Close interactions of the C-alpha-H---O type have been analyzed via X-ray crystallography and high-pressure infrared spectroscopy. The results demonstrate that the C-alpha-H---O interactions can offer an additional stability to the beta-sheet formation. X-ray structural data suggest that while 1-acetamido-3-(2-pyrimidinyl)-imidazolium bromide exhibits a bilayer stacking, the PF6- salt reveals a beta-sheetlike pattern. The appearance of the free-NH infrared absorption indicates that the conventional N-H---O or N-H---N hydrogen bonds do not fully dominate the packing for the PF6- salt. The high-pressure infrared study suggests that the C-alpha-H---O hydrogen bonds are the important determinants for the stability of the PF6- salt. This study also verifies that the imidazolium C-H stretching frequency shifts to a longer wavelength upon the formation of the C-H---O hydrogen bonds.