Kostjukov Victor V
Physics Department, Sevastopol State University, Universitetskaya St., 33, Sevastopol, 299053, Crimea.
J Mol Model. 2022 Mar 26;28(4):103. doi: 10.1007/s00894-022-05098-8.
The main goal of this work was the theoretical interpretation of the absorption spectra of neutral red in an aqueous solution (both neutral NR and protonated NR forms). To achieve this problem, TD-DFT/DFT calculations with different hybrid functionals, the IEFPCM solvent model, and the 6-31 + + G(d,p) basis set were used. MN12SX functional provided the best agreement with the experiment for both dye forms. While the absorption band of the cationic form of the dye in the visible region of the spectrum is due to one transition S → S (HOMO-LUMO), for its neutral form, there are two transitions S → S (HOMO → LUMO) and S → S (HOMO-1 → LUMO), with the latter having a higher intensity. The protonation of the dye chromophore introduces significant changes in HOMO shape. At the same time, LUMOs are almost the same for the protonated and neutral forms of the NR. During the transition from NR to the S state, its dipole moment increases more significantly than during the transition to the S state. Calculations confirmed the assumption of Singh et al. about the existence of two closely spaced excited states of NR, the first of which has a larger dipole moment. However, the hypothesis of these authors about the corresponding intramolecular charge transfer, as well as the huge value of the dipole moment of this excited state (~ 20 D) declared by them, was not confirmed by present calculations. It was shown that the photoinduced charge redistribution in both the neutral and cationic forms of the dye is local, and the corresponding dipole moment is ~ 10 D. This agrees with other early theoretical work by Aaron et al. The influence on the NR absorption spectrum of hydrating water molecules was also analyzed. It was found that the interplay of electrostatic and site-specific contributions leads to the fact that NR solvatochromism does not have a pronounced dependence on the polarity of the solvent.
这项工作的主要目标是对中性红在水溶液中的吸收光谱（包括中性NR和质子化NR形式）进行理论解释。为解决这个问题，使用了具有不同杂化泛函、IEFPCM溶剂模型和6-31++G(d,p)基组的TD-DFT/DFT计算。MN12SX泛函对两种染料形式都与实验结果吻合得最好。虽然染料阳离子形式在光谱可见光区域的吸收带归因于一个S→S（HOMO-LUMO）跃迁，但对于其中性形式，存在两个S→S（HOMO→LUMO）和S→S（HOMO-1→LUMO）跃迁，后者强度更高。染料发色团的质子化使HOMO形状发生显著变化。同时，NR的质子化形式和中性形式的LUMO几乎相同。从NR跃迁到S态时，其偶极矩的增加比跃迁到S态时更显著。计算结果证实了Singh等人关于NR存在两个紧密相邻的激发态的假设，其中第一个激发态的偶极矩更大。然而，这些作者关于相应分子内电荷转移的假设以及他们宣称的这个激发态的巨大偶极矩值（约20 D）未得到当前计算的证实。结果表明，染料的中性和阳离子形式中的光致电荷重新分布都是局部的，相应的偶极矩约为10 D。这与Aaron等人的其他早期理论工作一致。还分析了水合水分子对NR吸收光谱的影响。发现静电作用和位点特异性作用的相互影响导致NR溶剂化显色对溶剂极性没有明显依赖性。
