Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations

Abstract

This document is the Accepted Manuscript version of a Published Work that appeared in final form in the Journal of Physical Chemistry B, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.6b04066.

Determining the relative strengths of aromatic and aliphatic C―H⋅⋅⋅X hydrogen bonds in imidazolium ionic liquids through measurement of H/D isotope effects on 19F nuclear shielding

“…First, the larger isotope effects measured on the 19 F resonances of BF 4 − versus PF 6 − isotopologues correlate with the greater electron density on the F atoms in the former anion . In addition, the fact that the variations in Δ 19 F(H,D) values follow different trends in [C 4 mim]PF 6 and [C 4 mim]BF 4 isotopologues is likely related to differences in interaction geometries of these anions with the imidazolium cation . This suggests that isotope effect data could not only be employed to study H‐bond relative strength in N , N ′‐dialkylimidazolium salts, but also to scrutinize their local structure and ion ordering.…”

“…One involves the anion as the H‐bond acceptor and the C―H groups on the imidazolium ring as the H‐bond donors and is often referred to as aromatic or primary H‐bonds, while the other, known as aliphatic or secondary H‐bonds, is formed between the anion and C―H groups on the N ‐alkyl sidechains. While weaker, quantum theory of atoms in molecules (QTAIM) calculations have shown that the strengths of aliphatic C―H⋅⋅⋅X H‐bonds are in some cases comparable to those of their aromatic counterparts, suggesting that these interactions also play a central role in the ordering of the local structure of ILs . To date, however, there is virtually no experimental data that can corroborate these theoretical results.…”

“…[3][4][5] Indeed, these interactions have a critical impact on the liquid structure, dynamics, and solvating abilities of these materials, [3][4][5] and thus a gamut of experimental and theoretical methods have been employed to investigate them. [6][7][8][9][10][11][12][13] In systems based on N,N 0 -dialkylimidazolium cations, computational studies have identified two distinct types of Hbonds. [5,[10][11][12][13] One involves the anion as the H-bond acceptor and the C-H groups on the imidazolium ring as the H-bond donors and is often referred to as aromatic or primary H-bonds, while the other, known as aliphatic or secondary H-bonds, is formed between the anion and C-H groups on the N-alkyl sidechains.…”

“…[6][7][8][9][10][11][12][13] In systems based on N,N 0 -dialkylimidazolium cations, computational studies have identified two distinct types of Hbonds. [5,[10][11][12][13] One involves the anion as the H-bond acceptor and the C-H groups on the imidazolium ring as the H-bond donors and is often referred to as aromatic or primary H-bonds, while the other, known as aliphatic or secondary H-bonds, is formed between the anion and C-H groups on the N-alkyl sidechains. While weaker, quantum theory of atoms in molecules (QTAIM) calculations have shown that the strengths of aliphatic C-HÁÁÁX H-bonds are in some cases comparable to those of their aromatic counterparts, [11] suggesting that these interactions also play a central role in the ordering of the local structure of ILs.…”

“…While weaker, quantum theory of atoms in molecules (QTAIM) calculations have shown that the strengths of aliphatic C-HÁÁÁX H-bonds are in some cases comparable to those of their aromatic counterparts, [11] suggesting that these interactions also play a central role in the ordering of the local structure of ILs. [5,[10][11][12][13] To date, however, there is virtually no experimental data that can corroborate these theoretical results.…”