Vapor-phase mercury photosensitization

The general principles of Hg photosensitization are fully discussed in Calvert and Pitts text [1], and the early history of the subject is covered in a 1963 article by Gunning and Strauss [2]. Briefly, a low-pressure mercury-vapor lamp produces 254 nm radiation that is absorbed by trace mercury in the vapor phase of a quartz photoreactor. A drop of mercury can be added to the reactor to ensure enough Hg vapor is present. The mercury atom in the reactor is excited to the relatively long-lived (ca 10 7 s) 3P2 state, denoted Hg this is the reactive species that attacks the organic substrate. 

The mercury-photosensitized dehydrodimerization reaction has been known for nuuiy years, but it has only been made preparatively useful very recently. The key feature of the process is that the system is only active in the vapor phase, so that after condensation the product is protected from further conversion. This implies that the reaction can be run to essentially quantitadve conversion without a fall-off in yield. In order to run on a gram scale to tens of grams, all diat is needed is a quartz flask and a low pressure mercury lamp. Heating the substrate or substrates in the quartz flask with a small drop of mercury leads to smooth formation of the products. Aspects of the process are shown in equations (IS) to (18). 

Product isolation from a catal3d ic reaction may be continuous, or a batch process may be used. If the products are volatile, they can often be separated by distillation from the mixture, assuming the catalysts are involatile, as is often the case. There can also be unexpected selectivity advantages in vapor phase reactions. In mercury photosensitized reactions, alkanes can be converted to liquid functionalization products by a radical pathway. Normally, a radical route would... 

One of the first applications of this chopped-beam irradiation technitriplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

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