NDLI: Correlated/non-correlated ion dynamics of charge-neutral ion couples: the origin of ionicity in ionic liquids

Content Provider	Royal Society of Chemistry (RSC)
Author	Laaksonen, A. Driver, G. W. Wang, Y.-L. Huang, Y. Westlund, P.-O. Sparrman, T.
Copyright Year	2017
Abstract	Proton/fluoride spin–lattice (T1) nuclear magnetic relaxation dispersion (NMRD) measurements of 1-butyl-3-methyl-1H-imidazolium hexafluorophosphate, [C4mim][PF6], have been carried out using high field spectrometers and a fast-field-cycling instrument at proton Larmor frequencies ranging from 10 kHz to 40 MHz, at different temperatures. The NMRD profiles are interpreted by means of a simple relaxation model based on the inter- and intra-ionic dipole–dipole relaxation mechanism. Using an atomic molecular-ion dynamic simulation at 323 K the relevant spin dipole–dipole (DD) correlation functions are calculated. The results indicate that the NMRD profiles can be rationalized using intra- and inter-ionic spin DD interactions, however, anions are mainly modulated by ionic reorientation because of temporary correlations with cations, where modulation by translational diffusion plays a minor role. Reorientational dynamics of charge-neutral ion couples (i.e. [C4mim]⋯[PF6]) and [C4mim]+ ions are in the nano-second (ns) time range whereas the reorientation of [PF6]− is characterized by a reorientational correlation time in the pico-second (ps) regime. Based on the NMRD profiles we conclude that the main relaxation mechanism for [PF6]− is due to fast internal reorientational motion, a partially averaged F–F intra- and F–H inter-ionic DD coupling as the anion resides in close proximity to its temporary oppositely charged cation partner. The F-T1-NMRD data display a ns dispersion which is interpreted as being due to correlated reorientational modulations resulting from the H-containing charge-neutral ion couple [C4mim]⋯[PF6]. The analysis of ionicity is based on the free anion fraction, f, and it increases with temperature with f → 1 at the highest temperatures investigated. The fraction is obtained from the H–F NMRD profiles as correlated–non-correlated dynamics of the ions. The analysis of T1 relaxation rates of C, H, F and P at high fields cannot generally give the fraction of ions but is consistent with the interpretation based on the NMRD profiles with relaxation contributions due to DD-intra and -inter, CSA-intra (and -inter for C), including spin rotation for P. The investigation has led to a description of the mechanics governing ion transport in the title ionic liquid via identification of transient correlated/non-correlated ion dynamics.
Starting Page	4975
Ending Page	4988
Page Count	14
File Format	HTM / HTML PDF
ISSN	14639076
Volume Number	19
Issue Number	7
Journal	Physical Chemistry Chemical Physics
DOI	10.1039/c6cp05801a
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Open
Subject Keyword	Hexafluorophosphate Fluorine Butyl group Proton Ion Relaxation (NMR) Diffusion London dispersion force Ionic liquid
Content Type	Text
Resource Type	Article
Subject	Physics and Astronomy Physical and Theoretical Chemistry

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