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 PF₆^- 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 PF₆^- 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 C²–H...F interactions, combined with other weak hydrogen bonds, disturb the formation of N–H hydrogen bonds in the PF₆^- salt. This finding is consistent with the pressure-dependent results, which reveal that the C²–H...F interaction is enhanced upon increasing the pressure. In contrast to the PF₆^- 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 PF₆^- ions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]