Far-infrared spectra and two-dimensional potential energy surfaces involving the ring-puckering vibration of 2,5-dihydrothiophene
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abstract
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 9(Ag) in-plane ring angle bending vibration, which occurs at 509.9 cm-1. The ring-puckering levels in the 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 die energy of the puckering process, while the inplane ring bending is cooperative, facilitating the puckering motion.
