Theoretical Studies on Kinetics of Molecular Excited States

Kinetics on molecular excited states is a challenging subject in the field of theoretical chemistry. This thesis pays attention to theoretical studies on kinetics of photo-induced processes, including photo-chemical reactions, radiative and non-radiative transitions (intersystem crossing and internal conversion) in molecular and bio-related systems. Oneor multidimensional potential energy surfaces (PESs) not only provide qualitative mechanistic explanation for excited state decay, but also make it possible to perform kinetic simulations. We have constructed several types of PESs by using computational methods of high-accuracy for a variety of systems of interest. In particular, density functional theory (DFT) and couple cluster singles and doubles (CCSD) method are employed to build PESs of the ground and lowest triple states. For medium-sized molecules, the complete active space self-consistent (CASSCF) method is used for constructing the PESs of excited states. Various kinetic theories for the decay processes of excite states are briefly introduced, in particularly adiabatic and nonadiabatic Rice–Ramsperger–Kassel -Marcus (RRKM) approaches for the kinetics of nonradiative decay of excited 2-aminopridine molecule. Special attention has been devoted to Monte Carlo transition state theory which can provide an efficient way to predict the rate of nonradiative transitions of polyatomic molecules on multi-dimensional PESs. Examples of Monte Carlo simulations on the intersystem crossing of isocyanic acid and a model molecule of hexacoordinate heme, as well as internal conversion process for 2-amininopyridine dimer and the adenine-thymine base pair are presented. Preface The work presented in this thesis has been carried out at the Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Stockholm, Sweden. PaperI Zhang, F., Ai, Y. J., Luo, Y., Fang, W. H. Nonradiative decay of the lowest excited singlet state of 2-aminopyridine is considerably faster than the radiative decay, J. Chem. Phys., 2009, 130,144315. Paper II Zhang, F., Fang, W. H., Luo, Y., and Liu, R. Z. A multi-dimensional miocanonical Monte Carlo study of S0-T1 intersystem crossing of isocyanic acid, Sci China Ser B-Chem, 2009, 52,1885. Paper III Zhang, F., Ai, Y. J., Luo, Y., Fang, W. H. Nonadiabatic histidine dissociation of hexacoordinate heme in neuroglobin protein, J. Phys. Chem. A., 2010, doi: 10.1021/jp909887d. Paper IV Ai, Y. J., Zhang, F., Cui, G. L., Luo, Y., and Fang, W. H. Ultrafast deactivation processes in 2-aminopyridine dimer and the A-T base pair: similarity and difference? Submitted to J. Phys. Chem. A. Paper V Ai, Y. J., Zhang, F., Chen, S. F., Luo, Y., Fang, W. H. Importance of the intra-hydrogen bonding on the photochemistry of anionic hydroquinone (FADH) in DNA photolyase, J. Phys. Chem. Lett.,2010,doi: 10.1021/jz900434z. List of Papers not Included in this Thesis Paper VI Fang, Q., Zhang, F., Shen, L., Fang, W. H., and Luo, Y., Photodissociation of phosgene: Theoretical evidence for the ultrafast and synchronous concerted three-body process, J. Chem. Phys., 2009, 131,164306. Paper VII Cui, G. L., Zhang, F., Fang, W. H., Insights into the mechanistic photodissociation of methyl formate, J. Chem. Phys., 2010, 132, 034306. Paper VIII Zhang, F., Ding, W. J., Fang, W. H., Combined nonadiabatic transition-state theory and ab initio molecular dynamics study on selectivity of the α and β bond fissions in photodissociation of bromoacetyl chloride, J. Chem. Phys., 2006, 125,184305. Comments on My Contribution to the Papers Included · I was responsible for all the calculations and writing of paper I,II, III. · I was responsible for part of calculations and writing of paper IV. · I was responsible for the spectra simulation and part of writing and discussion of paper V.