We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water–vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at 1650 cm-1 and a positive peak at 1730 cm-1. Our results reveal that these widely (80 cm-1) separated peaks result from the interference of two closely spaced (29 cm-1) peaks of opposite sign. The positive peak at 1689 cm-1 originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at 1660 cm-1 from water with free O–H groups, pointing up. The small frequency difference of 29 cm-1 indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds.

J. Phys. Chem. Lett.

Nagata, Y., Hsieh, C.-S., Hasegawa, T., Voll, J., Backus, E., & Bonn, M. (2013). Water bending mode at the water-vapor Interface probed by sum-frequency generation spectroscopy : A combined molecular dynamics simulation and experimental study. J. Phys. Chem. Lett., 4(11), 1872–1877. doi:10.1021/jz400683v