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The rotational spectra of two small silicon sulfides, silanethione H2SiS and the disilicon sulfide ring Si2S, have been detected in the centimeter band by Fourier transform microwave spectroscopy of a molecular beam; lines of H2SiS were also observed in the millimeter band up to 377 GHz in a glow discharge. Precise rotational and centrifugal distortionconstants have been determined for the normal and a number of the more abundant rare isotopic species of both closed-shell molecules. Theoretical equilibrium (re) structures of H2SiS and Si2S were derived from coupled-cluster calculations that included triple and quadruple excitations, core correlation, and extrapolation to the basis-set limit. The re structures agree to within 5×10-4 Å and 0.1°with empirical equilibrium (reemp) structures derived from the experimental rotational constants, combined with theoretical vibrational and electronic corrections. Both H2SiS and Si2S are good candidates for radioastronomical detection in the circumstellar shells of evolved carbon-rich stars such as IRC+10216, because they are fairly polar and are similar in composition to the abundant astronomical molecule SiS. [ABSTRACT FROM AUTHOR]

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2–OCS, ortho-H2–OCS, HD–OCS, para-D2–OCS, and ortho-D2–OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2–OCS relative to ortho-H2–OCS increases exponentially with decreasing normal H2 component in H2/He gas mixtures, making the observation of para-H2–OCS in the presence of the more strongly bound ortho-H2–OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2–OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2–OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD–OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2–OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2–OCS. [ABSTRACT FROM AUTHOR]

Fourier transform microwave spectroscopy has been used to investigate vibrational excitation and relaxation of diatomic molecules produced by an electric discharge in the throat of a supersonic nozzle. Rotational transitions of SO, SiO, and SiS, in vibrational states up to v=33 for [sup 32]S[sup 16]O, v=45 for [sup 28]Si[sup 16]O, and v=51 for [sup 28]Si[sup 32]S in their ground electronic states have been detected. The isotopic species [sup 33]S[sup 16]O, [sup 34]S[sup 16]O, [sup 29]Si[sup 16]O, [sup 28]Si[sup 18]O, [sup 29]Si[sup 32]S, and [sup 28]Si[sup 34]S have also been observed in highly excited vibrational states. Microwave transitions include up to v=22 for the second lowest excited electronic state b [sup 1]Σ[sup +] of SO (∼10 510 cm-1 above ground) have also been detected. Effective vibrational temperatures have been derived for each species, and a general model is proposed to qualitatively explain the observations. Vibrational excitation is caused by inelastic collisions with the hot electrons produced in the discharge. The subsequent vibrational populations are largely determined by vibration–vibration energy transfer via molecule–molecule binary collisions. Two regions can be inferred from the data: one characterized by a temperature of around 1000 K and a second region with a temperature of several thousand degrees Kelvin. Improved Dunham coefficients and correction terms for the breakdown of the Born–Oppenheimer approximation have been determined for b [sup 1]Σ[sup +] SO, X [sup 1]Σ[sup +] SiO, and X [sup 1]Σ[sup +] SiS. Nuclear spin-rotation hyperfine structure for the [sup 29]Si isotopic species of SiO and SiS has been observed in all highly excited vibrational states. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]