Far-ultraviolet (FUV) spectra of <i>n</i>-alkanes (<i>n</i> = 5–14) and branched alkanes were measured in their liquid state by using a newly developed attenuated total reflection (ATR)-FUV spectrometer to investigate spectra–structure relationship in the FUV region. The <i>n</i>-alkanes show a broad band near 8.3 eV and a weak shoulder near 7.7 eV. The 8.3 eV band shows a lower energy shift with a significant intensity increase with the increase in the length of alkyl chain. We have assigned the 8.3 eV band to the overlap of two bands due to the transition from the highest occupied molecular orbital (HOMO) to 3p and that from the HOMO-1 to 3s based on the observation that the peak energy of the 8.3 eV band of the <i>n</i>-alkanes is proportional to the first ionization energy. The 7.7 eV shoulder may be due to the transition from HOMO to 3s. The intensity of the 7.7 eV band increases markedly in the order of alkanes without branch, with tertiary, and with quaternary carbon atoms. It is very likely that the forbidden transition from HOMO to 3s becomes allowed by the large decrease in symmetry upon going from the <i>n</i>-alkanes to the branched ones with the quaternary carbon, respectively.
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