Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

1 INTRODUCTION 283It is possible that the vibrationally excited hydroxyl radicals may decomposeozone’, and vibrationally excited nitrogen may also decompose ozonesi9N:+03 -, Nz+O2+O (2)although in both these cases there is some danger of misinterpretation of theresults”.A process of great importance in the atmosphere is the ultraviolet photolysis ofozone, and the reaction involves” a number of excited species, O(’D), Oz(’A0)and 0;03+hv + O(’D)+02(1A,)O(’D)+ 0 3 + 0; + 0 20: + o3 + 202 + o(’D)02(’Ag)+03 -, 202+O(3P)O(~P)+O, -, 202all of which react with ozone. 0; may possibly be a vibrationally excited moleculeof oxygen.Translationally “hot” species may also have enhanced reaction rates. Reubenand Linnett” have proposed that such “hot” oxygen atoms are formed in thethermal decomposition of nitrous oxide, and that these react at an accelerated ratewith further nitrous oxide. Williams and OggI3 have shown that hydrogen atomswith 42 kcal.mole- ’ of translational energy react at an enhanced rate with hydrogeniodide, and have demonstrated a similar effect in the reactivity of “hot” methylradicals.Spectroscopic methods of identification and estimation of excited species areclearly of great importance, although several other techniques have been employed(mass spectrometric, chemical, calorimetric, etc., and, for radicals, ESR).1.4 IONSMany inorganic and organic reactions in solution occur via ionic or semi-ionicintermediates. However, the stationary concentration of the intermediates is usuallyvery small, and methods of producing large instantaneous concentrations arenot of general applicability. In special cases suitable techniques are available-forexample, ESR spectroscopy may be used to detect unstable oxidation states oftransition metals, and optical spectroscopy may occasionally be used for the investigationof ionic organic species-although the method normally has to bedevised for the particular problem. It is intended, therefore, specifically to excludeionic intermediates in solution in the present discussion. The omission is probablyReferences pp. 336-342