Frequencies of HOH, HOD and DOD bending fundamentals in liquid water

The bending fundamentals ν 2 of the three isotopic species of water, H 2 O, D 2 O and HDO, were examined in the infrared spectra of liquid water at 0–99.9% D. The peak positions of the three bands vary linearly with the molar fractions of the three isotopic species. The half‐widths of ν 2 of H 2 O surrounded by D 2 O and of D 2 O surrounded by H 2 O, which are largely free from vibrational coupling, have been whown to reflect the distrbution of instantaneous hydrogen‐bond strengths in liquid water. The contribution to the ν 2 band width from dynamic processes is relatively small. Vibrational coupling, in the spectra of isotopically pure H 2 O or D 2 O, leads to additional broadening and to a sizable shift of the band center to lower frequencies. Small band shifts confirm that weak vibrational coupling also occurs between HDO and H 2 O and H 2 O and between HDO and D 2 O.