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

Abstract

Rotational spectra of several isotopomers of Ar₃-H₂O and Ar₃-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) Å and an Ar-c.m.(H₂X) distance of 3.675 (4.112) Å for H₂O (H₂S) complexes. The angle between the C₂ axis of the H₂X and the C₃ axis of the tetramer is estimated to be 74° for H₂O and 13° 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 Ar₃ 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.