An electron diffraction study of the molecular structure of the sodium chloride dimer

The structure of sodium chloride has been studied by gas-phase electron diffraction using photographic plates designed for high-temperature work [Kakumoto, Ino, Kodera & Kakinoki (1977). J. Appl. Cryst. 10, 100-103]. The molecular structure at about 1130 K has been determined by an analysis based on the new complex scattering factors for Na⁺ and Cl^- calculated recently by the present authors. The radial distribution function shows the existence of a considerable amount of dimer molecules in the vapor. The structures of the monomer and dimer have been analyzed by a least-squares method assuming that the dimer is of a planar diamond shape: for monomer r_a(Na--Cl) = 2.392 ± 0.028, for dimer r_a(Na-Cl) = 2.515 ± 0.017, r_a(Cl-Cl) = 3.893 ± 0.021 Å, ∠ClNaCl = 101.4 ± 0.8°. The present study indicates that the Na--Cl distance of the dimer is longer than that of the monomer and the determined structure parameters differ appreciably from existing theoretical predictions.