The energetics and electronic rearrangements associated with proton transfer between S atoms in (H2Sndash;Hndash;SH2)+are calculated usingab initiomolecular orbital methods and compared with similar data in the firsthyphen;row analog (H2Ondash;Hndash;OH2)+. The full potential energy surface of (S2H5)+, calculated as a function of the Hhyphen;bond length as well as the position of the proton, contains two equivalent minima separated by a small energy barrier, whereas the surface of (O2H5)+contains a single minimum corresponding to a symmetric position for the central proton. In both cases the energy barrier to transfer increases as the H bond is lengthened. This rise is noticeably less steep in the case of (S2H5)+, a fact attributed to the greater ease with which a proton may be pulled a given distance from each SH2subunit in the absence of the other. Enlargements of the proton transfer barriers also result from angular distortions of each H bond; these increases are qualitatively quite similar in the two systems. There is a great deal of resemblance also in the electronic redistribution patterns accompanying proton transfer in the two systems. However, the greater polarizability of SH2as compared to OH2leads to greater overall charge transfer between the subunits in (H2Sndash;Hndash;SH2)+and to larger extent of spatial regions of density change.
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