TYPE-I PHOTOSENSITIZED REACTIONS OF OXOPURINES - KINETICS AND THERMODYNAMICS OF THE REACTION WITH TRIPLET BENZOPHENONE BY TIME-RESOLVED PHOTOACOUSTIC-SPECTROSCOPY

Benzophenone photosensitized reactions of caffeine, theophylline and t heobromine were investigated in acetonitrile by time-resolved laser-in duced photoacoustics. In the three cases global quenching rate constan ts of triplet benzophenone were measured as a function of temperature and it was observed that this is a non-activated process. Besides, for theobromine and theophylline heats for NH hydrogen abstraction reacti ons were determined. In agreement with semiempirical calculation predi ctions, hydrogen abstraction is thermodynamically more favorable and f aster for theophylline (Delta H = - 265 kJ mol(-1) k(r) = 9.6 X 10(8) M-1 s(-1)) than for theobromine (Delta H = - 168 kJ mol(-1), k(r) = 3. 7 X 10(8) M-1 s(-1)). (C) 1998 Elsevier Science B.V. All rights reserv ed.