Elucidating Electronic Transition from σ Orbitals of Liquid n- and Branched Alkanes by Far-Ultraviolet(FUV) Spectroscopy and Quantumn Chemical Calculations
Elucidating Electronic Transition from σ Orbitals of Liquid n- and Branched Alkanes by Far-Ultraviolet(FUV) Spectroscopy and Quantumn Chemical Calculations
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通过远紫外 (FUV) 光谱和量子化学计算阐明液态正烷烃和支链烷烃 σ 轨道的电子跃迁
DOI：
10.1021/jp307634m
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发表时间：
2012
期刊：
影响因子：
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通讯作者：
Masahiro Ehara,and Yukihiro Ozaki
中科院分区：
文献类型：
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作者：
Yusuke Morisawa;Shin Tachibana;Masahiro Ehara,and Yukihiro Ozaki
用量子化学方法研究了液相中观察到的正构烷烃(CmH_2m+2)和支链烷烃的Rydberg态的衰减全反射远紫外光谱，目的是阐明液体烷烃和支链烷烃的σ轨道的电子跃迁。基于含时密度泛函理论(TD-DFT)和对称性适应的团簇组态相互作用(SAC-CI)计算，提出了新的指认，并详细分析了这些谱的差异。正构烷烃的FUV光谱在8.3 eV附近表现出广泛的不对称特征。通过TD-DFT和∼-CI计算，在Rydberg8.3 eV处的强带出现红移，强度随着碳链长度的增加而显著增加，这归因于从第三(或第四)最高占据轨道HOMO-2(或HOMO-3)和HOMO-1到Rydberg3py.这条谱带以前被分配给从HOMO到3P和从HOMO-1到3S的跃迁产生的两个峰的重叠，基于它们的项值。虽然最强烈的跃迁T1来自HOMO-2形式=5和6，HOMO-3形式在7-9范围内变化，但在所研究的所有烷烃中，T1的Kohn-Sham分子轨道形状是相似的。理论结果还表明，红移既源于Rydberg3P的稳定，也源于占据轨道的失稳。在7.7 eV下，支链烷烃的肩部强度显著增加，特别是那些具有2，2-二甲基丁烷等季碳原子的支链烷烃。这种强度的增加是由于支链烷烃对称性的降低，这导致到里德堡3S的禁止跃迁变成了允许的跃迁。通过这种方式，本研究为他们里德堡转变的存在以及转变中相关目标的形状提供了新的见解。
Attenuated total reflection far-ultraviolet (ATR-FUV) spectra containing Rydberg states ofn-alkanes (CmH2m+2;mvaries in the range 5–9) and branched alkanes observed in the liquid phase were investigated by quantum chemical calculations with the aim of elucidating electronic transitions from σ orbitals of liquidn-and branched alkanes. New assignments are proposed based on the time-dependent density functional theory (TD-DFT) and symmetry-adapted cluster configuration interaction (SAC-CI) calculations, and the differences in these spectra are analyzed in detail. The FUV spectra ofn-alkanes show a broad asymmetric feature near 8.3 eV. The strong band at ∼8.3 eV shows a red shift with a significant increase in intensity as the carbon chain length increases, which is attributed to the overlapping transitions from the third (or fourth) highest occupied molecular orbitals HOMO–2 (or HOMO–3) and HOMO–1 to Rydberg 3pyby the TD-DFT and SAC-CI calculations. This band was previously assigned to the overlap of two peaks arising from the transition from the HOMO to 3p and from the HOMO–1 to 3s based on their term values. Although the most intense transition, T1, is from HOMO-2 form= 5 and 6 and HOMO–3 formvarying in the range of 7–9, the shape of Kohn–Sham molecular orbital for T1 is similar among the all-alkanes investigated. The theoretical result also has demonstrated that the red shift originates in both stabilization of the Rydberg 3pyand destabilization of the occupied orbitals. The intensity of the shoulder at 7.7 eV drastically increases in the spectra of the branched alkanes, especially for those with quaternary carbon atoms such as 2,2-dimethyl butane. This increase in intensity is caused by a reduction in symmetry in the branched alkanes, which leads the forbidden transitions to Rydberg 3s to allowed transitions. In this way, the present study has provided new insight into the existence of their Rydberg transitions and the shape of the relevant MOs of the transitions.