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

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, Dj, and DJK 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) A and an Ar-c.m.(H2X) distance of 3.675 (4.112) A 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.