Microwave spectrum, dipole moment and conformation of 5,6-dihydro-2H-pyran-2-one

Abstract

The microwave spectrum of 5,6-dihydro-2H-pyran-2-one has been recorded from 18.0 to 40.0 GHz. Both a-type and b-type R-branch transitions in the ground state as well as a-type transitions in the first three excited states of the right-bending and the first excited state of the ring-deforming mode are assigned. The ground state effective rotational constants are determined to be A = 5143.00 ± 0.04, B = 2703.14 ± 0.02 and C = 1864.29 ± 0.01 MHz. Analysis of the measured Stark effects gives the dipole moment 5.08 ± 0.04 D with components of |μ a | = 4.96 ± 0.02, |μ b | = 1.01 ± 0.09 D and |μ c | = 0.45 ± 0.01 D. It is shown that the ring skeleton is non-planar from the magnitude of the μ c component of the dipole moment as well as from the value of I a + I b − I c . From the experimental data, the ring bending mode appears to be governed by a double minimum potential function with a barrier to the planarity larger than 250 cm −1.