Chiariello Maria Gabriella, Rega Nadia
Dipartimento di Scienze Chimiche , Università di Napoli Federico II, Complesso Universitario di M.S.Angelo , via Cintia , I-80126 Napoli , Italy.
Interdisciplinary Research Centre on Biomaterials (CRIB) Università di Napoli Federico II , Piazzale Tecchio 80 , I-80125 , Napoli , Italy.
J Phys Chem A. 2018 Mar 22;122(11):2884-2893. doi: 10.1021/acs.jpca.7b12371. Epub 2018 Mar 9.
Advances in time-resolved vibrational spectroscopy techniques provided a new stimulus for understanding the transient molecular dynamics triggered by the electronic excitation. The detailed interpretation of such time-dependent spectroscopic signals is a challenging task from both experimental and theoretical points of view. We simulated and analyzed the transient photorelaxation of the pyranine photoacid in aqueous solution, with special focus on structural parameters and low frequency skeleton modes that are possibly preparatory for the photoreaction occurring at later time, as suggested by experimental spectroscopic studies. To this aim, we adopted an accurate computational protocol that combines excited state ab initio molecular dynamics within an hybrid quantum mechanical/molecular mechanics framework and a time-resolved vibrational analysis based on the Wavelet transform. According to our results, the main nuclear relaxation on the excited potential energy surface is completed in about 500 fs, in agreement with experimental data. The rearrangement of C-C bonds occurs according to a complex vibrational dynamics, showing oscillatory patterns that are out of phase and modulated by modes below 200 cm. We also analyzed in both the ground and the excited state the evolution of some structural parameters involved in excited state proton transfer reaction, namely, those involving the pyranine and the water molecule hydrogen bonded to the phenolic O-H group. Both the hydrogen bond distance and the intermolecular orientation are optimized in the excited state, resulting in a tighter proton donor-acceptor couple. Indeed, we found evidence that collective low frequency skeleton modes, such as the out of plane wagging at 108 cm and the deformation at 280 cm, are photoactivated by the ultrafast part of the relaxation and modulate the pyranine-water molecule rearrangement, favoring the preparatory step for the photoreactivity.
时间分辨振动光谱技术的进展为理解由电子激发引发的瞬态分子动力学提供了新的推动力。从实验和理论的角度来看，对这种随时间变化的光谱信号进行详细解释是一项具有挑战性的任务。我们模拟并分析了水溶液中吡喃鎓光酸的瞬态光弛豫，特别关注结构参数和低频骨架模式，正如实验光谱研究所表明的，这些参数和模式可能为稍后发生的光反应做准备。为此，我们采用了一种精确的计算方法，该方法结合了混合量子力学/分子力学框架内的激发态从头算分子动力学和基于小波变换的时间分辨振动分析。根据我们的结果，激发态势能面上的主要核弛豫在约500飞秒内完成，这与实验数据一致。C-C键的重排按照复杂的振动动力学发生，呈现出异相的振荡模式，并由低于200厘米-1的模式调制。我们还在基态和激发态下分析了激发态质子转移反应中涉及的一些结构参数的演变，即那些涉及吡喃鎓和与酚羟基氢键相连的水分子的参数。氢键距离和分子间取向在激发态下都得到了优化，形成了更紧密的质子供体-受体对。事实上，我们发现证据表明，集体低频骨架模式，如108厘米-1处的面外摆动和280厘米-1处的变形，在弛豫的超快部分被光激活，并调节吡喃鎓-水分子的重排，有利于光反应性的准备步骤。
