Estimation of Heat Capacity and Properties of Water by Spectrum Decomposition of the Second Overtone Band of OH Stretching Vibration

The temperature dependency of the second overtone band assigned to an OH stretching vibration of water has been investigated by means of spectrum decomposition. The absorption bands of water and ice were decomposed into five and three Gaussian components, respectively, which were assigned to the vibration motion of molecules with a different number of hydrogen bonds. We found that the specific heat capacity of water could be explained by the extrapolated values of the heat capacity of ice and the hydrogen bonding potential calculated from our spectrum decomposition analysis. Errors for the estimation ranged from 0.1% to 1.8% in the temperature range from 273 K to 371 K. We also found that the five spectral components of water could be classified into two groups, namely strongly and weakly hydrogen-bonded groups. The temperature dependency of an equilibrium constant between these two groups suggested that a fast rotational motion of molecules in the weakly hydrogen-bonded group would be stimulated. In addition, the relationship between the average number of hydrogen bonds and the thermal properties of water in a super-cooled region suggested that the existence of the weakly hydrogen-bonded group would be an essential factor for characterising liquid water; however, the melting and freezing temperatures of water would be governed exclusively by a hydrogen-bonding condition of the strongly hydrogen-bonded group.