Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ANALYSIS 77tane, and CH, insertion reactions lead to the production of hot molecules andhot radicals’ 50.4.2.4 PhotosensitisationAnother way of producing hot species is by photosensitisation techniques. Extensivestudies have been made of mercury-photosensitised reactions. The subjecthas been reviewed by C~etanoviC’~~ and by Gunning and Strausz’”, whereparticular experimental techniques are referred to. Physical and chemical quenchingmay be treated mechanistically according to the Stern-Volmer scheme252 interms of quenching cross-sections. This involves physical quenching from Hg(63Pl)to Hg(63Po) and physical and chemical quenching of Hg(6’P1), Hg(63P1) andHg(63P0) to the ground state Hg(6’So). This may take one of three forms as depictedin reactions (48)-(50); the overall spin angular momentum should remainunaltered in the proces~’~Hg*+AB -+ Hg+AB*-+ Hg+A+B-+ HgA+BIn the sensitised decomposition of t-b~tanol’~~, the excess energy remains almostentirely in the t-BuO. radical in the form of vibrational energy.Mercury photosensitisation also provides a means of initiating a chemical reactionwhere species such as hydrocarbons do not absorb the wavelength available.It is also a means of placing a precise amount of energy into a molecule or radical,particularly when using isotopic photosensitisation’21. Photosensitised oxidationof hydrocarbons is important for the light thrown on the mechanism of combustion2”.Other atomic sensitisers that have been used are Na, Cd, Zn and inertgases, the latter particularly in radiochemical studies. Laidler’ 31 has shown howthe multiple use of Hg, Zn and Cd can be put to very good use in the estimationof bond energies and for mechanistic studies. Molecular photosensitisers in the gasphase include benzene, S02256, biacetyl, toluene, fluorobenzene, pyridine, pyrrole,styrenez5’ and presumably azomethane, certainly in solution258, and thereforealso in the gas phase.4.2.5 IsotopesAn important chemical method is that of isotopic labelling by both stable aswell as radioactive isotopes. The fate of different atoms of the same element in acompound may bz decided by isotopic substitution. Mechanistic problems are alsoReferences pp. 104-111