A laser flash photolysis system, capable of producing kinetic data of high precision, is described and detailed consideration is given to the methods of data analysis. Measurements of the allyl radical recombination reaction at the high-pressure limit show that the rate constant decreases slightly with temperature and may be described by the Arrhenius parameters A = (1.69 i 0.03) X lo-'' cm3 molecule-' s-' and E, = -(1.1 f 0.1) kJ mol-', over the temperature range 293-571 K, if van den Bergh and Callear's absorption cross section is employed and assumed independent of temperature. The errors quoted refer to one standard deviation and do not include the error in the absorption cross section. The reaction between C3H5 and NO is also studied, over the temperature range 295-400 K, and is found to be pressure dependent. The limiting high-pressure rate constant decreases with temperature, with a negative activation energy of -(3.4 f 0.1) kJ mol-'. IntroductionA knowledge of the kinetics of free radicals is of primary importance in the modeling of pyrolysis2 and combustion3 reactions. It also provides data necessary for the establishment of thermodynamic parameters for free radicals4 and for the development and testing of models of chemical reactions? All these applications demand accurate kinetic data over a wide range of temperatures. Kinetic studies

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The probabilities of formation of ortho-and para-hydrogen are incorrectly formulated in the above paper. In particular, the validity of eqn (5.37) and (5.38) is limited: the yS and Sy terms cannot be treated separately. For example two H atoms after an initial encounter in a To state with nuclear wavefunction 2-"2(yS + Sy) may subsequently re-encounter as singlets with 2-1'2( $3 -S y ) with probability (cf. section 5)

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