The endothermic ring opening of vibrationally excited cyclopentyl radicals generated in the gas phase by adding H to cyclopentene has been studied. The results obtained were combined with literature results to test activated complex models for the ring-opening reaction, and the data were fit by a model which gives a thermal A factor of 1014-0±°•4 sec-1, and a critical energy of 34.6 ± 0.6 kcal/mol. The exothermic ring opening of vibrationally excited methyIcyclobut-1 -yl generated by adding H to methylenecyclobutane in the gas phase was also studied. Assuming a model for the ring-opening activated complex which corresponds to a thermal A factor of 10141±0•4 sec-1, the critical energy is calculated to be 31.7 ± 0.7 kcal/mol. These results suggest that the critical energies for ring openings of cycloalkyl radicals may be relatively insensitive to ring strain.

A general method for the steady-state analysis of complex external activation systems with multiple isomerization and decompositions is developed. This method is used for both "strong" and "weak" collisions. The algorithm for such an analysis is discussed.

The homoallylic radicals, l-penten-4-yl and 3-methyl-l-buten-4-yl, were generated in the gas phase by adding H to 1,4-pentadiene. Evidence is given that they are in equilibrium even when thermalized at 0°, and that the isomerization critical energy is less than 15 kcal/mol. The decomposition of 3-methyl-lbuten-4-yl to 1,3-butadiene was studied, and a critical energy of 12 ± 1 kcal/mol for the addition of methyl to the 2 position of 1,3-butadiene was obtained. This is around 3 to 4 kcal/mol higher than the usual alkyl radical plus olefin critical energy; the extra energy is explained as being due to the effect of the conjugation stabilization of 1,3-butadiene being destroyed in the activated complex.

The reactions resulting from adding H atoms to 2-pentyne in the gas phase have been studied using several deactivating gases. Measured decomposition products are propyne, 1-butyne, and 1,2-butadiene, resulting from decompositions of initially formed radicals at rates consistent with thermal results. 1,3-Butadiene, resulting from isomerization of initially formed 2-penten-2-yl first via a 1,4 H shift and then via a cis-trans isomerization of homoallylic 2-penten-5-yl, was also formed. Information about the rates of these isomerizations was obtained. 1,4 H shift activated complex models could be tested using this work and literature results, and the use of looser activated complex models is supported. Using loose 1,4 H shift activated complex frequencies and a 0°K primary vinylic C-H bond dissociation energy of 110.2 kcal/mol, a 1,4 H shift critical energy of 18.9 kcal/mol, and a cis-trans homoallylic isomerization critical energy of 23.4 kcal/mol are obtained. In addition, there is evidence of a possible 1,3 H shift of 2-penten-3-yl occurring with a critical energy of about 30 kcal/mol.

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