Barriers to Internal Rotation in β-Methyl-γ-Butyrolactone, 3-Methylcyclopentanone, and γ-Valerolactone from Fourier Transform Microwave Spectroscopy

Abstract

The microwave spectrum of β-methyl-γ-butyrolactone has been analyzed using Stark modulation microwave spectroscopy in the frequency range 26.5–40 GHz. Spectra of the ground vibrational state and the first excited states of the ring bending, ring twisting, and methyl torsion have been assigned. The rotational constants are consistent with the molecule having a conformation similar to that of γ-butyrolactone, and with the methyl group occupying an equatorial position. A reinvestigation 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 frequency range 8–18 GHz using Fourier transform microwave spectroscopy on the ground vibrational state and the first excited state of the methyl torsion for β-methyl-γ-butyrolactone, 3-methylcyclopentanone, and γ-valerolactone in order to determine the barriers to internal rotation. No A–E splittings have been observed in the ground vibrational 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 β-methyl-γ-butyrolactoneV3= 17.2, for 3-methylcyclopentanoneV3= 14.76, and for γ-valerolactoneV3= 14.75.