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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Ground-State Tautomerism and Excited-State Proton-Transfer Processes in 4, 5-Dimethyl-2-(2'-hydroxyphenyl)imidazole in Solution: Fluorescence Spectroscopy and Quantum Mechanical Calculations
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Ground-State Tautomerism and Excited-State Proton-Transfer Processes in 4, 5-Dimethyl-2-(2'-hydroxyphenyl)imidazole in Solution: Fluorescence Spectroscopy and Quantum Mechanical Calculations

机译：4，5-二甲基-2-（2'-羟基苯基）咪唑在溶液中的基态互变异构和激发态质子转移过程：荧光光谱和量子力学计算

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摘要

Ground-state tautomerism and excited-state proton transfer of 4, 5-dimethyl-2-(2'-hydroxyphenyl)imidazole (DMHI) was investigated by means of UV-vis absorption spectroscopy, by steady-state and time-resolved fluorescence spectroscopy, and by quantum mechanical calculations. The behavior of DMHI in ethanol and water was studied in neutral, acidic, and basic conditions. Three ground-state species were detected in neutral solutions of DMHI: the cis-enol form with an intramolecular hydrogen bond, the trans-enol form that is hydrogen bonded to the solvent, and the keto tautomer. The relative proportions of these species depend strongly on the solvent: the keto tautomer was not detected in ethanol, whereas this tautomer is the predominant species in neutral water, where negligible amounts of the cis-enol form exist. These experimental findings could be reproduced by quantum mechanical calculations up to the B3LYP/6-311+G~(**) level of theory combined with semiempirical calculations of solvation free energies, which allowed the energy diagram in the gas phase and in solution to be known. A combination of the supermolecular approach and the SM_X methodology was necessary to account for the differential solvation of the different isomers in neutral aqueous solution. In the excited state, the cis-enol form always undergoes the fast excited-state intramolecular proton transfer to yield the keto tautomer. Upon photoexcitation of the trans-enol form and the keto tautomer, no excited-state reaction takes place. In acidic media, the photoexcited cation suffers deprotonation of the hydroxyl group, yielding the excited keto tautomer.
机译：通过紫外可见吸收光谱，稳态和时间分辨荧光光谱研究了4，5-二甲基-2-（2'-羟基苯基）咪唑（DMHI）的基态互变异构和激发态质子转移，并通过量子力学计算。在中性，酸性和碱性条件下研究了DMHI在乙醇和水中的行为。在DMHI的中性溶液中检测到三种基态物质：具有分子内氢键的顺式-烯醇形式，与溶剂氢键合的反式-烯醇形式和酮互变异构体。这些物种的相对比例在很大程度上取决于溶剂：在乙醇中未检测到酮互变异构体，而该互变异构体是中性水中的主要物种，那里存在少量的顺式-烯醇形式。这些实验结果可以通过理论上达到B3LYP / 6-311 + G〜（**）水平的量子力学计算与溶剂化自由能的半经验计算相结合，从而允许气相和溶液中的能量图被知道。必须结合使用超分子方法和SM_X方法，以解决中性水溶液中不同异构体的差异溶剂化问题。在激发态下，顺式-烯醇形式总是经历快速激发态的分子内质子转移，从而生成酮互变异构体。在反式-烯醇形式和酮互变异构体的光激发下，没有发生激发态反应。在酸性介质中，光激发的阳离子遭受羟基的去质子作用，产生了激动的酮互变异构体。

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