Terahertz laser vibration - Rotation tunneling spectroscopy of the water tetramer

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 A_g → B_g intermolecular vibration in the molecular symmetry group C_4h(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 water trimer VRT data, but not in evidence for (D₂O)₄, was found in the (H₂O)₄ spectrum in the form of broadened lines probably containing unresolved bifurcation tunneling splittings. Structural estimates from the data indicate that the interoxygen separations in both (D₂O)₄ and (H₂O)₄ are consistent with an exponential contraction in that parameter as a function of increasing cluster size.