Pressure-dependent studies on hydration of the C-H group in formic acid

The infrared spectroscopic profiles of HCOOD/D2O mixtures were measured as a function of pressure and concentration. The C-H bond of HCOOD shortens as the pressure is elevated, while the increase in C-H bond length upon dilut ing HCOOD with D2O was observed. Based on the experimental results, the shi ft in frequency of C-H stretching band is concluded to relate to the mechan ism of the hydration of the C-H group and the water structure in the vicini ty of the C-H group. The pressure-dependent results can be attributed to th e strengthening of C-H---O electrostatic/dispersion interaction upon increa sing pressure. The observations are in accord with ab initio calculation fo recasting a blueshift of the C-H stretching mode via C-H---O interaction in HCOOD-water/(HCOOD)(2)-(D2O) complexes relative to the noninteracting mono mer/dimer. Hydrogen-bonding nonadditivity and the size of water clusters ar e suggested to be responsible to cause the redshift in C-H stretching mode upon dilution HCOOD with D2O. (C) 2001 American Institute of Physics.