title: Probing C-H...X hydrogen bonds in amide-functionalized imidazolium salts under high pressure.
abstract: 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.
authors.affiliation: Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan.
authors.forename: Kwang Ming
authors.initials: KM
authors.lastname: Lee
authors.forename: Hai-Chou
authors.initials: HC
authors.lastname: Chang
authors.forename: Jyh-Chiang
authors.initials: JC
authors.lastname: Jiang
authors.forename: Li-Chuan
authors.initials: LC
authors.lastname: Lu
authors.forename: Chia-Jung
authors.initials: CJ
authors.lastname: Hsiao
authors.forename: Yi-Ting
authors.initials: YT
authors.lastname: Lee
authors.forename: Sheng Hsien
authors.initials: SH
authors.lastname: Lin
authors.forename: Ivan J B
authors.initials: IJ
authors.lastname: Lin