Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state
Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state
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DOI：
10.1039/c9cp04556b
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发表时间：
2019-10-14
影响因子：
3.3
通讯作者：
Zwier, Timothy S.
中科院分区：
文献类型：
--
作者：
Blodgett, Karl N.;Sun, Dewei;Zwier, Timothy S.
在气相喷射冷却条件下，收集了邻氨基苯甲酸甲酯（MA，2-氨基苯甲酸甲酯）及其含水络合物（MA-H2O）的激光诱导荧光（LIF）激发、分散荧光（DFL）、UV-UV-空穴燃烧和UV耗尽光谱。作为防晒剂的结构类似物，MA 由于在 UV-A 区域开始的 S-0-S-1 跃迁而具有强吸收，电子原点位于 28 852 cm(-1) (346.6 nm)。与大多数具有快速非辐射路径返回基态的防晒霜不同，MA 可以有效地发出荧光，激发态寿命为 27 ns。相对于苯甲酸甲酯，到三重态流形的系统间穿越在 MA 中被强分子内 NH 中心点 O=C H 键关闭，这将 (3)n pi* 态移至远高于 (1)pi pi* S-1 态。单电子振动能级 DFL 谱用于获得激发态电子振动结构的近乎完整的分配。激发光谱中的大部分振动结构是 Franck-Condon 活性，这是由于三种面内振动调节 NH2 和 CO2Me 基团之间的距离：nu(33) (421 cm(-1))、nu(34) (366 cm(-1)) 和 nu(36) (179 cm(-1))。基于TD-DFT B3LYP-D3BJ/def2TZVP理论水平上实验与理论的密切对应，评估了与电子激发相关的主要结构变化，得出主要运动是由分子内NH中心点O=C H键封闭的6元H键环的重新定向和收缩的结论。这导致 NH 中心点 O=C H 键距离从 1.926 埃缩短至 1.723 埃，相当于与全 H 原子转移相比，HMIDLINE HORIZONTAL ELLIPSISO 距离减少约 25%。因此，S-1原点附近的激发态过程是主要由重原子运动引起的氢原子位错，因为广泛的重原子运动导致氢键距离缩短，NH键长度相对于其基态值仅增加了0.03埃。
Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-H2O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S-0-S-1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm(-1) (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NH center dot center dot center dot O=C H-bond, which shifts the (3)n pi* state well above the (1)pi pi* S-1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck-Condon activity due to three in-plane vibrations that modulate the distance between the NH2 and CO2Me groups, nu(33) (421 cm(-1)), nu(34) (366 cm(-1)), and nu(36) (179 cm(-1)). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NH center dot center dot center dot O=C H-bond. This leads to a shortening of the NH center dot center dot center dot O=C H-bond distance from 1.926 angstrom to 1.723 angstrom, equivalent to about a 25% reduction in the HMIDLINE HORIZONTAL ELLIPSISO distance compared to full H-atom transfer. As a result, the excited state process near the S-1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 angstrom increase in the NH bond length relative to its ground state value.