Microwave spectrum of propionic acid. I. Spectrum, dipole moment, barrier to internal rotation, and low-frequency vibrations of <i>c</i> <i>i</i> <i>s</i>-propionic acid

Abstract

From its rotation spectrum propionic acid (CH3CH2COOH) is shown to exist predominantly in a cis−conformation in which the methyl group eclipses the carbonyl bond and with the hydroxyl hydrogen between the two oxygen atoms. The dipole moment of this rotamer is μ = 1.55±0.03 D (μa = 0.19 D, μb = 1.54 D). From the variation of its components under isotopic substitution the dipole moment deviates by (5.3±1.5) ° from the C = O bond toward the hydroxyl group. The barrier hindering internal rotation of the methyl group has been determined for four isotopic forms with the result V3 = 2340±30 cal/mole for CH3CH2COOH and CH3CH2COOD and V3 = 2370±30 cal/mole for CH3CD2COOH and CH3CD2COOD. Satellite spectra from 24 different vibrational levels have been analyzed by double resonance techniques. The five lowest vibrational modes of cis−propionic acid have been assigned: C−C torsion at 64±3 cm −1, CH3 internal rotation at 190.4 cm−1, CCC deformation at 270±15 cm−1, with tentative vibrational assignments of δ (CCO) at 530±30 cm−1, τ (OH) at 547±20 cm−1.