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

Abstract

Using high-resolution solid-state 15N cross-polarization magic angle spinning NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 15N-labeled porphycene were studied. Whereas at room temperature only a single 15N resonance is observed, indicating an equivalence of all nitrogen atoms arising from a quasi-degenerate fast proton transfer, four signals are observed at low temperatures, exhibiting temperature-dependent line positions. Their analysis is consistent with the presence of either (i) two different molecules A and B in the asymmetric unit, each of which is subject to a quasi-degenerate correlated double proton transfer, or (ii) a single molecule exhibiting all four possible near-degenerate tautomeric states, two trans- and two cis-tautomers, interconverting by fast single proton transfers. The average rate constants of the proton transfer processes are found to be in the nanosecond time-scale. These constants were obtained between 228 and 355K by analysis of the longitudinal 9.12 MHz 15N T1 relaxation times, which exhibit a minimum around 280 K. The relaxation analysis was performed in terms of a quasi-degenerate two-state proton transfer process which modulates the heteronuclear 1H–15N dipole–dipole interaction. From the value of T1 in the minimum, the crystallographic NN distance of 2.63 A and the hydrogen bond correlation for N—H···N hydrogen bonded systems, the two N···H distances of 1.10 and 1.60 A were obtained, i.e. a hydrogen bond angle of 152°, which are significantly different from the corresponding values of 1.03 and 2.28 A and 116° found for porphyrin. The analysis of the temperature dependence of the rate constants indicates tunneling as a major reaction 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 with porphyrin is rationalized in terms of the stronger intramolecular hydrogen bonds in porphycene. A strong coupling between these bonds would indicate that the proton tautomerism in porphycene corresponds to a correlated double proton transfer, in contrast to the stepwise transfer in porphyrin. Finally, a relation between the intrinsic 15N chemical shifts of porphyrinoids and the N···H distance was found, which might be useful for estimating geometries of porphyrinoids. Copyright © 2000 John Wiley & Sons, Ltd.