we analyze the rotational dynamics using a model derived from a previous molecular dynamics (MD) study on the functional form of the time correlation function for the rotational dynamics of water and benzene in IL.…”

“…In this study, we analyze the rotational dynamics using a model derived from a previous molecular dynamics (MD) study on the functional form of the time correlation function for the rotational dynamics of water and benzene in IL. 35,36 The longtime tail of the rotational time correlation function was found in Ref. 36 to be the major portion of the spectral density at the NMR Larmor frequency; the slow relaxation is responsible for determining T 1 and is diffusive with exponentially decaying time correlation function.…”

“…35,36 The longtime tail of the rotational time correlation function was found in Ref. 36 to be the major portion of the spectral density at the NMR Larmor frequency; the slow relaxation is responsible for determining T 1 and is diffusive with exponentially decaying time correlation function. The rotational time correlation function in IL can be thus modeled in a modified Debye form, Aexp(−t/τ ), where the constant A corresponds to the Lipari-Szabo order parameter used in the analysis of protein dynamics.…”