Department of Chemistry, School of Science and Engineering, Kinki University, Higashi-Osaka City, Osaka, Japan.
J Phys Chem A. 2012 Dec 6;116(48):11957-64. doi: 10.1021/jp307634m. Epub 2012 Nov 20.
Attenuated total reflection far-ultraviolet (ATR-FUV) spectra containing Rydberg states of n-alkanes (C(m)H(2m+2); m varies 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 liquid n- 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 of n-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 3p(y) by 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 for m = 5 and 6 and HOMO-3 for m varying 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 3p(y) and 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.
衰减全反射远紫外（ATR-FUV）光谱包含液态正构烷烃（C(m)H(2m+2)；m 范围为 5-9）和支链烷烃的里德堡态，这些光谱是通过量子化学计算观察到的，目的是阐明液态正构烷烃和支链烷烃 σ 轨道的电子跃迁。新的分配方案是基于含时密度泛函理论（TD-DFT）和对称自适应簇组态相互作用（SAC-CI）计算提出的，并详细分析了这些光谱的差异。正构烷烃的 FUV 光谱在 8.3eV 附近显示出宽不对称特征。在 8.3eV 附近的强带随着碳链长度的增加而发生红移，强度显著增加，这归因于 TD-DFT 和 SAC-CI 计算中从第三（或第四）最高占据分子轨道 HOMO-2（或 HOMO-3）和 HOMO-1 到里德堡 3p(y)的重叠跃迁。该带以前是根据其项值从 HOMO 到 3p 和从 HOMO-1 到 3s 的跃迁的两个峰的重叠来分配的。尽管最强烈的跃迁 T1 对于 m=5 和 6 来自 HOMO-2，对于 m 在 7-9 的范围来自 HOMO-3，但对于所有调查的烷烃，T1 的 Kohn-Sham 分子轨道的形状是相似的。理论结果还表明，红移既源于里德堡 3p(y)的稳定化，也源于占据轨道的去稳定化。在支链烷烃的光谱中，7.7eV 的肩峰强度急剧增加，特别是对于那些具有季碳原子的支链烷烃，如 2,2-二甲基丁烷。这种强度的增加是由于支链烷烃的对称性降低导致禁止跃迁到里德堡 3s 成为允许跃迁。通过这种方式，本研究为它们的里德堡跃迁的存在以及相关跃迁的 MO 形状提供了新的见解。
