Microwave spectrum, structure and dipole moment of methyl nitrate

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—H_i(in plane)= 1.08₈Å, C—H₀(out-of-plane)= 1.09₅Å, ∠O′CH_i= 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 µ_a= 3.07 D, µ_b= 0.23₅ D, µ_t= 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.