Density functional theory has been used to study the effects of solvent coordination on the reactivity of titanium hydroperoxo complexes in the epoxidation of the model olefin ethylene. Complexes possessing a variety of coordination environments have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. The Gibbs free energy of activation for ethylene epoxidation with a nonligated titanium hydroperoxo species is 25.7 kcal/mol. Addition of a single solvent ligand to the titanium center has little effect on the activation barrier unless that ligand participates in a hypervalent omega bond with the proximal oxygen (Oalpha) of the hydroperoxo moiety; in that case, the co bond inhibits the heterolytic cleavage of the Ti-Oalpha bond and increases the activation barrier for transfer of Oalpha to ethylene by 6-7 kcal/mol. Hydrogen bonding of the hydroperoxo moiety with a protic solvent ligand via a monodentate five-membered ring structure does not enhance the reactivity of the titanium hydroperoxo complex. The presence of two solvent ligands on the titanium center increases the activation barrier 11 - 16 kcal/mol by sterically hindering the attack of ethylene at Oalpha. The activation barrier is not affected by the presence of a hydrogen-bonded protic solvent molecule bridging the hydroperoxo moiety and a protic ligand on titanium. NBO analysis indicates that changes in the electrophilicity of the peroxo group caused by solvent coordination do not control the observed reactivity of the titanium hydroperoxo, complexes. The activation barriers for ethylene epoxidation with titanium hydroperoxo, methylperoxo, and trifluoromethylperoxo oxidants decrease with increasing positive charge on the atom bonded to the distal peroxo oxygen (Obeta) through stabilization of the developing negative charge on Obeta. Comparison of the Gibbs activation barriers for the formation of the titanium hydroperoxo intermediate and its subsequent reaction with ethylene indicates that the oxygen transfer step is the rate-determining step for the overall epoxidation mechanism.