Combination reactions of the methyl radical have been studied by following the decay of the absorbance of the methyl radical during the course of the reaction by means of kinetic spectroscopy. The limiting values of the second-order rate constants at high pressure were determined for two reactions at room temperature:k, = (2.6 f 0.3) x 1O1O 1 mole-1 sec-1 CH, + N O ( + M ) .+ CH,NO ( + M ) :The extinction coefficient of the methyl radical was found to have a maximum value of (1.02 f 0.06) x lo4 1 mole-l cm-l at 216.4 nm. Integration of the extinction coefficient over the absorption band of the methyl radical gave an oscillator strength of 1.0 x

The unusual pattern of the reactions shown by the cyclohexadienyl radical below 200' is consistent with the high stabilization energy and complex pattern of spin density in the radical, andwith itsclose relationship to benzene. We have investigated the scope of the reactions of the cyclohexadienyl radical in the gas phase, in the absence of significant concentrations of other radicals and in the presence of an excess of cyclohexadiene-1,4. Metathesis between these species would be revealed by a proces; of chain isomerization of cyclohexadiene-l,4 to cyclohexadiene-1,3, but this did not occur at a significant rate below 200". Mutual interaction was the only significant process of consumption of cyclohexadienyl radicals below lOO", and conformed to the pattern :-+C12H14 isomers (69 2,) between 63 and 101". At 136" cyclohexene was formed in this system, and the relative abundance of both cyclohexene and benzene increased rapidly as the reaction temperature was raised, until at 186" the rate of formation of each exceed the rate of the primary generation of cyclohexadienyl radicals. The results are consistent with the thermal dissociation of the cyclohexadienyl radical :CYClo-C6H7*+C6H6+H. ; Ea = 31 kcal/mole, followed by the formation of cyclohexene and the regeneration of the cyclohexadienyl radical in a chain sequence. The enthalpy of formation of the cyclohexadienyl radical wzs estimated as 44 kcall mole, and the stabilization energy as 25 kcal/mole.

The characteristics of the photolysis of 2,2'-azoisobutane imply that this system provides a good source for the study of the reactions of tert-butyl radicals below 100°C. The reactivity of tert-butyl radicals toward the parent azoalkane is extremely low; under typical conditions metathesis is negligible and addition to the double bond is sterically very unfavourable. The rate of generation of tat-butyl radicals is satisfactory although the quantum yield of nitrogen is dependent upon reaction conditions, implying that the collisional deactivation of electronically excited 2,2'-azoisobutane molecules is a significant but not a predominant process. The concentration of tert-butyl radicals may be estimated conveniently from the rate of formation of isobutene as the ratio of disproportionation to combinationis 2-3 f0-3 in thissystem. The applicationof this source to kineticstudies is illustrated by the measurement of Arrhenius parameters for the metathesis between the tert-butyl radical and cyclohexadiene-l,4 over the range 27-100". Azomethane was photolyzed under similar conditions to provide a basis for comparison and to supplement our knowledge of this system. The quantum yields of nitrogen and of intramolecular elimination of ethane were independent of temperature between 63 and 218", implying that collisional deactivation of electronically excited molecules is insignificant for azomethane. New estimates of the Arrhenius parameters for addition and metathesis between the methyl radical and azomethane are discussed.

The pattern of interaction of the cyclohexadienyl radical with each of a series of primary, secondary products CH3. MeCHz-MezCH. M e P and tertiary alkyl radicals R* is given by the percentages of the products :28 34 57The almost constant ratio of the products of combination implies that this pattern is determined almost exclusively by the properties of the cyclohexadienyl radical, and we have proposed that combination at the para position is favoured because the spin density is highest there. Conversely, the monotonic increase of the percentage of disproportionation with the degree of substitution at the a carbon atom of the allcyl radical parallels the behaviour of the alkyl radicals in mutual interaction. The significance of these results is discussed in relation to current models of the transition state.