School of Physics Engineering, Qufu Normal University, Qufu 273165, China.
School of Physics and Optoelectronics Engineering, Ludong University, Yantai 264025, China.
Molecules. 2023 Mar 1;28(5):2286. doi: 10.3390/molecules28052286.
In this contribution, we report a computational study of the vibrational Resonance Raman (vRR) spectra of cytosine in water, on the grounds of potential energy surfaces (PES) computed by time-dependent density functional theory (TD-DFT) and CAM-B3LYP and PBE0 functionals. Cytosine is interesting because it is characterized by several close-lying and coupled electronic states, challenging the approach commonly used to compute the vRR for systems where the excitation frequency is in quasi-resonance with a single state. We adopt two recently developed time-dependent approaches, based either on quantum dynamical numerical propagations of vibronic wavepackets on coupled PES or on analytical correlation functions for cases in which inter-state couplings were neglected. In this way, we compute the vRR spectra, considering the quasi-resonance with the eight lowest-energy excited states, disentangling the role of their inter-state couplings from the mere interference of their different contributions to the transition polarizability. We show that these effects are only moderate in the excitation energy range explored by experiments, where the spectral patterns can be rationalized from the simple analysis of displacements of the equilibrium positions along the different states. Conversely, at higher energies, interference and inter-state couplings play a major role, and the adoption of a fully non-adiabatic approach is strongly recommended. We also investigate the effect of specific solute-solvent interactions on the vRR spectra, by considering a cluster of cytosine, hydrogen-bonded by six water molecules, and embedded in a polarizable continuum. We show that their inclusion remarkably improves the agreement with the experiments, mainly altering the composition of the normal modes, in terms of internal valence coordinates. We also document cases, mostly for low-frequency modes, in which a cluster model is not sufficient, and more elaborate mixed quantum classical approaches, in explicit solvent models, need to be applied.
在这项工作中，我们基于含时密度泛函理论（TD-DFT）和 CAM-B3LYP 及 PBE0 泛函计算的势能面（PES），对水合胞嘧啶的振动共振拉曼（vRR）光谱进行了计算研究。由于激发频率与单个态准共振，因此需要采用一种通常的方法来计算 vRR，而胞嘧啶具有多个近邻且耦合的电子态，这对该方法构成了挑战。我们采用了两种最近开发的含时方法，一种基于耦合 PES 上的振子波包量子动力学数值传播，另一种基于忽略态间耦合的情况下的解析相关函数。通过这种方式，我们计算了 vRR 光谱，考虑了与八个最低能量激发态的准共振，从单纯的干涉不同贡献到跃迁极化率的不同状态中，分解了它们之间的相互作用。我们表明，在实验探索的激发能范围内，这些效应仅具有中等强度，并且可以从不同状态下平衡位置的简单位移分析来合理地解释光谱模式。相反，在更高的能量下，干涉和态间耦合起着主要作用，因此强烈推荐采用完全非绝热方法。我们还通过考虑氢键连接 6 个水分子的胞嘧啶簇，并将其嵌入极化连续体中，研究了特定的溶质-溶剂相互作用对 vRR 光谱的影响。我们表明，其包含可显著提高与实验的一致性，主要改变了内部价坐标的正则模式的组成。我们还记录了一些情况，主要是低频模式，对于这些情况，簇模型是不够的，需要应用更精细的混合量子经典方法，在显式溶剂模型中。
