Nonadiabatic Car-Parrinello Molecular Dynamics Studies of Photochemical Processes

我们将使用最新的从头计算分子动力学(MD)技术来研究两个重要的光化学现象，即DNA碱鸟嘌呤的激发态互变异构化和偶氮苯的光异构化。用密度泛函限制开壳Kohn-Sham方法确定了S激发电子态的优化几何构型和振动频率。将分析不同互变异构体的简谐和非简谐振动光谱，以寻找光谱‘指纹’，从而有可能指定实验红外光谱。用最近发展起来的非绝热CAR-Parrinello MD(NA-CP-MD)方法研究了激发态质子转移(ESPT)对酮和烯醇异构体的相互转化。这种方法进一步允许研究S电子激发态到SO基态的无辐射衰变。溶剂化对不同互变异构体的相对稳定性和ESPT势垒高度的影响应通过CP-MD及其在周期体系中使用显式水溶剂分子的非绝热延伸来研究。此外，这种方法可以研究溶剂介导的质子转移路径，这可能与在气相中观察到的路径非常不同。应使用NA-CP-MD方法在气相和溶液中研究偶氮苯的顺-反和反-顺光异构化反应。为了揭示最近原子力显微镜(AFM)实验的机理，将进行约束动力学模拟来计算收缩力及其与跨顺式异构化过程中电子结构变化的相互作用。

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.