Far-Infrared Spectra and Two-Dimensional Potential Energy Surfaces Involving the Ring-Puckering Vibration of 2,5-Dihydrothiophene

The far-infrared spectra of 2,5-dihydrothiophene not only show the principal ring-puckering series in the 87−127 cm -1 region but also possess two sideband series arising from the ring-twisting first and second excited states. A third series arises from puckering transitions in the excited state of the υ 9 (A g ) in-plane ring angle bending vibration, which occurs at 509.9 cm -1 . The ring-puckering levels in the υ 9 excited state are also confirmed by observed sum, difference, and hot bands. Observed double-quantum transitions confirm many of these assignments. Each of the four series can be fit well with a one-dimensional single minimum potential function possessing positive quartic and quadratic terms. Two-dimensional potential energy surfaces were also calculated to assess the interaction of the ring-puckering with the ring-twisting and in-plane ring bending modes. The twisting is anticooperative, raising the energy of the puckering process, while the in-plane ring bending is cooperative, facilitating the puckering motion.