Carbon tetrabromide as an electron acceptor in heterogeneous photocatalysis

Abstract

Three reactions: the CdS-mediated valence isomerization of hexamethyldewarbenzene to hexamethylbenzene, the cycloreversion of dianthracene, and the TiO2-mediated cis–trans isomerization of stilbene are known to be suppressed when the semiconductor excitation takes place in the absence of oxygen. The oxygen, when present, is presumed to trap the electron delaying electron-hole recombination. It has been found that CBr4 may replace oxygen and functions in a similar way. In addition, the concomitant formation of oxidation products in the TiO2-mediated aerobic reaction is avoided. The mechanism is discussed. Keywords: cadmium sulphide, carbon tetrabromide, dianthracene, heterogeneous photocatalysis, hexamethyldewarbenzene, titanium oxide.

Observation and Characterization of CH3CH2–MH, (CH2)2–MH2, CH2═CH–MH3, and CH3–C≡MH3– Produced by Reactions of Group 5 Metal Atoms with Ethane

“…(These bands are observed better using expanded absorbance scale.) Their relatively low frequencies and disappearance on addition of an electron scavenger (CBr 4 ) 28,29 support assignment to the A 1 and E NbH 3 stretching modes of the anionic Nb ethylidyene ). They correlate well with the computed values: the observed frequencies are 0.970 and 0.969 of the B3LYP frequencies (Tables 1 and S4), and the observed/calculated frequency ratios for Nb−H and Nb−D modes are 0.970 and 0.980 again with the higher value for the less anharmonic Nb−D mode.…”

“…The weak a absorptions decreased in the process of photolysis. They disappeared almost completely on the addition of CBr 4 in a separate experiment, which works as an electron scavenger in the condensing matrix. − This suggests that the product responsible for the a absorptions is an anionic species.…”

“…Laser-ablated Ta, Nb, and V atoms (Johnson-Matthey) produced by a Nd:YAG laser were codeposited with dilute mixtures (0.5–1.0%) of reagent gas (C 2 H 6 , Matheson; C 2 D 6 , MSD Isotopes) in argon onto a CsI window cooled to 10 K using a closed-cycle refrigerator (Air Products HC-2). In addition, CBr 4 (0.1%) , was included in the gas mixture to capture electrons produced in the laser-ablation process in complementary experiments: These methods have been described in detail elsewhere for matrix isolation experiments using CCl 4 as the electron trapping molecule. − After deposition, infrared spectra were recorded at a resolution of 0.5 cm –1 using a Nicolet 550 spectrometer with an MCT-B detector. Matrix samples were next irradiated by a mercury arc street lamp (175 W) with the globe removed (λ > 220 nm) with or without optical filters for 20 min and subsequently warmed and recooled to base temperature (annealed), and more spectra were recorded.…”

“…The a absorptions slightly increased on visible irradiation, but noticeably decreased on UV irradiation. Moreover, they disappeared almost completely on the addition of CBr 4 to the gas mixture, 28,29 indicating that an anionic product is generated, parallel to the Nb case.…”

“…While mixtures (0.5−1.0%) of reagent gas (C 2 H 6 , Matheson; C 2 D 6 , MSD Isotopes) in argon onto a CsI window cooled to 10 K using a closed-cycle refrigerator (Air Products HC-2). In addition, CBr 4 (0.1%) 28,29 was included in the gas mixture to capture electrons produced in the laser-ablation process in complementary experiments: These methods have been described in detail elsewhere for matrix isolation experiments using CCl 4 as the electron trapping molecule. 30−32 After deposition, infrared spectra were recorded at a resolution of 0.5 cm −1 using a Nicolet 550 spectrometer with an MCT-B detector.…”