Molecular Structure of Formamide as Studied by Gas Electron Diffraction

Abstract

The structure of formamide HCONH2 has been determined by gas electron diffraction. The rotational constants determined in previous microwave studies have also been used for determining the hydrogen parameters. From a joint least-squares analysis of the diffraction intensity and the rotational constants, where the out-of-plane vibrations are treated as finite-amplitude motions in the corrections for vibrational effects, the following rg bond distances and rz angles have been derived: C–N=1.368±0.003 Å, C=O=1.212±0.003 Å, ∠N–C=O=125.0±0.4°, N–H(average)=1.027±0.006Å, C–H=1.125±0.012 Å, ∠C–N–H(cis to C=O)=118.7±1.0°C, and ∠C–N–H (trans to C=O)=119.7±1.0°, where the uncertainties represent estimated limits of experimental error. The C–N bond is about 0.07 Å longer, whereas the C=O bond is about 0.04 Å shorter, in the free molecule than in the crystal studied by X-ray diffraction. The C–N and C=O bonds are about 0.01 Å or more shorter, and the N–C=O angle is about 3°C larger than the corresponding parameters in acetamide and N-methylacetamide.