Microwave Spectrum, Structure, Dipole Moment, and Quadrupole Coupling Constants of Formamide

Abstract

The microwave spectra of formamide, and the isotopic species 1, 2 N di-deutero formamide, 1 N mono-deutero formamide (cis) and (trans), have been investigated. The rotational constants obtained from the frequencies of assigned rotational transitions lead to the following effective structural parameters: for the bond distances, there results C–O = 1.243±0.007 A, C–N = 1.343±0.007 A, N–H = 0.995±0.007 A, and for the bond angles ∠HNH = 118.98±0.50°, ∠ NCO = 123.58±0.35°, ∠NCH = 103.9±1.2°. The molecule is planar. Stark effect measurements yield values for the dipole moment components which, when combined with assumed approximate bond moments and the structural parameters, give the magnitude and the direction of the dipole moment. The dipole moment is thus equal to 3.714±0.06 Debyes and makes an angle of 39.6° with the C–N bond. The quadrupole coupling constants, Xa = 1.9 mc, Xb = 1.7 mc, Xc = -3.6 mc, were calculated from the frequency shifts of the hyperfine splittings observed for several rotational transitions. The structural parameters and quadrupole coupling constants are discussed in terms of the bonding characteristic of the N–C–O linkage in formamide and related compounds.