Microwave spectrum and dipole moment of 3,6-dihydro-2<i>H</i>-pyran

Abstract

The microwave spectrum of 3,6-dihydro-2H-pyran has been studied in the R-band frequency range (26.5–40.0 GHz) with a Hewlett-Packard Model 8400 Stark-modulated microwave spectrometer. The spectrum is characterized by rather strong perpendicular transitions of a near oblate symmetric top. Transitions obeying a, b, and c dipole selection rules have been assigned and 24 low-J R -branch transitions have been used to derive the rigid rotor constants for the ground vibrational state: (in MHz) A = 5085.152±0.012; B = 4849.365±0.013; C = 2712.487±0.013. Stark displacement measurements were made for several transitions and the dipole moment components determined by least squares fitting the displacements; (in Debye) |μa|=0.892±0.006; |μb|=0.507±0.010; |μc|=0.771±0.002 yielding a total dipole moment μtot = 1.283±0.005. Model calculations to reproduce the rotational constants and the observed dipole moment components indicate that the data are consistent with a twisted ring skeleton with the oxygen atom on one side and carbon 2 on the other side of a plane formed by the other four carbon atoms. This oxygen atom and carbon 2 lie in a plane which passes halfway between carbons 3 and 6 and makes an angle τ=31.5° with the C3 − C4 = C5 − C6 plane. The valence angles within the ring are very near the values found in analogous open chain molecules.