Microwave Spectrum and Rotational Isomerism of Ethyl Formate

Abstract

The microwave spectrum of ethyl formate reveals the presence of at least two rotational isomers. Both isomers have the ethyl group cis to the carbonyl oxygen atom but they differ in the orientation of the methyl group about the –CH2–O bond. The trans isomer has the conformation with the heavy‐atom skeleton coplanar, while in the gauche isomer the methyl group is 95° out of the plane. The relative intensity measurements show the trans isomer to be lower in energy with the difference between the lowest states of the two isomers (gauche—trans) equal to 186±60 cal/mole. Five vibrationally excited states for the trans and one excited state of the gauche are assigned to the torsion about the –CH2–O bond. The relative intensities of these excited states and their rotational constants are used to derive, in the one‐vibration approximation, the potential function for the internal rotation of the ethyl group. This potential function yields an estimated 1100±250 cal/mole for the trans—gauche potential barrier. The dipole moment of both isomers is calculated from Stark‐effect measurements, yielding μ = 1.98±0.02 D for the trans and μ = 1.81±0.02 D for the gauche form.