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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >A Theoretical Study of the Low-Lying States of the Anionic and Protonated Ionic Forms of Urocanic Acid
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A Theoretical Study of the Low-Lying States of the Anionic and Protonated Ionic Forms of Urocanic Acid

机译：尿酸的阴离子和质子化离子形式的低依赖态的理论研究

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摘要

A multistate second-order perturbation theory (MS-CASPT2) study of the lowest lying states in the electronic spectra of urocanic acid in vacuo is presented. The anionic trans and cis isomers, as well as the biologically important trans protonated ionic structure, are considered. The vertical and 0-0 excitation spectra were computed for each system at the MS-CASPT2/ANO-L level, describing the lowest lying #pi##pi#~* and n#pi#~* singlet and triplet states. In all three systems, a weakly absorbing #pi##pi#~* singlet state was observed at approx4.0 eV in the vertical excitation spectrum, suggesting both a novel assignment and an alternative explanation for the previously described wavelength dependent photochemistry of this molecule. The trans anion vertical spectrum otherwise comprises three intense #pi##pi#~* transitions, at 4.14, 4.59, and 5.00 eV, whereas that of the cis anion shows two, at 4.52 and 5.01 eV. Conversely, the vertical spectrum of the protonated trans system is dominated by a single, intensely absorbing #pi##pi#~* state, computed at 4.76 eV. The wave functions of the #pi##pi#~* states all show a multiconfigurational character, which is most pronounced in the protonated ioic structure. The lowest vertical singlet states are of n#pi#~* character (trans anion, 4.13 eV; cis anion, 3.81 eV; trans protonated, 3.19 eV), although the lowest triplet states are due to #pi##pi#~* transitions (trans anion, 3.40 eV; cis anion, 3.39 eV; trans protonated, 2.56 eV). In each of the anionic systems,the origin of the most intense singlet #pi##pi#~* transition in the vertical spectrum (trans anion, 4.03 eV; cis anion, 3.81 eV) lies below the corresponding lowest singlet n#pi#~* vertical excitation energy, further suggesting that the latter state may not be directly and efficiently populated. Good agreements with available experimental data are noted throughout.
机译：提出了一种多态二阶微扰理论（MS-CASPT2），用于研究在真空中尿烷酸电子光谱中的最低价态。考虑了阴离子反式和顺式异构体，以及生物学上重要的反式质子化离子结构。在MS-CASPT2 / ANO-L级别为每个系统计算了垂直和0-0激发光谱，描述了最低的＃pi ## pi＃〜*和n＃pi＃〜*单重态和三重态。在所有三个系统中，在垂直激发光谱中大约4.0 eV处观察到弱吸收的＃pi ## pi＃〜*单重态，这表明该分子先前依赖于波长的光化学具有新颖的归属和替代解释。否则，跨阴离子的垂直光谱包括三个强烈的＃pi ## pi＃〜*跃迁，分别为4.14、4.59和5.00 eV，而顺式阴离子的跃迁在4.52和5.01 eV处显示两个跃迁。相反，质子化反式系统的垂直光谱由一个以4.76 eV计算的强烈吸收的＃pi ## pi＃〜*状态控制。 ＃pi ## pi＃〜*状态的波动函数都显示出多构型特征，这在质子化的ioic结构中最为明显。尽管最低的三重态是由于＃pi ## pi＃〜*引起的，最低的垂直单重态是n＃pi＃〜*字符（反阴离子4.13 eV;顺式阴离子3.81 eV;反质子化的3.19 eV）。过渡（反式阴离子为3.40 eV;顺式阴离子为3.39 eV;反式质子化为2.56 eV）。在每个阴离子体系中，垂直光谱中最强的单重态＃pi ## pi＃〜*跃迁的起源（反式阴离子为4.03 eV;顺式阴离子为3.81 eV）低于相应的最低单线态n＃pi＃ *垂直激发能，进一步表明后者状态可能无法直接有效地填充。全文中都指出与可用实验数据的良好一致性。

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