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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Computer Simulation of the Excited State Dynamics of Betaine-30 in Acetonitrile
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Computer Simulation of the Excited State Dynamics of Betaine-30 in Acetonitrile

机译：甜菜碱30在乙腈中激发态动力学的计算机模拟。

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摘要

Time-dependent studies of the excited state dynamics of betaine-30 in acetonitrile at room temperature have been carried out using a mixed classical/quantum molecular dynamics simulation methodology. The #pi#-electron system of the solute molecule is treated quantum mechanically using the semiempirical Pariser-Parr-Pople Hamiltonian, including the solvent influence on electronic structure. The remaining interactions are treated via empirical potentials. Transition probabilities between adiabatic electronic states are evaluated using surface hopping methods, including all nuclear degrees of freedom in the coupling. The dynamics treats the (rigid) solvent and the dihedral angles for relative rotation of rings of an otherwise rigid solute classically. The contribution of all remaining solute intramolecular vibrations is included in the nonadiabatic coupling via an approximate, but purely quantum mechanical, treatment. Analysis of the dynamics reveals that, after excitation to the first excited state, the energy gap between ground and first excited states of the molecule exhibits an ultrafast (approx 100 fs) decrease due to the inertial response of the solvent that accounts for about 70% of the solvent response, followed immediately by a further subpicosecond solvent component. The times and amplitudes of these solvation components are in accord with the results inferred from resonance Raman spectra, and the solvent contribution to the Stokes shift observed is in accord with values inferred from ground state absorption spectral line shape analysis. However, we also find that the energy gap exhibits a slower picosecond time scale response of comparable magnitude due to relative rotation of the central phenolate and pyridinium rings. This relaxation has not been previously noted or incorporated in corresponding electron transfer models. Analysis of contributions to the electronic nonadiabatic coupling shows that this is dominated by a small set of high-frequency intramolecular modes of the betaine-30 molecule, with the solvent making a relatively very small contribution, also in agreement with previous experimental inference.
机译：使用混合经典/量子分子动力学模拟方法对室温下甜菜碱30在乙腈中的激发态动力学进行时变研究。使用半经验的Pariser-Parr-Pople哈密顿量对溶质分子的＃pi＃-电子体系进行量子力学处理，包括溶剂对电子结构的影响。剩余的相互作用通过经验潜力进行处理。绝热电子态之间的跃迁概率是使用表面跳变方法评估的，包括耦合中的所有核自由度。动力学经典地处理（刚性）溶剂和二面角，以使原本为刚性的溶质的环相对旋转。通过近似但纯粹的量子力学处理，所有剩余的溶质分子内振动的贡献都包括在非绝热耦合中。动力学分析表明，激发到第一个激发态后，由于溶剂的惯性响应，分子的基态和第一个激发态之间的能隙呈现出超快的下降（约100 fs），这大约占溶剂的70％溶剂响应后，立即加入亚皮秒级的溶剂。这些溶剂化组分的时间和幅度与从共振拉曼光谱推断的结果一致，并且溶剂对观察到的斯托克斯位移的贡献与从基态吸收光谱线形分析推断的值一致。但是，我们还发现，由于中心酚盐和吡啶鎓环的相对旋转，能隙表现出了相当数量级的较慢的皮秒时间尺度响应。先前尚未注意到这种弛豫，也未将其纳入相应的电子转移模型。对电子非绝热偶合的贡献的分析表明，这主要由少量的甜菜碱30分子的高频分子内模式决定，溶剂的贡献相对很小，这也与先前的实验推论一致。

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