CHAIR CONFORMATION AND BARRIER TO RING PUCKERING IN 1,3-BENZODIOXOLE

The rotational spectrum of 1,3-benzodioxole has been investigated in t he frequency range 30-40 GHz. Six vibrational satellites have been obs erved and assigned to excited states of the puckering and twisting mod es of the five-membered ring moiety. On the basis of the observed zigz ag behaviour of the rotational constants upon vibrational excitation, the puckering has been shown to be controlled by a double-minimum pote ntial. Similar changes upon vibrational excitation have been observed for the centrifugal distortion constant Delta(JK) and for the dipole m oment component mu(a). These properties have been interpreted by a one -dimensional flexible model for the ring-puckering motion. A two-dimen sional model that accounts for both the puckering and the butterfly mo tion of the two rings contained in the molecule was used to analyse th e available far-infrared data and has led to the assignment of 19 obse rved absorption lines. This analysis has yielded a ring-puckering barr ier of 126 cm(-1) and an equilibrium structure in which the oxygen nuc lei and the methylenic carbon nucleus are on different sides of the pl ane that was assumed to be formed by the benzene ring.