Charge-enhanced C-H-O interactions of a self-assembled triple helical spine probed by high-pressure

C-H-O interactions of a self-assembled triple helix based on the 1-acetamido-3(2-pyrazinyl)-imidazolium cation has been probed by high pressure. The infrared spectroscopic profiles and ab initio calculations allow us to make a vibrational assignment of this compound. The C-H bonds forming C-H-O interactions shorten as the pressure was elevated, while free C-H vibration modes show low sensitivity to high pressure. The pressure-dependent results can be attributed to the strengthening of C-H-O electrostatic-dispersion interactions upon increasing pressure. The appearance of the free-NH infrared absorption indicates that the conventional N-H-O hydrogen bond does not dominate the inter-strand packing in the compound. It is proposed that the charge-enhanced C-H-O interactions, forming a helical hydrogen-bonding network, disturb the formation of inter-strand N-H-O hydrogen-bonding in order to form a maximum number of hydrogen bonds. Applying high-pressure seems not to change the C-O bond length in contrast to the trend of blue-shift in frequency of C-H vibrations. London dispersion energy is suggested to be required for understanding the pressure-dependent results, although more additional terms, such as the effect in the presence of charge, are needed for the correct explanation. This work demonstrates that high-pressure studies may have the potential to provide insight into the C-H-O structural properties of biological related systems. (C) 2002 American Institute of Physics.