The structures of some small molecules. Ab initio molecular orbital calculations versus low temperature neutron diffraction crystal structure analyses

Abstract

The results of ab initio MO calculations of fully optimized molecular geometries at the HF/3-21G level of approximation are compared with the results of single crystal neutron diffraction structure analyses, carried out between 9 and 23 K. The molecules discussed are acetamide, monofluoroacetamide, thioacetamide, formamide oxime, N, N′-diformylhydrazide, glyoxime, and 1,2,4-triazole. The theoretical results are corrected for basis-set deficiency and electron correlation by consideration of more extensive calculations on simpler molecules. The experimental results are corrected for thermal and zero-point energy motion by segmented-body thermal motion analysis. Allowance for hydrogen-bonding effects is made by means of calculations on simpler model systems at the HF/STO-3G level. After applying these corrections, the disagreement between the theoretical and experimental bond lengths ranges between 0 and 0.028 Å, depending upon the type of bond. With one exception, the difference between the experimental and theoretical bond angles does not exceed 2.6° and the mean difference is 1.4°. The phenomenon of π-bond anisotropy leading to pyramidalization of the C sp 2 atoms is calculated and observed in the acetamide and thioacetamide molecules.