Far infrared ring puckering vibration of 3,3-difluoroxetane: Effect of fluorine substitution on the ring puckering potential

Abstract

The far infrared spectrum of the ring puckering vibration of 3,3-difluoroxetane has been observed and the potential determined to be V(Z) = 0.644×104Z2 + 0.425×106Z4 cm−1, where Z is in angstroms. The reduced mass for the vibration is 151 amu. From the vibrational dependence of the rotational constants, the dynamics of the ring vibration are found to shift from 1.5 times more bending about the C–O axis in trimethylene oxide to 1.5 times more bending about the C–C axis in difluoroxetane. An analysis of the changes in the barrier to inversion in cyclobutane, 1,1-difluorocyclobutane, trimethylene oxide, and difluoroxetane is based on the assumption of a model potential. The model potential for the ring puckering vibration is approximated as the sum of angle deformations and torsions about the ring bonds with a ``torsional strain'' parameter included which is analogous to the angle strain contribution to the deformation. Using literature values for the unstrained threefold contribution to the potential, the torsional strain contribution is found to decrease by 1.9±0.1 kcal mole−1 with difluoro substitution on both cyclobutane and trimethylene oxide.