Nonadiabatic Car-Parrinello Molecular Dynamics Studies of Photochemical Processes

• Applicant: Professor Dr. Nikos L. Doltsinis
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
• Term: from 2003 to 2009
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5397860

State-of-the-art as well as novel ab initio molecular dynamics (MD) techniques will be employed in order to study two photochemical phenomena of fundamental importance, namely excited state tautomerism in the DNA base guanine and photoisomerisation of azobenzene. A density functional restricted openshell Kohn-Sham method will be used to determine optimized geometries as weil as vibrational frequencies in the S, excited electronic state. Both harmonic and anharmonic vibrational spectra will be analysed for different tautomers in search of spectral 'fingerprints' making possible the assignment of experimental IR spectra. Excited state proton transfer (ESPT) interconverting keto and enol tautomers will be investigated by means of the recently developed nonadiabatic Car-Parrinello MD (na-CP-MD) method. This approach further allows studying radiationless decay of the S, electronically excited state to the So ground state. Effects of solvation on the relative stability of the different tautomers and on the barrier heights for ESPT shall be studied by CP-MD and its nonadiabatic extension using explicit water solvent molecules in a periodic system. Furthermore, such an approach allows studying solvent-mediated proton transfer pathways which might be very different from the ones observed in the gas phase. Cis-trans and trans-cis photoisomerisation of azobenzene shall be studied both in the gas phase and in solution using the na-CP-MD approach. Constraint dynamics simulations will be performed to compute the contraction force and its interplay with changes of the electronic structure during trans-cis isomerisation in order to uncover the mechanistic underlying recent atomic force microscopy (AFM) experiments.

• DFG Programme: Research Grants