C−H- - -O Hydrogen Bonds in β-Sheetlike Networks: Combined X-ray Crystallography and High-Pressure Infrared Study

Close interactions of the Cα−H- - -O type have been analyzed via X-ray crystallography and
high-pressure infrared spectroscopy. The results demonstrate that the Cα−H- - -O interactions can offer
an additional stability to the β-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 β-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 Cα−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.