Direct determination of reaction heat in intramolecular hydrogen abstraction of excited 2-methylbenzophenone

Time-resolved thermal lensing (TRTL) technique was applied to the determination of reaction heats of intramolecular hydrogen abstraction for excited 2-methylbenzophenone (MBP). Excited MBP abstracts a hydrogen atom from the neighboring methyl group to form cis- and trans-enols, whose lifetimes were 3.8 μs and 5.8 ms, respectively. Heats of reaction for the production of cis- and trans-enols were successfully determined to be 116 and 202 kJ/mol, respectively. The large energy gap (86 kJ/mol) implies that the structure of the trans form should be less planar because of vinyl/phenyl steric hinderance.