Terahertz Laser Vibration−Rotation Tunneling Spectroscopy of the Water Tetramer

Abstract

A vibration−rotation-tunneling (VRT) spectrum of (H2O)4 has been recorded near 2.04 THz (67.9 cm-1). The band origin of this ΔK = 0 symmetric rotor spectrum is shifted by 0.1 cm-1 to the blue of that of a (D2O)4 band reported previously [Science, 1996, 271, 59]. Similar to that spectrum, each transition in the (H2O)4 spectrum exhibits a regular doublet splitting. The (D2O)4 and (H2O)4 doublet spacings, 5.6 and 2260 MHz, respectively, are constant as a function of rotation, indicative of degenerate tunneling splittings. They have been rationalized in terms of a concerted flipping motion of the H atoms that do not participate in hydrogen bonding in the S4 equlibrium structure, analogous to ammonia inversion. Both bands have been assigned to the lowest Ag→ Bg intermolecular vibration in the molecular symmetry group C4h(M). Additional K = 2 splittings, which increase nonlinearly with increasing J, were found in both D2O and H2O tetramer spectra. Evidence for the bifurcation tunneling motion observed in the wa...