N-15 and H-2 NMR relaxation and kinetics of stepwise double proton and deuteron transfer in polycrystalline tetraaza14annulene

We have studied the dynamics of nano- to picosecond proton transfer processes in the N-15 labeled polycrystalline TTAA molecule (1,8-dihydro-5,7,12,14-tetramethyldibenzo(b,i)-N-15(4)-(1,4,8,11)-tetraazacyclotetra-deca-4,6,11,13-tetraene) by a combination of 9.12 MHz N-15 T-1 relaxation time measurements under CPMAS conditions (CP = cross polarization and MAS = magic angle spinning) and by 46 MHz H-2 T-1 relaxation time measurements of a static sample of polycrystalline doubly deuterated TTAA-d(2). By an analysis of the temperature dependent isotropic N-15 chemical shifts of the four inequivalent N-15 atoms in TTAA evidence was obtained for a network of proton transfer steps between two trans-tautomers 1 and 2 and two cis-tautomers 3 and 4 which interconvert by single proton transfers. However, in the temperature range between 100 and 400 K tautomer 4 is not formed to an observable extent. Only a single spin diffusion averaged N-15 T-1 relaxation time for all nitrogen atoms was observed, whereas the two deuterons in TTAA-N-15(4) give rise to two different H-2 T-1 relaxation times. An appropriate N-15 and H-2 relaxation theory in the presence of the reaction sequence 1 reversible arrow 3 reversible arrow 2 was developed and used to convert the relaxation data into the rate constants including the H/D isotope effects of the two reaction steps. Some N-15 relaxation data obtained for TTAA at natural N-15 abundance allowed us to assign a larger barrier to the reaction step 1 reversible arrow 3 and a smaller barrier to the step 3 reversible arrow 2 which dominates the longitudinal N-15 and H-2 relaxation. The Arrhenius diagram including the kinetic isotope effects showed that tunneling is operative at low temperatures. The results are discussed in comparison to those obtained previously for related intramolecular proton transfers in porphyrin, porphycene and the related DTAA molecule (1,8-dihydro-6,13-dimethyldibenzo(b,i)-N-15(4)-(1,4,8,11)-tetraazacyclotetra-deca-4,6,11,13- tetraene)

Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates