Probing C-H...X hydrogen bonds in amide-functionalized imidazolium salts under high pressure.
We have probed under high pressure the C-H hydrogen bonds formed by N,N(')-disubstituted imidazolium ions having PF(6) (-) and Br(-) counterions. High-pressure infrared spectral profiles, x-ray crystallographic analysis, and ab initio calculations allow us to make a vibrational assignment of these compounds. The appearance of a signal for the free-NH unit (or weakly bonded N-H...F unit) in the infrared spectrum of the PF(6) (-) salt indicates that conventional N-H...O and N-H...N hydrogen bonds do not fully dominate the packing. It is likely that the charge-enhanced C(2)-H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N-H hydrogen bonds in the PF(6) (-) salt. This finding is consistent with the pressure-dependent results, which reveal that the C(2)-H...F interaction is enhanced upon increasing the pressure. In contrast to the PF(6) (-) salt, the imidazolium C-H bonds of the Br(-) salt have low sensitivity to high pressure. This finding suggests that the hydrogen bonding patterns are determined by the relative hydrogen bond acceptor strengths of the Br(-) and PF(6) (-) ions.
Lee
Kwang Ming
KM
Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan.
Chang
Hai-Chou
HC
Jiang
Jyh-Chiang
JC
Lu
Li-Chuan
LC
Hsiao
Chia-Jung
CJ
Lee
Yi-Ting
YT
Lin
Sheng Hsien
SH
Lin
Ivan J B
IJ