BARRIERS TO INTERNAL-ROTATION IN BETA-METHYL-GAMMA-BUTYROLACTONE, 3-METHYLCYCLOPENTANONE, AND GAMMA-VALEROLACTONE FROM FOURIER-TRANSFORM MICROWAVE SPECTROSCOPY

The microwave spectrum of beta-methyl-gamma-butyrolactone has been ana lyzed using Stark modulation microwave spectroscopy in the frequency r ange 26.5-40 GHz. Spectra of the ground vibrational state and the firs t excited states of the ring bending, ring twisting, and methyl torsio n have been assigned. The rotational constants are consistent with the molecule having a conformation similar to that of gamma-butyrolactone , and with the methyl group occupying an equatorial position. A reinve stigation of the microwave spectrum of 3-methylcyclopentanone has led to a reassignment of the first excited states of the ring twisting and methyl torsion vibrations. Measurements have been made in the frequen cy range 8-18 GHz using Fourier transform microwave spectroscopy on th e ground vibrational state and the first excited state of the methyl t orsion for beta-methyl-gamma-butyrolactone, 3-methylcyclopentanone, an d gamma-valerolactone in order to determine the barriers to internal r otation. No A-E splittings have been observed in the ground vibrationa l state. Analyses of A-E splittings in the first excited state of the methyl torsion using the IAM method give the following barrier heights (in kJ mol(-1)): for beta-methyl-gamma-butyrolactone V-3 = 17.2, for 3-methylcyclopentanone V-3 = 14.76, and for gamma-valerolactone V-3 = 14.75. (C) 1996 Academic Press, Inc.