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 eigh t hydrogen bonds. The isolated clusters are produced in a pulsed super sonic slit jet. Striking similarities are found between the structure and the average interoxygen distance R0-0 (2.82 Angstrom) of the hexam er cage and those of the basic unit of ice VI, The hybrid perpendicula r 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 singl e-donor single-acceptor monomers about their donor hydrogen bonds, the reby causing changes in the dipole moments from each monomer to be ort hogonal to each other as well as to be perpendicular to the approximat e symmetry a-axis. Triplet spectral patterns accompanying each rovibra tional transition with line spacings of 1.9 MHz and intensity ratios o f 9:6:1 are attributed to the degenerate quantum tunneling that involv es the exchange of protons within two similar monomers of the cluster. The Stark effect of the degenerate asymmetry doublets of K-a greater than or equal to 3 has been analyzed to yield vibration- and (J,K-a)-d ependent electric dipole moment components ranging from 1.82 to 2.07 D along the a-axis of this near prolate rotor. The selection rules esta blish that this dipole moment component preserves the sign upon vibrat ional excitation. A reasonable agreement is found between the dipole m easurement and the results of a model calculation using an iterated in duction expansion including the quadrupole-induced dipole. The same mo del has also been applied to extract the individual monomer dipole mom ents for the dimer and the cage and cyclic hexamers. The trends of two molecular properties-the contraction of the R0-0 distance and enhance ment of the average individual monomer dipole with increasing cluster size up to the cyclic hexamer-are found to converge exponentially to t he bulk phase values. In both cases, the cage properties deviate from the trends established by the dimer and cyclic clusters.