Cyclization/fission and fragmentation/recombination mechanisms for the 1,2 shift in free radicals:: A computational study of H2C•-CH2X (X = -C≡CH, -C≡N, -CH=CH2, and -CH=NH) and H2C•-CH2CY=O (Y = -H, -F, -Cl, -CH3, -CN, -SH, -SCH3, -OH, and -O-)

The role of the cyclopropyl ring structure in the cyclization/fission mechanism for the 1,2 shift of the title radicals, whether las a local minimum on the potential energy surface or as a transition state, has been investigated using density functional theory calculations at the B3LYP/6-31+G*//B3LYP/6-31+G* computational level. The three-membered ring structure, C-(1)-C-(2)-C-(3), has been identified for all the substituent groups, and the alteration in ring geometry that accompanies the changeover in its role has been characterized-notably an increase in the C-(1)-C-(3) and C-(2)-C-(3) bond lengths beyond a certain threshold value. The free; energy of activation at 298 K for the cyclization/fission mechanism has been compared with that for the alternative fragmentation/recombination mechanism, in which breaking the C-(2)-C-(3) bond in the initial extended chain structure of the radical, giving an olefin and the radical derived from the substituent group; is followed by C-(3)-C-(1) bond formation. The difference, delta DeltaG(double dagger) = DeltaG(double dagger)(frag/recomb) - DeltaG(double dagger)(cycliz/fiss), whether positive or negative, determines which mechanism is the more likely. With -C-(3)=CH, -C-(3)=N, and -C(3)H=CH2 as the substituent group, delta DeltaG(double dagger) is positive, indicating that the cyclization/fission mechanism is more favored, whereas with most of the carbonyl substituent groups delta DeltaG(double dagger) is negative, showing the fragmentation/recombination mechanism to be the more favored.