Studies of the unimolecular decomposition o f 4-methylpent-2-yne (M2P) and 4,4-dimethylpent-2-yne (DMZP) have been carried out over the temperature range of 903-1246 K using the technique of very-low pressure pyrolysis (VLPP). T h e primary reaction for both compounds is fission of the C-C bond adjacent to the acetylenic group producing the resonance-stabilized methyl-substituted propargyl radicals, CH:3C=CCH(CH3) from M2P and CH&=Cc(CH& from DM2P. RRKM calculations were performed in conjunction with both vibrational and hindered rotational models for the transition state. Employing the usual assumption of unit efficiency for gas-wall collisions, the results show that only the rotational model with a temperature-dependent hindrance parameter gives a proper fit to the V L P P data over the entire experimental temperature range. T h e high-pressure Arrhenius parameters a t 1100 K are given by the rate expressions log k 2 (sec-l) = (16.2 f 0.3) -(74.4 f 1.5)/0 for M2P and log k3 (sec-I) = (16.4 f 0.3) -(71.4 f 1.5)/0 for DM2P where 0 = 2.303RT kcal/mol. The A factors were assigned from the results of recent shock-tube studies of related alkynes. Inclusion of a decrease in gas-wall collision efficiency with temperature would lower both activation energies by -1 kcal/mol. The critical energies together with the assumption of zero activation energy for recombination of the product radicals a t 0 K lead to DH'[CH&CCH(CHn)-CHx] = 76.7 f 1.5, AHO,[CH:&XcH(CH:I)] = 65.2 f 2.3. AHO,[CHnCCC(CH&] = 53.0 f 2.3, and T)H"[CH:ICCC(CH,I)2-H] = 82.3 f 2.7
, where all quantities are in kcal/mol a t 300 K. The resonance stabilization energies of the 1.3-dimethylpropargyl and 1,1,3-trimethylpropargyl radicals are 7.7 f 2.9 and 9.7 f 2.9 kcal/mol a t 300 K. Comparison with results obtained previously for other propargylic radicals indicates that methyl substituents on both the radical center and the terminal carbon atom have little effect on the propargyl resonance energy.