title: Elucidating electronic transitions from σ orbitals of liquid n- and branched alkanes by far-ultraviolet spectroscopy and quantum chemical calculations.
abstract: 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.
authors.affiliation: Department of Chemistry, School of Science and Engineering, Kinki University, Higashi-Osaka City, Osaka, Japan. morisawa@chem.kindai.ac.jp
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Morisawa
authors.forename: Shin
authors.initials: S
authors.lastname: Tachibana
authors.forename: Masahiro
authors.initials: M
authors.lastname: Ehara
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Ozaki
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D004583:Electrons; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000473:Alkanes