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)