MOLECULAR GEOMETRIES BY THE EXTENDED-HUCKEL MOLECULAR-ORBITAL METHOD .2. HYDROCARBONS AND ORGANIC-MOLECULES CONTAINING O, N, AND S

Bond lengths and bond angles of hydrocarbons and of organic molecules containing oxygen, nitrogen, and sulfur have been investigated by the extended-Huckel method in its improved ASED (atom superposition and electron delocalization) form. We have examined in detail bond lengths and bond angles of hydrocarbons - aliphatic, conjugated, rings, and aromatic - and we have also studied reaction enthalpies. Both properties can be calculated resonably well, if a small adjustment of the parameter K in the distance dependent Wolfsberg-Helmholz formula is accepted. We have also found that moderate contraction of the usually applied 2s-oxygen, the 2s-nitrogen, and the 3s-sulfur Slater exponents is sufficient to obtain astonishingly good geometries for important classes of organic molecules containing these elements. The three-membered rings (CH2)2X (X = CH2, NH, O, and S) which have attracted much interest and stimulated theoretical studies have been investigated, and we have found that their geometries can be reproduced nicely. It is important that geometry calculation of the investigated molecules can be carried out without losing transparency and well established prediction capabilities of the original EHMO procedure. The extended-Huckel method in its improved ASED form is, therefore, a useful tool for combining the information of the EHMO results with good geometry calculation.