Eric Herbst

An experimentally derived torsional potential function for HSSH

Abstract A variety of milimeter-wave and far-infrared spectral data on the internal rotor HSSH has been used to determine the cis and trans barrier heights to torsional motion. Our values of 2800(90) and 1990(15) cm −1 for V cis and V trans ,respectively, are in good agreement with the results of ab initio calculations.

The high-resolution rotational and torsional spectra of HSSH

Abstract High-resolution spectroscopic measurements on both the torsional and the rotational spectra of disulfane (HSSH) are reported. The infrared torsional spectrum has been measured using a high-resolution Fourier transform spectrometer in the range 300–550 cm −1 . Approximately 2000 torsional-rotational spectral lines belonging to the band systems v t = 1 ← 0, 2 ← 1, and 3 ← 2 have been analyzed and fitted. In addition, new rotational transitions are reported, especially involving the second excited ( v t = 2) torsional state. The torsional potential function of Herbst and Winnewisser ( Chem. Phys. Lett. 155, 572–575 (1989)) has been refined.

Ab initio determination of mode coupling in HSSH: the torsional splitting in the first excited S-S stretching state

A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S-S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S-S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.

New calculations on the ion-molecule processes C2H2++H2→C2H3++H and C2H2++H2→C2H4+

New high-level quantum chemical calculations have been undertaken to understand the rates and mechanisms of the reactive and associative channels for the reactants C2H2(+) + H2. The reactive channel, which produces C2H3(+) + H, has been shown to be slightly endothermic, confirming earlier calculations at a somewhat lower level and in agreement with some recent experimental work. The associative channel, leading to C2H4+, has been shown to proceed via a transition state with negative energy relative to the reactants, so that association is predicted to be efficient. This result is in conflict with an earlier theoretical study but in agreement with low-temperature experimental measurements.

Chemistry and Star Formation

The normal gas phase chemistry of dense interstellar clouds is strongly affected by the process of star formation. Three mechanisms in which star formation impacts the chemistry of the surrounding material have been discussed in the literature. These are: evaporation of grain mantles due to heating, mixing of material driven by stellar winds into the ambient interstellar gas, and interstellar shock waves. There is evidence for each of these mechanisms in the Orion Molecular Cloud and it also appears that the mechanisms operate in physically distinct sections of the cloud.