Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene
Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene
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DOI：
10.1039/c7sc02027a
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发表时间：
2017-08-01
期刊：
影响因子：
8.4
通讯作者：
Zwier, Timothy S.
中科院分区：
文献类型：
--
作者：
Hewett, Daniel M.;Bocklitz, Sebastian;Zwier, Timothy S.
研究了气相喷冷条件下正戊基至正十一烷基苯的构象选择。结合激光诱导荧光激发光谱、烷基CH伸展区的荧光-浸渍红外光谱和拉曼光谱，对观察到的构象进行了指认。密度泛函理论计算在B3LYP-D3BJ/Def2TZVP水平上提供了相对能量和简正模振动，作为早期工作中介绍的关于n=2-4烷基苯的非简谐局部模理论的输入。该模型显式地考虑了CH伸缩模式与CH2和CH3链中CH2和CH3基团剪刀/弯曲模式的非简谐混合，并用于指认和解释单构象红外光谱。在辛基苯中，一对从全反式电子起点移位-92和-78 cm(-1)的LIF跃迁具有独特的烷基CH伸缩跃迁，该跃迁由局部模型拟合为g1g3g4构象，其中烷基链折叠回到芳环pi云上。它的计算能量仅比全反式全局最小值高1.0kJ·mol(-1)。这一折叠的烷基链长不到纯烷烃的一半(n=18)，这与G1二面体的较小能量消耗和链与p云的色散相互作用的增加是一致的。来自局域模式模型的整组构象的局域位置频率显示出‘边缘效应’，它提高了CH2(1)和CH2(2)由于苯环的位置频率和CH2(n-1)由于甲基的位置频率。G1g3g4构象还显示，由于与pi云的相互作用，CH2(3)和CH2(6)的局部位置频率上移。
The conformational preferences of pentyl-through decylbenzene are studied under jet-cooled conditions in the gas phase. Laser-induced fluorescence excitation spectra, fluorescence-dip infrared spectra in the alkyl CH stretch region, and Raman spectra are combined to provide assignments for the observed conformers. Density functional theory calculations at the B3LYP-D3BJ/def2TZVP level of theory provide relative energies and normal mode vibrations that serve as inputs for an anharmonic local mode theory introduced in earlier work on alkylbenzenes with n = 2-4. This model explicitly includes anharmonic mixing of the CH stretch modes with the overtones of scissors/bend modes of the CH2 and CH3 groups in the alkyl chain, and is used to assign and interpret the single-conformation IR spectra. In octylbenzene, a pair of LIF transitions shifted -92 and -78 cm(-1) from the all-trans electronic origin have unique alkyl CH stretch transitions that are fit by the local model to a g1g3g4 conformation in which the alkyl chain folds back over the aromatic ring pi cloud. Its calculated energy is only 1.0 kJ mol(-1) above the all-trans global minimum. This fold is at an alkyl chain length less than half that of the pure alkanes (n = 18), consistent with a smaller energy cost for the g1 dihedral and the increased dispersive interaction of the chain with the p cloud. Local site frequencies for the entire set of conformers from the local mode model show 'edge effects' that raise the site frequencies of CH2(1) and CH2(2) due to the phenyl ring and CH2(n - 1) due to the methyl group. The g1g3g4 conformer also shows local sites shifted up in frequency at CH2(3) and CH2(6) due to interaction with the pi cloud.