Far-Ultraviolet Spectra of n-Alkanes and Branched Alkanes in the Liquid Phase Observed Using an Attenuated Total Reflection—Far Ultraviolet (ATR-FUV) Spectrometer

Abstract

Far-ultraviolet (FUV) spectra of n-alkanes (n = 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 n-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 n-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 n-alkanes to the branched ones with the quaternary carbon, respectively.

Far‐Ultraviolet ( FUV ) Spectroscopy in the Solid and Liquid States, Principle, Instrumentation, and Application of

“…Figure 21(a) compares ATR/FUV spectra in the 8.5-6.6 eV region of neat n-hexane, 2-methylpentane (2MP), 3-methylpentane (3MP), 2,2-dimethyl butane (2,2-DMB), and 2,3-dimethyl butane (2,3-DMB). (39) Note that all the branched alkanes show a lower energy shift of the 8.3 eV band and that the intensity of the 7.7 eV band increases markedly in the order of alkanes without branch, with tertiary, and with quaternary carbon atoms. shows the corresponding spectra of n-hexane, 2MP, 3MP, 2,2-DMB, and 2,3-DMB calculated by TD-CAM-B3LYP.…”

“…(54) Figure 15 displays ATR/FUV spectra in the region of 8.55-6.53 eV (145-190 nm) of 10 n-alkanes (n = 5-14) in the liquid phase. (39) All the alkanes show a broad feature near 8.3 eV and the feature yields a lower energy shift with a significant intensity increase as the alkyl chain length increases. (39) In order to elucidate the band assignments of FUV spectra, the causes of the band shift and intensity increases, and the electronic structure of n-alkanes in the liquid phase, we carried out quantum chemical calculations of TD-CAM-B3LYP and SAC-CI.…”

“…(39) All the alkanes show a broad feature near 8.3 eV and the feature yields a lower energy shift with a significant intensity increase as the alkyl chain length increases. (39) In order to elucidate the band assignments of FUV spectra, the causes of the band shift and intensity increases, and the electronic structure of n-alkanes in the liquid phase, we carried out quantum chemical calculations of TD-CAM-B3LYP and SAC-CI. (54) Figure 16(a) shows spectra in the 9.5-6.5 eV region of n-alkanes calculated at the TD-CAM-B3LYP/cc-pVTZ level.…”

“…It may be considered that the forbidden transition from HOMO to 3s becomes allowed by the symmetry decrease on going from the n-alkanes to the branched alkanes with lower symmetry. (39,54) In this way, we have made an assignment of absorption bands for n-alkanes observed and branched alkanes in the FUV region by comparing experimental results with quantum chemical calculations. The combination of ATR-FUV spectroscopy may open a novel area of 'chemistry'.…”

“…The study has also provided new insight into Rydberg states and transition of liquid alkanes. (39,54) Encyclopedia of Analytical Chemistry, Online …”