MOLECULAR-ORBITAL ESTIMATION OF THE ACTIVATION ENTHALPIES FOR INTRAMOLECULAR HYDROGEN TRANSFER AS FUNCTIONS OF SIZE OF THE CYCLIC TRANSITION-STATE AND C-H-C ANGLE

The AM1 molecular orbital method was used to compute the activation enthalpies for H-transfer between carbons as a function of C-H-C angle and size of the cyclic transition state for intramolecular H-transfer. In the case of intramolecular H-transfer, reactions of a primary radical site with primary, secondary, and tertiary C-H's were considered for H-shifts to C1 from C3-C8. The activation enthalpies are insensitive to C-H-C angle in the range 145-180-degrees. Activation enthalpies are lowest for intramolecular H-transfers involving 1-5 and 1-6 H-shifts. The higher activation enthalpies for the other internal H-transfers are attributed to C-H-C strain for 1-3 and 1-4 H-transfers only, and conformational effects other than C-H-C angle in the transition states.