The free-OH stretching frequencies of 3-coordinated H2O in water clusters and on ice surfaces

The free-OH stretching of 3-coordinated H2O on ice and water surfaces typically resonates at 3690 cm−1, but it blue-shifts to 3715 cm−1 in small- to medium-sized water clusters. This research attempts to account computationally for the frequency difference using ab initio calculations for a number of benchmark systems. Systematic investigations indicate that the 25 cm−1 difference is primarily due to the disparity in molecular structures between water clusters and crystalline ice. We attribute the blue-shifting in water clusters to the reduction of hydrogen bond directionality and the presence of nearby water molecules in the form of double proton donors that are absent in crystalline ice.