Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

322 DETECTION AND ESTIMATION OF INTERMEDIATESrepresents the situation fairly faithfully. Similar observations on the overall stoichiometryof the reaction have been presented by Phillips and Schiff '.The recombination of atomic nitrogen gives rise to a yellow chemiluminescence'",which is the main component of the glow of 'active' nitrogenN+N+M + N,*+M (42)Nf + N2+hv (43)The disappearance of the yellow emission can be used, therefore, to mark the endpointof reaction (37) when nitric oxide is used as titrant. It may be noted thatsince the titration reaction produces one atom of oxygen for every molecule ofnitric oxide consumed, it also provides a means of producing known atomicoxygen concentrations.The detection of excited species by chemical means will now be considered.If a species is energetically rich, it may take part in a chemical reaction more rapidlythan does the normal species. In particular, if a reaction is exothermic for an excitedspecies, but endothermic for the ground state reactant, then it may proceedisothermally only with the excited species. Typical of such reactions is that ofatomic oxygen with water vapour; the reactionO(3P)+H20 + 20H (44)is 17 kcal . mole-' endothermic, while the reactionO('D)+H20 + 20H (45)is 29 kcal . mole-' exothermic. A water vapour-ozone mixture shows absorptiondue to hydroxyl radicals on flash photolysis", suggesting that excited, 'D, oxygenatoms are produced in the photolysis of ozone by ultraviolet radiation. Directmeasurement of the quantum yield for ozone decomposition gives a similar result:the presence of water vapour greatly increases the quantum yield for photolysis byultraviolet radiation, and it is believed that hydroxyl radicals produced in (45) arethe chain carriers". Water does not affect the quantum yield for photolysis byred light'42, at which wavelength there is only sufficient energy for the formationof o(3~).Foote and Wexler' 43 have investigated the reaction of singlet molecular oxygen(in the 'A,, or 'X: states) with a number of olefins. Sodium hypochlorite andhydrogen peroxide were added to solutions of the olefin: these compounds areknown to react with the formation of singlet oxygen'44. The products observedare identical with those seen in the photoinitiated autoxidation of the olefins, thussuggesting that singlet molecular oxygen is important in the autoxidations. Two