Date of Award
Program or Major
Doctor of Philosophy
Frank L Pilar
Organolithium compounds are important reagents of widespread use in synthetic organic chemistry. Structures of organolithium hydrocarbons do not follow classical rules; replacement of a hydrogen by lithium in their molecules almost always results in a major change in geometry, electronic state, or both. The structures of many of these compounds still remain unknown and a subject of controversy.
This study investigated the geometries and electronic states of the ground states of dilithioacetylene and dilithiomethane molecules by using ab initio molecular orbital calculations. The geometries of both the molecules have been fully optimized at both the single determinant Hartree-Fock self consistent field (HF SCF) and the multiconfiguration self consistent field (MC SCF) levels with a variety of basis sets. Linear and planar bridged forms for dilithioacetylene and tetradedral-like, cis planar, and trans planar forms for dilithiomethane were studied. The electronic states examined were the singlet state for both molecules and the triplet state for dilithiomethane.
It has been found that the equilibrium geometries, a linear and a planar bridged structures for dilithioacetylene molecule, and a cis planar and a tetrahedral-like forms for the singlet and triplet states of dilithiomethane molecule, are rather sensitive to the choice of the basis set at the Hartree-Fock level of theory. Calculations at the MC-SCF level of theory behave the same way but in smaller active multi configuration spaces, and do not seem to depend on the basis set in larger active spaces. Both the molecules have a relatively flat potential energy surface and do not exhibit strong preferences for the optimized structures. This lack of preference for the optimized structures is especially true for the MCSCF calculations on the multiplicity of the ground state structures for dilithiomethane molecule.
Su, Shu-Jun, "Ab initio molecular orbital investigations of molecular structures for lithiated hydrocarbons" (1988). Doctoral Dissertations. 1547.