Microwave spectrum and conformation of 5,6-dihydro-2H-thiopyran

Abstract

The microwave spectrum of 5,6-dihydro-2H-thiopyran has been observed between 12.4 and 40 GHz. Rotational and quartic centrifugal distortion constants have been obtained for the ground and seven excited vibrational states. The fundamental wavenumbers of the two lowest vibrations have been obtained from relative intensity measurements as ν36= 134 (12) cm–1 and ν35= 219 (18) cm–1. The electric dipole moment of the ground vibrational state has been determined from Stark effect measurements as [in D, 1 D (Debye)≈ 3.335 64 × 10–30 C m], µa= 1.35 (1), µb⩽ 0.1, µc= 0.74 (2), with a total dipole moment of µ= 1.54 (2). The rotational constants and relatively large µc component of the electric dipole moment are consistent with 5,6-dihydro-2H-thiopyran having a twisted or half-chair conformation. A molecular structure has been derived from the ground vibrational state rotational constants and planar moments of inertia using the method of predicate observations. The equilibrium twist angle of the ring is determined as τ= 32.5 (1.5)°.