Microwave Spectrum, Barrier to Internal Rotation, and Structure of Methyl Formate

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Abstract
Rotational constants have been assigned to the following isotopic species of methyl formate by investigation of their microwave spectra: HCOOCH3, DCOOCH3, HCOOCH2D, HCOOCD3, HC13OOCH3, HCOOC13H3, HCO18OCH3, and HCOO18CH3. The following structural parameters were obtained from these rotational constants: r (C=O) = 1.200±0.01 A, r (C–O carboxyl) = 1.334±0.01 A, r (C–O methoxyl) = 1.437±0.01 A, r (C–H carboxyl) = 1.101±0.01 A, r (C–H methyl) = 1.086±0.015 A, ∠O=C–O=125°52′±1°, ∠H–C–O (carboxyl) = 109°18′±1°, ∠C–O–C = 114°47′±1°, and ∠H–C–H = 110°40′±1.5°. The carbon‐oxygen skeleton is planar with the methyl group cis to the carboxyl oxygen. The methyl group axis has been assumed to be along the C–O (methoxyl) bond, although some evidence suggests that it may be tilted. The barrier to internal rotation of the methyl group was calculated from the splittings observed in the rotational transitions of the ground torsional state for several isotopic species and found to be 1190±40 cal/mole. The dipole moment was calculated from the observed Stark effect and found to be 1.77±0.03D. The dipole moment lies at an angle of 39.4±2° from the C=O bond and roughly on the line between the ester oxygen and the carboxyl oxygen.
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