Rotational spectra and structures of the Ar3-H2O and Ar3-H2S symmetric tops

Rotational spectra of several isotopomers of Ar-3-H2O and Ar-3-H2S tetramers were obtained with a Balle-Flygare Fourier transform microwave spectrometer. Both were found to be symmetric tops, the former being an oblate and the latter a prolate one. The rotational constants B, D-j, and D-JK were determined to be 1172.1323(1) MHz, 7.199(1) kHz, and -5.545(2) kHz for the H2O and 819.0385(1) MHz, 3.346(1) kHz, and +3.145(2) kHz for the H2S containing tetramer. Substitution analysis with the rotational constants of various isotopomers led to an Ar-Ar distance of 3.848 (3.865) Angstrom and an Ar-c.m.(H2X) distance of 3.675 (4.112) Angstrom for H2O (H2S) complexes. The angle between the C-2 axis of the H2X and the C-3 axis of the tetramer is estimated to be 74 degrees for H2O and 13 degrees for the H2S complex. No evidence for any excited tunneling/internal rotor states was found for either of the tetramer. MMC calculations show that the equilibrium geometry has the H2X positioned above the plane of the Ar-3 with both the protons pointing towards one Ar each. The barrier for the "pseudorotation" in which the protons hop between the argons is determined to be about 6 (8) cm(-1) only for H2O (H2S) making the H2X moiety very mobile and effectively making both the tetramers symmetric tops. Rigid body diffusion quantum Monte Carlo (RBDQMC) calculations with the MMC potential have been carried out for vibrational analysis. (C) 2001 American Institute of Physics.