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 C-2-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 C-2-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. (C) 2004 American Institute of Physics.