Electron-diffraction study of water and heavy water

Abstract

The structures of gaseous H2O and D2O were investigated using the sector-microphotometer method of electron diffraction. Structural parameters and estimated standard errors for H2O included the mean bond length rv(OH) = 0.976±0.003 Å, rms amplitude lg(OH) =0.0666±0.003 Å, and mean HOH angle ag= 107.2°±3°. Corresponding values for D2 were 0.970±0.0025 Å, 0.564±0.002 Å, and 104.2°±3°. Results agreed with spectroscopic values to within the larger limits of error of the diffraction results. Analyses were based on the radial distribution method and on a new automatic "smooth-background" method. Observed atomic intensities deviated significantly at small scattering angles from intensities calculated using the conventional approximations. The resulting discrepancy led to a greater uncertainty in structural parameters than did the random experimental intensity errors. This almost universally neglected source of error is briefly discussed.