Photofragment studies of carbon-oxygen and carbon-hydrogen bond activation by transition metal cations
Gas-phase transition metal cations can activate C-C and C-H bonds in hydrocarbons at room temperature. They can even activate C-O bonds in CO2. They thus serve as a relatively simple model for C-C and C-H activating catalysts. A better understanding of the crucial steps and a complete characterization of possible intermediates in these reactions is desirable in order to improve catalysts. Brief introductions to gas-phase transition metal chemistry and to the experimental techniques employed in these studies are given in Chapter 1. Chapter 2 describes the details of the apparatus, laser systems and data acquisition.
In Chapters 3 and 4, we investigate the spectroscopy and photodissociation dynamics of V+(OCO). Electronic spectra of gas-phase V +(OCO) are measured in the near-infrared and the visible using photofragment spectroscopy. The visible band shows clearly resolved vibrational progressions in the metal-ligand stretch and rock, and in the OCO symmetric stretch and bend. We measure the OCO antisymmetric stretch frequencies in the ground and excited state of V+(OCO) using vibrationally mediated photodissociation (VMP). Photodissociation produces V+ + CO2 (non-reactive pathway) and VO+ + CO (reactive pathway). One-photon dissociation studies confirm mode selectivity and they are extended to higher energy. The effect of OCO antisymmetric stretch vibrations on reactivity is investigated using VMP. Exciting the antisymmetric stretch leads to a three-fold increase in the reactive VO+ channel, and combination bands involving the antisymmetric stretch all show similar enhanced reactivity. Electronic structure calculations were performed to characterize the dissociation pathways and excited electronic states of V+(OCO). In addition, spin-orbit coupling of quintet states to triplet states was calculated and used to compute intersystem crossing rates, which reproduce many of the observed mode-selective trends. The V+-OCO stretch and OCO antisymmetric stretch appear to enhance reactivity by increasing the intersystem crossing rate.
In Chapter 5, photodissociation spectra of Fe+(CH 4)3 and Fe+(CH4)4 entrance channel complexes in the C-H stretching region are discussed. Vibrational spectroscopy reveals that bonding in these complexes is not merely electrostatic, but includes significant covalency, which weakens the C-H bonds and significantly lowers the C-H stretching frequencies. In addition, to get structural information from the experiments, we calculated the geometries and vibrational spectra of low-energy isomers.