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 diluting HCOOD with D2O was observed. Based on the
experimental results, the shift in frequency of C–H stretching band is concluded to relate to the
mechanism of the hydration of the C–H group and the water structure in the vicinity of the C–H
group. The pressure-dependent results can be attributed to the strengthening of C–H---O
electrostatic/dispersion interaction upon increasing pressure. The observations are in accord with
ab initio calculation forecasting a blueshift of the C–H stretching mode via C–H---O interaction in
HCOOD-water/~HCOOD!2-~D2O! complexes relative to the noninteracting monomer/dimer.
Hydrogen-bonding nonadditivity and the size of water clusters are suggested to be responsible to
cause the redshift in C–H stretching mode upon dilution HCOOD with D2O.