Determination of the cationic conformational structure of tetrahydrothiophene by one-photon MATI spectroscopy and Franck–Condon fitting

The conformers of tetrahydrothiophene (THT) in the neutral (S0) and cationic (D0) ground states have attracted significant attention in terms of the conformational interconversion through pseudorotation. Herein, these conformers were explored by utilising one-photon mass-analysed threshold ionization (MATI) spectroscopy using the coherently tunable vacuum ultraviolet laser pulse generated by four-wave difference-frequency mixing in Kr medium, which allowed the acquisition of the vibrational spectrum of the corresponding cation. To identify the conformer corresponding to the measured MATI spectrum, the potential energy surfaces associated with pseudorotation in the S0 and D0 states were constructed at the B3LYP/cc-pVTZ level, where the twisted conformer with C2 symmetry in both states lies at the global minimum, while the Cs and C2v conformations were located at the saddle points. Although most of the peaks observed in the spectrum could be assigned as the ionic transitions between the twisted conformers (C2 symmetry) in the S0 and D0 states, distinct nontotally symmetric modes could not be assigned to any allowed vibration. Hence, Franck–Condon fitting was applied for the vibrational assignments in the observed spectrum. This revealed that the cationic conformer had a bent-like twist conformation of C1 symmetry instead of C2 symmetry. Furthermore, the geometrical changes induced by the removal of an electron from the non-bonding orbital of the sulfur atom gave prominent overtones and combination bands of the ring out-of-plane modes associated with pseudorotation as well as the stretching of 2C–1S–3C in the ring.