Instituto de Física, Universidade Federal de Alagoas, 57072-970 Maceió, AL, Brazil.
J Chem Phys. 2011 Oct 14;135(14):144103. doi: 10.1063/1.3644894.
A combination of the polarizable continuum model (PCM) and the hybrid quantum mechanics/molecular mechanics (QM/MM) methodology, PCM-MM/QM, is used to include the solute electronic polarization and then study the solvent effects on the low-lying n→π(∗) excitation energy and the (15)N nuclear magnetic shielding of pyrazine and pyridazine in aqueous environment. The results obtained with PCM-MM/QM are compared with two other procedures, i.e., the conventional PCM and the iterative and sequential QM/MM (I-QM/MM). The QM calculations are made using density functional theory in the three procedures. For the excitation energies, the time-dependent B3LYP/6-311+G(d) model is used. For the magnetic shielding, the B3LYP/aug-pcS2(N)/pcS2(C,O,H) is used with the gauge-including atomic orbitals. In both cases, i.e., PCM-MM/QM and I-QM/MM, that use a discrete model of the solvent, the solute is surrounded by a first shell of explicit water molecules embedded by an electrostatic field of point charges for the outer shells. The best results are obtained including 28 explicit water molecules for the spectral calculations and 9 explicit water molecules for the magnetic shielding. Using the PCM-MM/QM methodology the results for the n→π(∗) excitation energies of pyridazine and pyrazine are 32,070 ± 80 cm(-1) and 32,675 ± 60 cm(-1), respectively, in good agreement with the corresponding I-MM/QM results of 32,540 ± 80 cm(-1) and 32,710 ± 60 cm(-1) and the experimental results of 33,450-33,580 cm(-1) and 32,700-33,300 cm(-1). For the (15)N magnetic shielding, the corresponding numbers for the gas-water shifts obtained with PCM-MM/QM are 47.4 ± 1.3 ppm for pyridazine and 19.7 ± 1.1 ppm for pyrazine, compared with the I-QM/MM values of 53.4 ± 1.3 ppm and 19.5 ± 1.2 ppm and the experimental results of 42-54 ppm and 17-22 ppm, respectively. The agreement between the two procedures is found to be very good and both are in agreement with the experimental values. PCM-MM/QM approach gives a good solute polarization and could be considered in obtaining reliable results within the expected QM/MM accuracy. With this electronic polarization, the solvent effects on the electronic absorption spectra and the (15)N magnetic shielding of the diazines in water are well described by using only an electrostatic approximation. Finally, it is remarked that the experimental and theoretical results suggest that the (15)N nuclear magnetic shielding of any diazine has a clear dependence with the solvent polarity but not directly with the solute-solvent hydrogen bonds.
采用极化连续模型（PCM）和混合量子力学/分子力学（QM/MM）方法的组合，即 PCM-MM/QM，用于包括溶质的电子极化，然后研究溶剂对吡嗪和哒嗪的低能 n→π(）激发能和（15）N 核磁共振屏蔽的影响在水环境中。与两种其他程序，即常规 PCM 和迭代和顺序 QM/MM（I-QM/MM）相比，使用 PCM-MM/QM 获得的结果。在这三种程序中，使用密度泛函理论进行 QM 计算。对于激发能，使用时间相关的 B3LYP/6-311+G(d)模型。对于磁屏蔽，使用 B3LYP/aug-pcS2（N）/pcS2（C，O，H），带有包含原子轨道的规范。在这两种情况下，即 PCM-MM/QM 和 I-QM/MM，使用溶剂的离散模型，溶质被包围在由静电点电荷场嵌入的第一壳层的显式水分子中。对于光谱计算，最好的结果是使用 28 个显式水分子，对于磁屏蔽，最好的结果是使用 9 个显式水分子。使用 PCM-MM/QM 方法，吡嗪和哒嗪的 n→π(）激发能的结果分别为 32,070 ± 80 cm(-1)和 32,675 ± 60 cm(-1)，与相应的 I-MM/QM 结果 32,540 ± 80 cm(-1)和 32,710 ± 60 cm(-1)和实验结果 33,450-33,580 cm(-1)和 32,700-33,300 cm(-1)一致。对于（15）N 磁屏蔽，使用 PCM-MM/QM 获得的气-水位移的相应数值为 47.4 ± 1.3 ppm 用于吡嗪和 19.7 ± 1.1 ppm 用于哒嗪，与 I-QM/MM 值 53.4 ± 1.3 ppm 和 19.5 ± 1.2 ppm 以及实验结果 42-54 ppm 和 17-22 ppm 相比。两种方法之间的一致性被发现非常好，并且都与实验值一致。PCM-MM/QM 方法可以很好地极化溶质，并且可以在预期的 QM/MM 精度内获得可靠的结果。使用这种电子极化，仅使用静电近似就可以很好地描述二嗪在水中的电子吸收光谱和（15）N 磁共振屏蔽的溶剂效应。最后，值得注意的是，实验和理论结果表明，任何二嗪的（15）N 核磁共振屏蔽具有与溶剂极性的明显依赖性，但与溶质-溶剂氢键没有直接关系。
