Colloidal semiconductor nanocrystals are materials with intriguing properties that make them useful for a diverse array of applications such as photocatalysts, light-absorbing materials in solar cells, light emitting diodes and luminescent biological tags, to name only a few. Performance of nanomaterials in these applications is directly related to the size, shape and stoichiometry of the nanocrystals. Strategies exist to control these characteristics during colloidal synthesis, but they tend to rely on certain surfactants, additives, or multi-step procedures to achieve desirable properties. This thesis describes new directions in the synthesis of colloidal nanomaterials that use computational chemistry as a guide. Using new and efficient methods in density functional theory (DFT) to reliably calculate bond dissociation energies (BDEs) of organodichalcogenide (sulfide or selenide) precursors enables the rational synthesis of dot, rod and tetrapod morphology cadmium chalcogenide nanocrystals. Precursors with weaker C-E (E = S, Se) bonds and stronger E-E bonds yielded dot-shaped nanocrystals, while precursors with stronger C-E and weaker E-E bonds afforded rod or tetrapod shapes. This methodology readily extends to the BDE calculation of tertiary phosphine chalcogenides with substituted phenyl, alkyl, perfluoroalkyl moieties or Verkade-type cage structures. In these systems the BDE of a series of P--S or P--Se bonds increases with slightly increasing bond distance, although the BDE of P--Se bonds is significantly lower than P--S bonds.

Another promising method in colloidal nanocrystal synthesis is photochemical decomposition of precursors to access unusual phases or shapes. This thesis also describes the photochemical synthesis of cobalt(III) oxyhydroxide, Co(O)OH, nanocrystals from chloropentaamminecobalt(III) salts in aqueous solution. Compared to the thermal decomposition of the starting material in the absence of light, the photochemically-synthesized material exhibits a smaller size with a lower-temperature phase transition to cobalt(II,III) oxide, Co3O4.