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

The rotational spectrum of 1,3-benzodioxole has been investigated in the frequency range 30-40 GHz. Six vibrational satellites have been observed and assigned to excited states of the puckering and twisting modes of the five-membered ring moiety. On the basis of the observed zigzag behaviour of the rotational constants upon vibrational excitation, the puckering has been shown to be controlled by a double-minimum potential. Similar changes upon vibrational excitation have been observed for the centrifugal distortion constant Delta(JK) and for the dipole moment component mu(a). These properties have been interpreted by a one-dimensional flexible model for the ring-puckering motion. A two-dimensional model that accounts for both the puckering and the butterfly motion of the two rings contained in the molecule was used to analyse the available far-infrared data and has led to the assignment of 19 observed absorption lines. This analysis has yielded a ring-puckering barrier of 126 cm(-1) and an equilibrium structure in which the oxygen nuclei and the methylenic carbon nucleus are on different sides of the plane that was assumed to be formed by the benzene ring.