A systematic pairwise comparison of geometric parameters obtained by X-ray and neutron diffraction

Abstract

The effects of X-ray atomic asphericity on derived molecular geometry have been examined for ten common substructures involving C, N, O. The Cambridge Structural Database has been used to locate pairs of X-ray (X) and neutron (N) structure determinations of the same compound. Rigid criteria based on experimental temperature, refinement procedures, crystalline form and precision have been applied in selecting suitable X, N structure pairs. Corresponding X and N values for derived parameters have been paired for 46 chemically unique subsets (39 bond-length sets and 7 valence-angle sets). The statistical significance of the distribution of X N differences (Di) has been assessed in each case via a Wilcoxon matched-pairs signed-ranks test. The signed magnitude of the mean difference (/5) is large, as expected, for bonds to H, ranging from -0.096 (7) A for C-H to -0.155 (10) A for O-H. Some highly significant differences are also observed for bonds involving C, N, O alone. Representative /) values (A) are -0.0096 (9) for C--C (ethylenic), -0.0052(9) for C---C (benzenoid), -0.0078 (16) for C_----N, +0.0035 (12) for C-OH (carboxyl), +0.0046 (10) for N---O (nitro), +0.0054 (8) 0108-7681 / 86/050515-08 $01.50 for C-O (carbohydrate) and +0.0082 (21) for C-C (C-C----N); the significance of the Di distribution for C = O (keto) was marginal. No significant differences were observed for C = O (COOH and COO-), nor for any of the valence angles tested. Some implications for the combination or comparison of structural results obtained from different techniques are noted.