Eugene Switkes

Localized Bonds in SCF Wavefunctions for Polyatomic Molecules. III C–H and C–C Bonds

Localized molecular orbitals which minimize the exchange energy have been obtained for CH4, C2H6, C2H4, C2H2, CH3CCH, C3H6, HCN, and H2CO. These objectively determined orbitals correspond to the inner shells, lone pairs, and two‐center bonds of classical bonding theory. In each case where double or triple bonds occur, the local orbitals correspond to equivalent bent bonds. The hybrids in the C–C bonds of cyclopropane form angles of 28° with the internuclear direction. The local orbitals are analyzed in terms of hybridization, polarity, bond moments, bond directions, and delocalization. Calculation of the curvature of the self‐repulsion energy surface provides an indication of the uniqueness of the results. Sigma–pi separability and the sensitivity of the local orbitals to changes in basis set are also discussed.

Localized Bonds in SCF Wavefunctions for Polyatomic Molecules. I. Diborane

Molecular SCF orbitals of B2H6 have been computed from optimized minimum basis sets which employ isotropic or anisotropic atomic 2p orbitals. These SCF wavefunctions have been transformed to localized MOs which maximize the self‐energy, D = Σi(φiφi | φiφi). This objective procedure strongly supports the three‐center bond for each BHB bridge. The resulting hybrids are sp2.5, with ∠Ht–B–Ht=125° and ∠Hb–B–Hb=93° for terminal and bridge Hs, respectively.

Polyatomic SCF Calculations Utilizing Anisotropic Basis Sets of Slater‐type Orbitals

Fully optimized SCF wavefunctions using minimum Slater‐type basis sets have been obtained for BH3, NH3, C2H2, C2H4, HCN, and H2CO. Calculations employing anisotropic minimum basis sets are reported for NH3, H2O, HCN, and H2CO. The lower SCF energies of the calculations for anisotropic sets are primarily due to a reduction in electron–electron repulsion. Improvements in calculated dipole moments occur when independent p‐orbital components are used to describe lone pairs and covalent bonds. A geometry‐optimized calculation for H2O is also reported.

Localized Bonds in SCF Wavefunctions for Polyatomic Molecules. IV. Ethylene, Butadiene, and Benzene

Ab initio molecular‐orbital wavefunctions for ethylene, butadiene, and benzene have been analyzed in terms of the localized orbitals (LMOs) defined by minimizing molecular exchange energy. The effect of conjugation on the LMOs is studied in terms of the delocalization of LMOs and the curvature of the energy surface which determines the LMOs. The degree of preference for bent double‐bond LMOs over σ–π double‐bond LMOs is also examined. In all cases, the former LMOs represent the correct solutions, while the latter LMOs, obtained by imposing the constraint of σ–π separation, correspond to saddle‐point solutions. The double‐bond LMOs for ethylene and butadiene are quite similar, whereas in the case of benzene, appreciably greater delocalization is found, and the preference for bent bonds over σ–π bonds is greatly reduced.