Instituto de Física, Universidade Federal de Alagoas, 57072-970, Maceió, AL, Brazil.
Phys Chem Chem Phys. 2010 Nov 14;12(42):14023-33. doi: 10.1039/c0cp00122h. Epub 2010 Sep 20.
The solvent effects on the low-lying absorption spectrum and on the (15)N chemical shielding of pyrimidine in water are calculated using the combined and sequential Monte Carlo simulation and quantum mechanical calculations. Special attention is devoted to the solute polarization. This is included by an iterative procedure previously developed where the solute is electrostatically equilibrated with the solvent. In addition, we verify the simple yet unexplored alternative of combining the polarizable continuum model (PCM) and the hybrid QM/MM method. We use PCM to obtain the average solute polarization and include this in the MM part of the sequential QM/MM methodology, PCM-MM/QM. These procedures are compared and further used in the discrete and the explicit solvent models. The use of the PCM polarization implemented in the MM part seems to generate a very good description of the average solute polarization leading to very good results for the n-π* excitation energy and the (15)N nuclear chemical shield of pyrimidine in aqueous environment. The best results obtained here using the solute pyrimidine surrounded by 28 explicit water molecules embedded in the electrostatic field of the remaining 472 molecules give the statistically converged values for the low lying n-π* absorption transition in water of 36 900 ± 100 (PCM polarization) and 36 950 ± 100 cm(-1) (iterative polarization), in excellent agreement among one another and with the experimental value observed with a band maximum at 36 900 cm(-1). For the nuclear shielding (15)N the corresponding gas-water chemical shift obtained using the solute pyrimidine surrounded by 9 explicit water molecules embedded in the electrostatic field of the remaining 491 molecules give the statistically converged values of 24.4 ± 0.8 and 28.5 ± 0.8 ppm, compared with the inferred experimental value of 19 ± 2 ppm. Considering the simplicity of the PCM over the iterative polarization this is an important aspect and the computational savings point to the possibility of dealing with larger solute molecules. This PCM-MM/QM approach reconciles the simplicity of the PCM model with the reliability of the combined QM/MM approaches.
采用组合和顺序蒙特卡罗模拟和量子力学计算方法，计算了溶剂对嘧啶在水中的低能吸收光谱和（15）N 化学位移的影响。特别关注了溶剂化效应。通过先前开发的迭代程序包括在内，其中通过静电平衡使溶剂与溶质平衡。此外，我们验证了一种简单但尚未探索的替代方法，即将极化连续体模型（PCM）与混合 QM/MM 方法相结合。我们使用 PCM 获得平均溶质极化，并将其包含在顺序 QM/MM 方法的 MM 部分中，PCM-MM/QM。比较并进一步使用离散和显式溶剂模型。在 MM 部分中使用 PCM 极化似乎可以很好地描述平均溶质极化，从而使嘧啶在水溶液中的 n-π激发能和（15）N 核化学位移得到很好的结果。使用周围有 28 个显式水分子的溶质嘧啶嵌入其余 472 个分子的静电场中，获得了水中低能 n-π吸收跃迁的统计收敛值，分别为 36900±100（PCM 极化）和 36950±100cm-1（迭代极化），彼此之间以及与实验值（带最大处为 36900cm-1）非常吻合。对于核屏蔽（15）N，使用周围有 9 个显式水分子的溶质嘧啶嵌入其余 491 个分子的静电场中，得到统计收敛值分别为 24.4±0.8 和 28.5±0.8ppm，与推断的实验值 19±2ppm 相比。考虑到 PCM 相对于迭代极化的简单性，这是一个重要方面，计算节省了处理更大的溶质分子的可能性。这种 PCM-MM/QM 方法将 PCM 模型的简单性与组合 QM/MM 方法的可靠性结合起来。
