The Microwave Spectrum of 3,4-Dihydro-l^-Pyran

The rotational transitions of 3,4-dihydro-l,2-pyran in the ground state and six vibrationally excited states have been assigned. The rotational constants for the ground state (A = 5198.1847(24), B = 4747.8716(24) and C = 2710.9161(24) have been derived by fitting fia, ßh and /<c-type transitions. The dipole moment was determined from Stark displacement measurements to be 1.400(8) D with its principal axis components na =1.240(2), nb = 0.588(10) and ßc = 0.278(8) D. A model calculation to reproduce the ground state rotational constants indicates that the data are consistent with a twisted ring conformation. The average intensity ratio gives vibrational separations between the ground and excited states of the ring-bending and ring-twisting modes of ~ 178 and ~ 277 cm 1 respectively.