Pentane, Hexane, Cyclopentane, Cyclohexane, 1-Hexene, 1-Pentene,cis-2-Pentene, Cyclohexene, and Cyclopentene at Vapor/α-Alumina and Liquid/α-Alumina Interfaces Studied by Broadband Sum Frequency Generation

Abstract

The molecular orientation and structure of adsorbates at oxide interfaces is driven by surface-molecule and molecule-molecule interactions and is useful for predicting reactivity and product selectivity in heterogeneous chemical reactions, including those important in catalytic processes. Using broadband vibrational sum-frequency generation spectroscopy, we probed cyclic and acyclic alkanes and olefins at buried vapor/solid and liquid/ solid interfaces of the R-alumina (0001) surface under ambient conditions. The spectroscopic signatures of the adsorbates were measured and compared with bulk phase spectra and spectra of model surfaces containing hydrocarbons covalently linked to glass slides using silane chemistry. By utilizing appropriate polarization combinations, the orientations of hydrocarbons adsorbed from the vapor were evaluated and compared to the hydrocarbon orientations at the liquid/solid interface. The data support a proposed orientation for n-alkanes and cycloalkanes at the liquid/solid interface in which the hydrocarbons lie with the planes of their carbon backbones parallel to the surface, whereas at the vapor/solid interface, the adsorbates are oriented with their carbon backbones in an orientation neither parallel nor perpendicular to the surface. The surface vibrational spectra of olefins at both types of interfaces indicate that their orientations differ from saturated counterparts, and their unique spectral signatures allow differentiation of adsorbed olefins from alkanes.

Contrasting organic aerosol particles from boreal and tropical forests during HUMPPA-COPEC-2010 and AMAZE-08 using coherent vibrational spectroscopy

“…This finding is consistent with the fact that the three methyl groups are tightly interlocked in the bridged six-membered ring structure. Given the rigid structure of α-pinene, the relative position of the oscillators is quite inflexible, and the SFG response is a result of strongly coupled coherences, which is consistent with the fact that we obtain a higher signal-to-noise for the terpenes than for more flexible systems such as hexane, pentane, cyclohexane, or cyclopentane (Buchbinder et al, 2010b), which have similar vapor pressures at room temperature and, given their general hydrophobic nature, thus most likely a similar surface coverage. Figure 2 also shows that the CH stretches of the two methyne and methylene CH stretches, which should occur at 2900 cm −1 and 2920 and 2850 cm −1 , respectively, are not resolved in the α-pinene spectrum shown.…”

“…Like the α-pinene SFG spectrum, the SFG response is dominated by one peak, which now occurs at 2950 cm −1 , while the SFG signal contribution at 2880 cm −1 is not shifted from that of unreacted α-pinene. In addition, the SFG spectrum of the α-pinene-derived secondary organic aerosol particles on the filter exhibits some signal contribution at 2920 cm −1 , which is characteristic of methylene asymmetric CH stretches in alkyl chains (Voges et al, 2004;Buchbinder et al, 2010b). Given the strong asymmetric CH stretching contributions observed for the α-pinenederived secondary organic aerosol particles, we conclude that the chemical composition of the particles is related to that of α-pinene in the sense that it contains rigidly arranged methyl groups.…”

“…The vibrational SFG spectra of cis-2-pentene, n-hexene, n-pentene, cyclohexene, and cyclopentene vapor in contact with an α-alumina optical window, which have been analyzed, assigned, and discussed in our previous work (Buchbinder et al, 2010b), show substantial signal intensity in the symmetric CH stretching region below 2900 cm −1 , and this unique response distinguishes those compounds from the terpenes.…”

“…We conclude from these results that the SFG spectrum of (+)-α-pinene exhibits just the asymmetric and symmetric methyl CH stretches at 2960 cm −1 and 2880 cm −1 as well as a methyl Fermi resonance at 2940 cm −1 , which dominates the spectral response. In α-and β-pinene, the methylene asymmetric stretch of the CH 2 on the four-membered ring may be shifted to higher frequencies (2950-2990 cm −1 ) due to ring strain, and contribute to the signal observed in this range (Galabov et al, 1972;Durig et al, 1990Durig et al, , 2008Ball, 1997;Buchbinder et al, 2010a).…”

Probing Electronic Structures of Organic Semiconductors at Buried Interfaces by Electronic Sum Frequency Generation Spectroscopy

“…We emphasize this equation differs from the formula for vibrational sum frequency generation (VSFG) 4,[41][42][43][44][45][46][47][48][49][50] , in which one of the two incoming pulses are tuned in resonant with molecular transitions. First, all states are electronic states and the contribution from vibrational states are treated implicitly.…”