KINETIC HH HD DH DD ISOTOPE EFFECTS ON NONDEGENERATE STEPWISE REVERSIBLE DOUBLE PROTON-TRANSFER REACTIONS - NMR-STUDY OF THE TAUTOMERISM OF MESO-TETRAPHENYLCHLORIN

The tautomerism of meso-tetraphenylchlorin-(N4)-N-15 (TPC) has been studied by dynamic H-1 NMR spectroscopy. Only two degenerate tautomers of TPC, AC and CA, are observed whose structure corresponds to 5,10,15,20-tetraphenyl-7,8-dihydroporphyrin. Thus, the IUPAC name 5,10,15,20-tetraphenyl-2,3-dihydroporphyrin proposed for TPC is incorrect. Both tautomers interconvert via a mutual proton exchange process along two different routes. The rate constants of the tautomerism are given by k(AC --> CA)HH = 10(11.4 +/- 0.2) exp(-58.4 +/- 1.4 kJ mol-1/RT), 298 K less-than-or-equal-to T less-than-or-equal-to 406 K, k(AC --> CA)HH (298) almost-equal-to 15 s-1, and k(AC --> CA)HD = 10(11.5 +/- 0.3) exp(-61 +/- 2 kJ mol-1/RT), 329 K less-than-or-equal-to T less-than-or-equal-to 411 K, k(AC --> CA)HD (298) almost-equal-to 5.8 s-1, with an unusually small kinetic HH/HD isotope effect of k(AC --> CA)HH/k(AC --> CA)HD almost-equal-to 2.6 at 298 K. A theory of kinetic HH/HD/DD isotope effects for nondegenerate stepwise reversible double proton transfer reactions is described that can accommodate these kinetic results. Thus evidence is obtained that the proton transfer mechanism in TPC is stepwise as in the case of TPC-H-2 but nonequivalent in the case of TPC-HD. In the HD reaction a H isotope is in flight in the rate-limiting step of route i but a D isotope is in flight in the rate-limiting step of route ii. Therefore, the D route contributes only to a minor extent to the HD reaction. The observed kinetic HH/HD isotope effect arises, therefore, not from an intrinsic kinetic isotope effect but from the circumstance that in the HH case there are two equivalent H reaction routes and in the HD case there is only one H reaction route.