Terahertz Laser Vibration−Rotation Tunneling Spectroscopy and Dipole Moment of a Cage Form of the Water Hexamer

Tunable terahertz laser vibration−rotation-tunneling spectroscopy has been employed to characterize the structure and hydrogen bond network rearrangement dynamics of a cage form of the water hexamer having eight hydrogen bonds. The isolated clusters are produced in a pulsed supersonic slit jet. Striking similarities are found between the structure and the average interoxygen distance R_O-O (2.82 Å) of the hexamer cage and those of the basic unit of ice VI. The hybrid perpendicular band of b- and c-types is observed near 2.491 THz (83.03 cm^-1) and rationalized to originate from the torsional motions of the two single-donor single-acceptor monomers about their donor hydrogen bonds, thereby causing changes in the dipole moments from each monomer to be orthogonal to each other as well as to be perpendicular to the approximate symmetry a-axis. Triplet spectral patterns accompanying each rovibrational transition with line spacings of 1.9 MHz and intensity ratios of 9:6:1 are attributed to the degenerate quantum tunneling that involves the exchange of protons within two similar monomers of the cluster. The Stark effect of the degenerate asymmetry doublets of K_a ≥ 3 has been analyzed to yield vibration- and (J,K_a)-dependent electric dipole moment components ranging from 1.82 to 2.07 D along the a-axis of this near prolate rotor. The selection rules establish that this dipole moment component preserves the sign upon vibrational excitation. A reasonable agreement is found between the dipole measurement and the results of a model calculation using an iterated induction expansion including the quadrupole-induced dipole. The same model has also been applied to extract the individual monomer dipole moments for the dimer and the cage and cyclic hexamers. The trends of two molecular propertiesthe contraction of the R_O-O distance and enhancement of the average individual monomer dipole with increasing cluster size up to the cyclic hexamer−are found to converge exponentially to the bulk phase values. In both cases, the cage properties deviate from the trends established by the dimer and cyclic clusters.