Solid-state NMR spectroscopy of benzene adsorbed on .eta.-alumina

Abstract

peak at 304 nm, and the corresponding fluorescence spectra were broad and featureless with a maximum at 395 nm and a lifetime of 55 ns. The absorption spectra were consistent with a disordered, glasslike adlayer, and the broad featureless fluorescence spectra were attributed to excimer emission. The relatively long 55-11s fluorescence decay times were also consistent with excimer emission.The absorption spectra changed abruptly and resembled the crystalline absorption spectra when the adlayer was annealed to 200 K. The main feature of this change was a dramatic reduction of the absorbance of the So -S2 electronic transition. After annealing above 195 K, the fluorescence spectra also revealed vibrational structure characteristic of the crystalline fluorescence spectra originating at 354 nm.The sudden changes in the absorption and fluorescence spectra at 200 K were interpreted as a disorder-to-order transition in the phenanthrene adlayer on A1203( 1120). During this transition, the phenanthrene film evolves from a glasslike to a crystalline state. Polarized absorption spectra indicated that this crystalline phenanthrene adlayer was oriented with the ab plane of the phenanthrene crystal aligned parallel to the AI,O,( 1 120) surface.Energy-transfer studies revealed that excimer defect sites in the adlayer were very effective traps for excited electronic energy. Energy-transfer studies performed with anthracene impurities in the phenanthrene adlayer demonstrated that electronic energy was immobilized at excimer traps in the disordered film. In contrast, the electronic energy was observed to be mobile in the ordered crystalline adlayer. Acknowledgment.

NMR and IR Study of Fluorobenzene and Hexafluorobenzene Adsorbed on Alumina

“…It is not clear exactly how C 6 F 6 interacts with the alumina surface. Benzene coordinates via π-electrons to act as a proton acceptor or π-donor . Hexafluorobenzene on the other hand has been shown through ab initio calculations and spectroscopic investigations to act as a lone-pair acceptor and interacts with the oxygen center of H 2 O, , and it is a reasonable proposition that this type of interaction can also take place on the surface of alumina.…”

“…Clearly, an amorphous alumina surface must be a very complex mixture of these possibilities. Adsorption of aromatic molecules to the surface of alumina is believed to occur through interaction of the aromatic π-system with hydroxyl and Al 3+ centers at the oxide surface …”

Methionine bound to Pd/γ-Al2O3 catalysts studied by solid-state 13C NMR

“…in CO on transition metals. [26][27][28] On the other hand, several studies [29][30][31][32][33][34] have shown that for carbon not directly bonded to the metal, e.g. separated by a heteroatom such as S 30 or physisorbed, [32][33][34] the change in peak position is more moderate, < 20 ppm, and can be used to monitor adsorption.…”

“…[26][27][28] On the other hand, several studies [29][30][31][32][33][34] have shown that for carbon not directly bonded to the metal, e.g. separated by a heteroatom such as S 30 or physisorbed, [32][33][34] the change in peak position is more moderate, < 20 ppm, and can be used to monitor adsorption. [29][30][31][32][33][34] One recent study demonstrated that the chemical shift of formate on different metal nanoparticles correlated well with the calculated dband center, formate decomposition rate, metal work function, and specific activity.…”

“…separated by a heteroatom such as S 30 or physisorbed, [32][33][34] the change in peak position is more moderate, < 20 ppm, and can be used to monitor adsorption. [29][30][31][32][33][34] One recent study demonstrated that the chemical shift of formate on different metal nanoparticles correlated well with the calculated dband center, formate decomposition rate, metal work function, and specific activity. 31 In metaloxide catalysts, 13 C NMR of acetone has been shown to be a viable means for measuring solidacid strength.…”