The disproportionation-combination reactions of radicals 4,4-dimethylpentyl-2 and 2,4-dimethylpentyl-2, produced by collisional stabilization of the respective chemically activated species, have been studied at 298"K, with and without getter radicals. Use of getters t-C4H9 and CzHs with 4,4-dimethylpentyl-2, and of iso-C3HT with 2,4-dimethylpentyl-2 yielded disproportionation-combination ratios A for parent-getter and t-C4H9-C 2H6 cross reactions, and for the mutual reaction of t-CqH9 radicals. The unstabilized hot radicals decompose, and these experiments also yielded values for the high-pressure rate constant k,, of 1.7OX 108sec-1 and 2.11 X 105ec-' for 4,4-dimethylpentyl-2 and 2,4-dimethylpentyl-2, respectively. The results, compared to those for other large alkyl radical systems obtained earlier, suggest that steric effects influence the accessibility of H atoms for transfer .

Notes index field can be recorded and reconstructed at any time for evaluation by various optical methods chosen at convenience.

Collisional deactivation of chemically activated 2-pentyl radicals having ~44.3 kcal mole-I vibrational energy has been studied in the low-pressure turnup region for the bath gases H., CH., and CF4• A stochastic analysis of the data yielded information about (AE), the average amount of energy transferred per collision and (300;, the practical high-pressure relative collisional efficiency of inert gas j. The approximate values of (AE) for H., CH., and CF. are 1.2, 3.0, and 4.5 kcal mole-I, respectively. These correspond to average values of (300; =0.20, 0.64, and 0.85, respectively. The preferred collisional transition probability model for H. is not definite, but an exponential form gives best fit; a stepladder or, more probably, a Gaussian model seems in order for the most efficient gas.

Collisional de-excitation of vibrationally excited (∼45 kcal mole−1) 2,4-dimethylpentyl-2* and 2,4-dimethylpentyl-2-d1* alkyl radicals by bath molecules H2 and D2, respectively, was studied in both the ``high'' and ``low'' pressure regions. The behavior with the stronger collider CF4 was also investigated at ``high'' pressures. Both symmetry and unimportance of the endothermic (5ts) isomerization of this radical promoted greater accuracy of experimentation. Comparison of experimental rate behavior with model stochastic computations yields values of 〈 Δ E〉, the average amount of energy transferred per collision, of 1.5, 2.2, and 4.6 kcal mole−1 for H2, D2, and CF4, respectively. Independently of any uncertainty in collision cross sections, the data establish that the preferred forms of the distribution of the transition probabilities pij are exponential (or related type) for weak colliders, and stepladder (or Gaussian-like) for CF4. The data support earlier conclusions that gas kinetic (viscosity)-derived collision cross sections are appropriate for this type of energy transfer process; that inefficient bath gases are not such because of a preponderance of elastic collisions.

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