Molecular Structure of Nitrogen Dioxide and Nitric Acid by Electron Diffraction

Abstract

New electron diffraction photographs have been taken of NO2 extending the region previously investigated to include larger angles of scattering. An interference ring was found at (1/λ) sin ½θ = 0.49 followed by another ring appearing at 0.94 as determined by visual measurements. The outer portion of the pattern consists of two rather broad rings and two well-defined minima. Theoretical intensities of scattering were computed for various nitrogen valence angles, assuming the positions of the two oxygens to be equivalent. The best fit, and probably the correct structure, gives the angle O–N–O = 130±2° with the N–O distance 1.21±.02A. Photographs were obtained from pure nitric acid vapor at 70°—85°C. The interference maxima were measured visually as far out as the eighth maximum at (1/λ) sin ½θ = 1.83; a prominent minimum was seen at 1.54. Theoretical intensities were computed for various likely models, disregarding the scattering by the hydrogen atom. Good agreement was obtained for a planar model having an NO2 group with the same structure found for nitrogen dioxide. The third oxygen atom O′ is located at a distance of 1.41±0.02A from the nitrogen atom and equidistant from the other oxygen atoms. A model having the nitrogen atom slightly out of the plane containing the oxygen atoms also gave good agreement with the experimental results. This model however is considered less probable in view of Raman spectra data which apparently require a planar structure for O′–NO2.