Reis H, Papadopoulos M G, Grzybowski A
Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, Vasileos Constantinou 48, GR-11635 Athens, Greece.
J Phys Chem B. 2006 Sep 21;110(37):18537-52. doi: 10.1021/jp063529c.
This is the second part of a study to elucidate the local field effects on the nonlinear optical properties of p-nitroaniline (pNA) in three solvents of different multipolar character, that is, cyclohexane (CH), 1,4-dioxane (DI), and tetrahydrofuran (THF), employing a discrete description of the solutions. By the use of liquid structure information from molecular dynamics simulations and molecular properties computed by high-level ab initio methods, the local field and local field gradients on p-nitroaniline and the solvent molecules are computed in quadrupolar approximation. To validate the simulations and the induction model, static and dynamic (non)linear properties of the pure solvents are also computed. With the exception of the static dielectric constant of pure THF, a good agreement between computed and experimental refractive indices, dielectric constants, and third harmonic generation signals is obtained for the solvents. For the solutions, it is found that multipole moments up to two orders higher than quadrupole have a negligible influence on the local fields on pNA, if a simple distribution model is employed for the electric properties of pNA. Quadrupole effects are found to be nonnegligible in all three solvents but are especially pronounced in the 1,4-dioxane solvent, in which the local fields are similar to those in THF, although the dielectric constant of DI is 2.2 and that of the simulated THF is 5.4. The electric-field-induced second harmonic generation (EFISH) signal and the hyper-Rayleigh scattering signal of pNA in the solutions computed with the local field are in good to fair agreement with available experimental results. This confirms the effect of the "dioxane anomaly" also on nonlinear optical properties. Predictions based on an ellipsoidal Onsager model as applied by experimentalists are in very good agreement with the discrete model predictions. This is in contrast to a recent discrete reaction field calculation of pNA in 1,4-dioxane, which found that the predicted first hyperpolarizability of pNA deviated strongly from the predictions obtained using Onsager-Lorentz local field factors.
这是一项研究的第二部分，该研究采用溶液的离散描述方法，来阐明在三种具有不同多极特性的溶剂（即环己烷（CH）、1,4 - 二氧六环（DI）和四氢呋喃（THF））中，局部场对对硝基苯胺（pNA）非线性光学性质的影响。通过使用分子动力学模拟得到的液体结构信息以及由高水平从头算方法计算得到的分子性质，在四极近似下计算了对硝基苯胺和溶剂分子上的局部场以及局部场梯度。为了验证模拟和诱导模型，还计算了纯溶剂的静态和动态（非）线性性质。除了纯THF的静态介电常数外，对于这些溶剂，计算得到的折射率、介电常数和三次谐波产生信号与实验结果之间取得了良好的一致性。对于溶液，发现如果采用简单的pNA电性质分布模型，高达四极以上两个量级的多极矩对对硝基苯胺上的局部场影响可忽略不计。在所有三种溶剂中都发现四极效应不可忽略，尤其在1,4 - 二氧六环溶剂中更为显著，其中局部场与THF中的相似，尽管DI的介电常数为2.2，模拟的THF的介电常数为5.4。用局部场计算得到的溶液中pNA的电场诱导二次谐波产生（EFISH）信号和超瑞利散射信号与现有实验结果吻合良好到较好。这证实了“二氧六环异常”对非线性光学性质也有影响。实验人员应用的基于椭球Onsager模型的预测与离散模型预测非常吻合。这与最近对1,4 - 二氧六环中pNA的离散反应场计算结果形成对比，后者发现预测的pNA第一超极化率与使用Onsager - Lorentz局部场因子得到的预测结果有很大偏差。
