稀有气体低温基质中胸腺嘧啶的构象结构、红外光谱和光诱导转化

Photochem Pub Date : 2022-06-07 DOI:10.3390/photochem2020028
A. Lopes Jesus, Cláudio M. Nunes, I. Reva
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引用次数: 2

摘要

利用量子化学计算方法研究了天然产物百里香酚(2-异丙基-5-甲基苯酚)在B3LYP和MP2水平上的构象空间,发现存在四种不同取向的异丙基和羟基构象。在惰性气体(Ar和Xe)基质(15k)中分离到百里香酚单体,并用红外光谱对其进行了表征。在B3LYP谐波振动计算的支持下,在低温矩阵中确定了两个最稳定的反式oh构象,它们在异丙基取向上不同。两个不太稳定的顺- oh构象没有被检测到,因为它们将经历快速隧穿到最稳定的构象。在高达75 K的Xe基体中退火实验没有导致两个孤立构象之间的任何转换,这与计算出的大体积异丙基旋转化的显著能量势垒(~24 kJ mol−1)一致。在孤立构象的2ν(OH)频率下,宽带或窄带辐照促进的振动激发也没有导致任何转化,这可以解释为羟基旋转化比异丙基旋转化更有效的能量转移(与较低的能垒和轻的h原子有关)。宽带紫外辐射实验(λ > 200 nm)导致基质分离的百里香酚迅速转化,光谱证据表明形成异构烷基取代环己二烯酮,杜瓦异构体和开链共轭烯酮。光化学机理解释与报道的类似苯酚衍生物一致。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Conformational Structure, Infrared Spectra and Light-Induced Transformations of Thymol Isolated in Noble Gas Cryomatrices
The conformational space of the natural product thymol (2-isopropyl-5-methylphenol) was investigated using quantum chemical calculations at the B3LYP and MP2 levels, which revealed the existence of four types of conformers differing in the orientation of the isopropyl and hydroxyl groups. Thymol monomers were isolated in noble gas (Ar and Xe) matrices (at 15 K) and characterized by IR spectroscopy. With the support of B3LYP harmonic vibrational calculations, the two most stable trans-OH-conformers, differing in the isopropyl orientation, were identified in the cryomatrices. The two less stable cis-OH conformers were not detected as they shall undergo fast tunneling to the most stable ones. Annealing experiments in a Xe matrix up to 75 K did not lead to any conversion between the two isolated conformers, which is in accordance with the significative energy barrier computed for rotamerization of the bulky isopropyl group (~24 kJ mol−1). Vibrational excitation promoted by broadband or by narrowband irradiation, at the 2ν(OH) frequencies of the isolated conformers, did not lead to any conversion either, which was interpreted in terms of a more efficient energy transfer to the hydroxyl rotamerization (associated with a lower energy barrier and a light H-atom) than to the isopropyl rotamerization coordinate. Broadband UV irradiation experiments (λ > 200 nm) led to a prompt transformation of matrix isolated thymol, with spectroscopic evidence suggesting the formation of isomeric alkyl-substituted cyclohexadienones, Dewar isomers and open-chain conjugated ketenes. The photochemical mechanism interpretation concords with that reported for analogous phenol derivatives.
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CiteScore
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