Andrzejak Marcin, Zams Joanna, Goclon Jakub, Kolek Przemysław
K. Gumiński Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, 31-007 Kraków, Poland.
Doctoral School of Exact and Natural Sciences, Jagiellonian University, 31-007 Kraków, Poland.
Molecules. 2024 Nov 25;29(23):5562. doi: 10.3390/molecules29235562.
The main purpose of this study is to characterize the nature of the low-energy singlet excited states of the anthranilic acid homodimer (AA) and their changes (symmetry breaking) caused by deformation of the centrosymmetric, ground state structure of AA towards the geometry of the S state. We employ both the correlated ab initio methods (approximate Coupled Clusters Singles and Doubles-CC2 and CASSCF/NEVPT2) as well as the DFT/TDDFT calculations with two exchange-correlation functionals, i.e., B3LYP and CAM-B3LYP. The composition of the wavefunctions is investigated using the one-electron transition density matrix and difference density maps. We demonstrate that in the case of AA, small asymmetric distortions of geometry bring about unproportionally large changes in the excited state wavefunctions. We further provide comprehensive characterization of the AA electronic structure, showing that the excitation is nearly completely localized on one of the monomers, which stands in agreement with the experimental evidence. The excitation increases the π-electronic coupling of the substituents and the aromatic ring, but only in the excited monomer, while the changes in the electronic structure of the unexcited monomer are negligible (after geometry relaxation). The increased electronic density strengthens both intra- and intermolecular hydrogen bonds formed by the carbonyl oxygen atom of the excited monomer, making them significantly stronger than in the ground state. Although the overall pattern of changes remains qualitatively consistent across all methods employed, CC2 predicts more pronounced excitation-induced modifications of the electronic structure compared to the more routinely used TDDFT approach. The most important deficiency of the B3LYP functional in the present context is locating two charge-transfer states at erroneously low energies, in close proximity of the S and S states. The range-corrected CAM-B3LYP exchange-correlation functional gives a considerably improved description of the CT states at the price of overshot excitation energies.
本研究的主要目的是表征邻氨基苯甲酸同二聚体（AA）低能单重激发态的性质，以及由AA中心对称基态结构向S态几何形状变形所引起的变化（对称性破缺）。我们采用了相关的从头算方法（近似耦合簇单双激发-CC2和CASSCF/NEVPT2）以及使用两种交换相关泛函（即B3LYP和CAM-B3LYP）的DFT/TDDFT计算。使用单电子跃迁密度矩阵和差分密度图研究波函数的组成。我们证明，在AA的情况下，几何形状的小不对称畸变会导致激发态波函数发生不成比例的大变化。我们进一步对AA的电子结构进行了全面表征，表明激发几乎完全局限于其中一个单体上，这与实验证据一致。激发增加了取代基与芳环之间的π电子耦合，但仅在激发的单体中，而未激发单体的电子结构变化可忽略不计（几何弛豫后）。增加的电子密度增强了激发单体的羰基氧原子形成的分子内和分子间氢键，使其比基态时明显更强。尽管在所采用的所有方法中，变化的总体模式在定性上保持一致，但与更常规使用的TDDFT方法相比，CC2预测电子结构的激发诱导修饰更为明显。在当前背景下，B3LYP泛函最重要的缺陷是将两个电荷转移态定位在错误的低能量处，紧邻S和S态。范围校正的CAM-B3LYP交换相关泛函以激发能过高为代价，对CT态给出了显著改进的描述。
