Microwave spectrum, structure and dipole moment of methyl nitrate

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Abstract
The microwave spectra of the normal species and seven isotopic species of methyl nitrate have been measured. The detailed structure of the molecule, which has a planar heavy-atom skeleton, with the methyl group staggered with respect of the cis oxygen atom, is determined as : C—O′= 1.437 Å, N—O′= 1.402 Å, N—O (cis)= 1.205 Å, N—O(trans)= 1.208 Å, C—Hi(in plane)= 1.088Å, C—H0(out-of-plane)= 1.095Å, ∠O′CHi= 103.4°, ∠O′CH0= 110.4°, ∠H0CH0= 109.8°, ∠CO′N = 112.72°, ∠O′NO(cis)= 118.10°, ∠O′NO(trans)= 112.38°. The methyl group and nitro group are tilted away from each other; the former is inclined at 4.8° to the C—O′ bond direction, and the latter at 2.9° to the N—O′ direction. The components of the dipole moment have been determined, from Stark effect measurements, as µa= 3.07 D, µb= 0.235 D, µt= 3.08 D, and similar measurements on the trans-18O species have shown that the dipole moment in the normal species is oriented at 39.6° to the bridging N—O′ bond. The structure and bonding of methyl nitrate is discussed together with some related molecules; its structure relates to nitric acid as methyl formate does to formic acid.

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