Ring-Puckering Potential Energy Functions and Structures for Trimethylene Sulfide and Its Monovalent Cation*

The vibrational spectra of trimethylene sulfide (TMS) and the vacuum ultraviolet mass-analyzed threshold ionization spectra of the trimethylene sulfide cation (TMS$^{\mathrm{+}})$ have been reported. Theoretical computations have been carried out to calculate the structures of both TMS and TMS$^{\mathrm{+}}$ using ab initio (MP2/cc-pVTZ) and DFT (B3LYP/cc-pVTZ) methods. These calculations were used to predict the coordinate dependent kinetic energy functions for the ring-puckering of both molecules. The kinetic energy functions were then used to calculate the refined ring-puckering potential energy functions for both species based on the previously published spectra. The refined ring-puckering potential energy functions fit the experimental data very well. TMS has an energy barrier of 271 cm$^{\mathrm{-1}}$ and energy minima at ring-puckering angles of \textpm 29 degree. TMS$^{\mathrm{+}}$ has a barrier of 60 cm$^{\mathrm{-1}}$ and the energy minima at ring-puckering angles of \textpm 21 degree. The lower barrier for TMS$^{\mathrm{+}}$ shows that the cation has lower ring angle strain than TMS itself.