| The research presented in this thesis may be divided into three areas: derivative theory, transition metal chemistry, and solvation effects.;Analytic derivatives. A derivation of the closed shell frozen-core analytic gradient for second order Moller Plesset perturbation (MP2) theory is presented, as well as a derivation of an expression for the localized orbital dipole polarizability. Both methods have been implemented in the electronic structure code GAMESS and are currently in wide use.;Transition metal chemistry. Ab initio electronic structure theory methods have been used to investigate the chemistry of simple titanium compounds. These include the closed shell TiH$sb4$ dimer: Ti$sb2$H$sb8$; the diradical TiH$sb3$ dimer: Ti$sb2$H$sb6$; and the closed shell TiX$sb4$ dimers: Ti$sb2$X$sb8$ (X = F, Cl, Br). Both the hydride dimers contain 2 electron, 3 center bonds analogous to those in diborane. Ti$sb2$H$sb6$ is a model compound for more complex di-titanium(III) bridged systems. We have accurately calculated the isotropic singlet-triplet energy gap in Ti$sb2$H$sb6$ (1.43 kcal/mol) and in addition calculated anisotropic effects due to spin-orbit coupling between ground and excited states. Calculations on TiX$sb4$ dimers establish the possibility of halide exchange and thereby explain unexpected NMR line widths observed for TICl$sb4.$.;Solvation studies. The effective fragment potential (EFP) method is used to study the effects of solvating water molecules on the Menshutkin reaction between ammonia and methyl bromide. The effects, relative to the gas phase reaction, of two, four, six, and eight water molecules are examined. The EFP method is found to reproduce all ab initio calculations very accurately at a fraction of the cost. |
