Existence of weakly interacting OH bond at air/water interface

Ab initio molecular dynamics simulations at the air/water interface are carried out and elucidate a clear bump-like shoulder band at ∼3600 cm^-1 in the imaginary part of the second order nonlinear susceptibility measured by phase-sensitive or heterodyne-detected vibrational sum frequency generation spectroscopy. The structure of the weakly interacting (WI) OH bond producing this band is found by first-principles simulation. WI OH is the OH bond directing toward the vapor phase and is somewhat buried in the Gibbs dividing surface of water, which is a characteristic structure at the air/water interface. The WI OH vibration tends to couple with the combination band between a neighboring hydrogen-bonded OH vibration and its bonding intermolecular oxygen-oxygen vibration.