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 principa l ring-puckering series in the 87-127 cm(-1) region but also possess two si deband series arising from the ring-twisting first and second excited state s. A third series arises from puckering transitions in the excited state of the v(9)(A(g)) in-plane ring angle bending vibration, which occurs at 509. 9 cm(-1). The ring-puckering levels in the v(9) excited state are also conf irmed by observed sum, difference, and hot bands. Observed double-quantum t ransitions confirm many of these assignments. Each of the four series can b e fit well with a one-dimensional single minimum potential function possess ing positive quartic and quadratic terms. Two-dimensional potential energy surfaces were also calculated to assess the interaction of the ring-puckeri ng 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.