Microwave spectrum of propionic acid. II. Structure of c i s-propionic acid by double resonance modulation microwave spectroscopy

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Abstract
Microwave double resonance modulation has been used for the first time to observe all the isotopic species spectra that are needed for a structure determination by the microwave method. From spectra of the normal and thirteen isotopically labelled species of cis−propionic acid the substitution structure of this molecule has been determined completely. Bond lengths (in Å) are as follows; Cketo−Cmethylene = 1.509±0.002, Cmethylene−Cmethyl = 1.523±0.003, C=O = 1.210±0.001, C−O = 1.352±0.002, O−H = 0.970±0.001, (C−H)methylene = 1.098±0.002, (C−H)methyl = 1.088±0.002. The nonbonded distance between the oxygen atoms is 2.245±0.001 Å. The bond angles are (in degrees): & CCC = 112.7±0.1, & (HCH)methylene = 106.4±0.2, & (HCH)methyl = 108.6±0.2, & (CCH)methyl =110.0±0.3, & C=O = 125.8±0.2, & CC−O = 111.8±0.1 and & COH = 105.8±0.2. The plane HCH of the methylene hydrogens is tilted by 4.9±0.1° from the external bisector of the CCC−angle towards the hydroxyl oxygen atom. Spectra corresponding to the first excited state of the large amplitude, torsional vibration around the central carbon−carbon bond have been analyzed for all 14 isotopic forms, enabling the extrapolation to torsional equilibrium moments. Structure calculations based on these moments show that the torsional vibration has no significant influence on the ground state structure. For the normal and four deuterated species the effects of the three lowest vibrations could be eliminated from the ground state constants. The corresponding structure calculations suggest that the out−of−plane carbon−hydrogen bonds derived from ground state constants contain a contribution of 0.005 and 0.002 Å, respectively, for the methylene and methyl hydrogens due to the combined zero−point effects of these three vibrations. The heavy skeleton structure of propionic acid appears little affected, however.