Microwave spectrum, barrier to internal rotation of the methyl group, chlorine-35 nuclear quadrupole coupling constants, and molecular conformation of methyl chloroformate

Abstract

The microwave spectrum of methyl chloroformate vapour has been analysed and found to be consistent only with a planar heavy atom skeleton with the methyl group cis to the carbonyl group. Both µ_a and µ_b lines were observed. The three-fold barrier to internal rotation of the methyl group was calculated from splittings in µ_b Q branch lines to be 1250 ± 30 cal mol^–1(5230 ± 126 J mol^–1). The ³⁵Cl nuclear quadrupole constants were measured from the µ_a 2â†�1 transitions to be _Xaa=–58.8 ± 0.2 MHz and _Xbb= 31.1 ± 0.5 MHz.