Molecular interactions and photochemical processes

These projects of dr. G.C. Groenenboom were selected twice by CW/NWO as a "young chemist" support position. Questions of great importance in atmospheric chemistry related to ozone depletion and the greenhouse effect have prompted advanced experimental studies of the photodissociation reactions OClO+photon --> OCl+O and N_2O+photon --> N_2 + O. For OClO, fine-structure resolved experimental data so far defies interpretation and as a result this data cannot help to resolve existing questions regarding the photodissociation mechanism. We propose a computational study in which we focus initially on constructing a proper long range model, which we believe to be a key part missing in previous theoretical studies of this photodissociation reaction. A semiclassical dynamical study is needed to draw the first conclusions. Photo-induced isotopic fractionation of N_2O could possibly resolve the persistent imbalance in sinks and sources in atmospheric models. However, its mechanism is only partly understood. We propose to attack the problem by developing a generalized Renner-Teller model to describe the initial excitation step, based on ab initio calculations. Nonadiabatic and spin-orbit couplings play an important role in chemical reactions when near degeneracies of electronic states occur. In reactions involving open shell species this occurs in the long range, where electronic states become asymptotically degenerate. Also, in electronically excited molecules near degeneracies and associated couplings are common. In order to have the full benefit of the experiments currently ongoing complementary theoretical/numerical studies are crucial. Our ultimate goal is to arrive at a complete quantum dynamical description of both photodissociation reactions by performing time-dependent wave packet calculations including all relevant couplings. The potential energy surfaces and the couplings will be obtained by matching the long range models with ab initio calculations at the MCSCF and contracted MRCI level. MCSCF, MRCI and nonadiabatic couplings may now be obtained with several ab initio program packages. For example, we have been using the MOLPRO package, which allows the calculation of nonadiabatic couplings at the MCSCF and MRCI level.