Ribeiro Raphael F, Marenich Aleksandr V, Cramer Christopher J, Truhlar Donald G
Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431.
J Chem Theory Comput. 2009 Sep 8;5(9):2284-300. doi: 10.1021/ct900258f.
Although continuum solvation models have now been shown to provide good quantitative accuracy for calculating free energies of solvation, questions remain about the accuracy of the perturbed solute electron densities and properties computed from them. Here we examine those questions by applying the SM8, SM8AD, SMD, and IEF-PCM continuum solvation models in combination with the M06-L density functional to compute the (14)N magnetic resonance nuclear shieldings of CH3CN, CH3NO2, CH3NCS, and CH3ONO2 in multiple solvents, and we analyze the dependence of the chemical shifts on solvent dielectric constant. We examine the dependence of the computed chemical shifts on the definition of the molecular cavity (both united-atom models and models based on superposed individual atomic spheres) and three kinds of treatments of the electrostatics, namely the generalized Born approximation with the Coulomb field approximation, the generalized Born model with asymmetric descreening, and models based on approximate numerical solution schemes for the nonhomogeneous Poisson equation. Our most systematic analyses are based on the computation of relative (14)N chemical shifts in a series of solvents, and we compare calculated shielding constants relative to those in CCl4 for various solvation models and density functionals. While differences in the overall results are found to be reasonably small for different solvation models and functionals, the SMx models SM8, and SM8AD, using the same cavity definitions (which for these models means the same atomic radii) as those employed for the calculation of free energies of solvation, exhibit the best agreement with experiment for every functional tested. This suggests that in addition to predicting accurate free energies of solvation, the SM8 and SM8AD generalized Born models also describe the solute polarization in a manner reasonably consistent with experimental (14)N nuclear magnetic resonance spectroscopy. Models based on the nonhomogeneous Poisson equation show slightly reduced accuracy. Scaling the intrinsic Coulomb radii to larger values (as has sometimes been suggested in the past) does not uniformly improve the results for any kind of solvent model; furthermore it uniformly degrades the results for generalized Born models. Use of a basis set that increases the outlying charge diminishes the accuracy of continuum models that solve the nonhomogeneous Poisson equation, which we ascribe to the inability of the numerical schemes for approximately solving the nonhomogeneous Poisson equation to fully account for the effects of electronic charge outside the solute cavity.
尽管连续介质溶剂化模型现已被证明在计算溶剂化自由能方面具有良好的定量准确性，但对于微扰溶质电子密度及其所计算出的性质的准确性仍存在疑问。在此，我们通过将SM8、SM8AD、SMD和IEF - PCM连续介质溶剂化模型与M06 - L密度泛函相结合，来计算CH3CN、CH3NO2、CH3NCS和CH3ONO2在多种溶剂中的(14)N磁共振核屏蔽，以此来研究这些问题，并分析化学位移对溶剂介电常数的依赖性。我们研究了计算所得化学位移对分子腔定义（包括联合原子模型和基于叠加单个原子球的模型）以及三种静电处理方式的依赖性，这三种静电处理方式分别是具有库仑场近似的广义玻恩近似、具有不对称去屏蔽的广义玻恩模型以及基于非均匀泊松方程近似数值解方案的模型。我们最系统的分析基于一系列溶剂中相对(14)N化学位移的计算，并将各种溶剂化模型和密度泛函相对于CCl4中的屏蔽常数计算结果进行比较。虽然不同溶剂化模型和泛函的总体结果差异被发现相对较小，但使用与计算溶剂化自由能时相同的腔定义（对于这些模型而言意味着相同的原子半径）的SMx模型SM8和SM8AD，对于所测试的每种泛函，与实验结果的一致性最佳。这表明，除了预测准确的溶剂化自由能外，SM8和SM8AD广义玻恩模型还以一种与实验(14)N核磁共振光谱合理一致的方式描述了溶质极化。基于非均匀泊松方程的模型显示出略低的准确性。将固有库仑半径按比例增大到更大的值（如过去有时所建议的那样）并不会对任何一种溶剂模型的结果都产生一致的改善；此外，它会一致地降低广义玻恩模型的结果。使用增加外围电荷的基集会降低求解非均匀泊松方程的连续介质模型的准确性，我们将其归因于近似求解非均匀泊松方程的数值方案无法充分考虑溶质腔外电子电荷的影响。
