Ring inversion in tetrahydrothiophen-3-one. A microwave study

Abstract

The microwave spectrum of tetrahydrothiophen-3-one has been observed in the frequency region 26.5–50.0 GHz. Rotational transitions have been assigned for the ground state, four excited states of the lowest-frequency ring-puckering mode (ca. 70 cm^–1) and two excited states of the highest-frequency ring-puckering mode (ca. 200 cm^–1). From the ground-state rotational constants and the magnitude of µ_c, the electric dipole moment component [µ_a= 1.36(2) D, µ_b= 1.31(2) D and µ_c= 0.56(4) D] a non-planar ring equilibrium configuration has been found. The µ_c rotation inversion transitions have been found to be unsplit for the ground and excited states of the vibration at ca. 70 cm^–1. For the first excited state of the vibration at ca. 200 cm^–1 an inversion splitting of 0.3 MHz has been measured directly from these transitions. The high-J, Q-branch transitions of the second excited state of this vibration appear as well resolved inversion doublets. These splittings, as well as the variations in the rotational constants, indicate that the vibration at ca. 70 cm^–1 is associated with a single-minimum potential function and that at ca. 200 cm^–1 with a double-minimum potential function. A one-dimensional reduced potential function has been derived for this vibration and gives a barrier to ring inversion of 1050 ± 230 cm^–1.