Kolek Przemysław, Andrzejak Marcin, Uchacz Tomasz, Goclon Jakub, Pogocki Dariusz, Kisała Joanna, Bankiewicz Barbara, Szlachcic Paweł, Tulej Marek
Institute of Physics, University of Rzeszów, 1 Pigonia Street, 35-310 Rzeszów, Poland.
K. Gumiński Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland.
Spectrochim Acta A Mol Biomol Spectrosc. 2024 Oct 15;319:124491. doi: 10.1016/j.saa.2024.124491. Epub 2024 May 21.
This study aims to investigate the impact of the π → π* excitation localised in one monomer on the equilibrium geometry and oscillations of the AA dimer. Several low-frequency vibrations appear in pairs in the LIF spectrum because oscillations involving intermolecular hydrogen bonds are coupled, generating approximately symmetric and antisymmetric combinations (especially the COOH rocking modes, LIF: 295 and 301 cm). Furthermore, quantitative evaluation based on the TDDFT(B3LYP) results indicates that a dozen among 90 intramolecular oscillations are strongly coupled. In contrast, most vibrations are decoupled or weakly coupled, since they involve remote parts of the monomers. This makes several single vibrations active in the LIF spectrum (including the bending mode of the NH···O intramolecular hydrogen bond associated the strongest vibronic band 442 cm), while the other in each pair remains inactive. The reason for decoupling of oscillations and symmetry breaking is that the π → π* electronic excitation is entirely localised within one of the monomers, which makes them no longer equivalent in terms of geometry and dynamics. Additionally, the excitation of one monomer induces strengthening and shortening by 6 pm of only one intermolecular hydrogen bond linking the carboxylic groups of both molecules. This causes the 1.7° in-plane distortion of the dimer and lowering of its symmetry to C group (from C for the S state). The distortion induces the activity of two low-frequency in-plane intermolecular vibrations, i.e. the geared oscillation (LIF: 58 cm) and the shearing motion (99 cm) of the monomers.
本研究旨在探究局限于一个单体中的π→π激发对AA二聚体平衡几何结构和振动的影响。在激光诱导荧光（LIF）光谱中，几个低频振动成对出现，这是因为涉及分子间氢键的振动相互耦合，产生了近似对称和反对称的组合（特别是COOH摇摆模式，LIF：295和301厘米）。此外，基于含时密度泛函理论（TDDFT，B3LYP）结果的定量评估表明，90种分子内振动中有12种强烈耦合。相比之下，大多数振动是解耦的或弱耦合的，因为它们涉及单体的远端部分。这使得LIF光谱中几种单一振动活跃（包括与最强振动带442厘米相关的NH···O分子内氢键的弯曲模式），而每对中的另一个振动保持不活跃。振动解耦和对称性破缺的原因是π→π电子激发完全局限于其中一个单体中，这使得它们在几何结构和动力学方面不再等效。此外，一个单体的激发仅导致连接两个分子羧基的一个分子间氢键加强并缩短6皮米。这导致二聚体在平面内发生1.7°的扭曲，并使其对称性降低到C群（从S态的C2群）。这种扭曲引发了两种低频平面内分子间振动的活性，即单体的齿轮状振荡（LIF：58厘米）和剪切运动（99厘米）。
