N-15 NMR study of proton localization and proton transfer thermodynamics and kinetics in polycrystalline porphycene

Using high-resolution solid-state N-15 cross-polarization magic angle spinn ing NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline N-15-labeled porphycene were studied. Whereas at room temperature only a single N-15 resonance is observed, indic ating an equivalence of all nitrogen atoms arising from a quasi-degenerate fast proton transfer, four signals are observed at low temperatures, exhibi ting temperature-dependent line positions. Their analysis is consistent wit h the presence of either (i) two different molecules A and B in the asymmet ric unit, each of which is subject to a quasi-degenerate correlated double proton transfer, or (ii) a single molecule exhibiting all four possible nea r-degenerate tautomeric states, two trans- and two cis-tautomers, interconv erting by fast single proton transfers. The average rate constants of the p roton transfer processes are found to be in the nanosecond time-scale. Thes e constants were obtained between 228 and 355 K by analysis of the longitud inal 9.12 MHz N-15 T-1 relaxation times, which exhibit a minimum around 280 K. The relaxation analysis was performed in terms of a quasi-degenerate tw o-state proton transfer process which modulates the heteronuclear H-1-N-15 dipole-dipole interaction. From the value of T-1 in the minimum, the crysta llographic NN distance of 2.63 Angstrom and the hydrogen bond correlation f or N--H ... N hydrogen bonded systems, the two N ... H distances of 1.10 an d 1.60 Angstrom were obtained, i.e. a hydrogen bond angle of 152 degrees, w hich are significantly different from the corresponding values of 1.03 and 2.28 Angstrom and 116 degrees found for porphyrin. The analysis of the temp erature dependence of the rate constants indicates tunneling as a major rea ction pathway, involving a barrier of about 32 kJ mol(-1). The finding of a larger NH distance and a smaller barrier for proton transfer as compared w ith porphyrin is rationalized in terms of the stronger intramolecular hydro gen bonds in porphycene. A strong coupling between these bonds would indica te that the proton tautomerism in porphycene corresponds to a correlated do uble proton transfer, in contrast to the stepwise transfer in porphyrin. Fi nally, a relation between the intrinsic N-15 chemical shifts of porphyrinoi ds and the N ... H distance was found, which might be useful for estimating geometries of porphyrinoids. Copyright (C) 2000 John Wiley & Sons, Ltd.