Rotational spectroscopy and equilibrium structures of S3 and S4 -: art. no. 054326

The sulfur molecules thiozone S-3 and tetrasulfur S-4 have been observed in a supersonic molecular beam in the centimeter-wave band by Fourier transform microwave spectroscopy, and in the millimeter- and submillimeter-wave bands in a low-pressure glow discharge. For S-3 over 150 rotational transitions between 10 and 458 GHz were measured, and for S-4 a comparable number between 6 and 271 GHz. The spectrum of S-3 is reproduced to within the measurement uncertainties by an asymmetric top Hamiltonian with three rotational and 12 centrifugal distortion constants; ten distortion constants, but an additional term to account for very small level shifts caused by interchange tunneling, are required to reproduce to comparable accuracy the spectrum of S-4. Empirical equilibrium (r(e)(emp)) structures of S-3 and S-4 were derived from experimental rotational constants of the normal and sulfur-34 species and vibrational corrections from coupled-cluster theory calculations. Quantum chemical calculations show that interchange tunneling occurs because S-4 automerizes through a transition state with D-2h symmetry which lies about 500 cm(-1) above the two equivalent C-2 upsilon minima on the potential energy surface. (C) 2005 American Institute of Physics.