A Comparative Study of the Rotational Dynamics of PF₆^– Anions in the Crystals and Liquid States of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Results from ³¹P NMR Spectroscopy

Abstract

The rotational dynamics of the hexafluorophosphate anion (PF(6)(-)) in the crystalline and liquid states of the archetypal room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) are investigated using (31)P NMR spectroscopy line shape analyses and spin-lattice relaxation time measurements. The PF(6)(-) anion performs isotropic rotation in all three polymorphic crystals phases α, β, and γ as well as in the liquid state with a characteristic time scale that ranges from a few ps to a few hundred ps over a temperature range of 180-280 K. The rotational correlation time τ(c) for PF(6)(-) rotation follows the sequence γ-phase < α-phase ≈ liquid < β-phase. On the other hand, in the liquid state, all local motions in the cation as well as its global rotational reorientation are characterized by time scales that are slower compared to that for the PF(6)(-) anion rotation. The time scale τ(c) and the activation energy of PF(6)(-) rotation in this RTIL are found to be comparable with those observed in ordinary alkali and ammonium salts despite the large counterion size and low melting point of the former. The high sphericity of the PF(6)(-) ion is hypothesized to play an important role in the decoupling of its rotational dynamics that appear to be practically independent of the averaged cation-anion interaction.

A review of NMR methods used in the study of the structure and dynamics of ionic liquids

“…On the other hand, the 31 P NMR relaxation studies of [PF 6 ] − anion of [BMIM][PF 6 ] show that the anions are rotating in an isotropic manner in all of their three polymorphic crystals . The rotational correlation time of the [PF 6 ] − anion calculated from their T 1 relaxation data shows the order as β ‐phase > α ‐phase ≈ liquid phase > γ ‐phase . This order indicates that the γ ‐crystals are in its maximum stability because of its slowest correlation time .…”

“…The rotational correlation time of the [PF 6 ] − anion calculated from their T 1 relaxation data shows the order as β ‐phase > α ‐phase ≈ liquid phase > γ ‐phase . This order indicates that the γ ‐crystals are in its maximum stability because of its slowest correlation time . The temperature dependent dynamics of anion of [BMIM][PF 6 ] has also been studied by the same research group by studying their T 1 relaxation times as represented in Fig.…”

Fast solute diffusivity in ionic liquids with silyl or siloxane groups studied by the transient grating method

“…33 Full geometry optimization analyses in the gas phase were carried out using 6-311+G(d,p) basis sets based on Becke's three-parameter hybrid method 34 with LYP correlation (B3LYP). [35][36] No imaginary frequencies were produced by the optimized structures, ensuring the presence of an energetic minimum. Ionic volumes of the ions were estimated using DFT calculations.…”

Calorimetric study of polymorphism in 1-butyl-3-methylimidazolium hexafluorophosphate

“…Kabo et al [21] obtained the heat capacity curve from 5 K to T fus and no trace of a solid-solid phase transition was found. Endo et al [18] reported two solidsolid phase transitions at T = 250 K and T = 276 K. The first transition corresponds to irreversible process, as reported in a subsequent paper from the same group [22]. Additionally, the enthalpy of fusion reported by Endo et al [18] is 32 % lower than the value obtained by adiabatic calorimetry [21].…”