15N and 2H NMR relaxation and kinetics of stepwise double proton and deuteron transfer in polycrystalline tetraaza[14]annulene

We have studied the dynamics of nano- to picosecond proton transfer processes in the 15N labeled polycrystalline TTAA molecule (1,8-dihydro-5,7,12,14-tetramethyldibenzo(b,i)-15N4-(1,4,8,11)-tetraazacyclotetra-deca-4,6,11,13-tetraene) by a combination of 9.12 MHz 15N T1 relaxation time measurements under CPMAS conditions (CP≡cross polarization and MAS≡magic angle spinning) and by 46 MHz 2H T1 relaxation time measurements of a static sample of polycrystalline doubly deuterated TTAA-d2. By an analysis of the temperature dependent isotropic 15N chemical shifts of the four inequivalent 15N 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 15N T1 relaxation time for all nitrogen atoms was observed, whereas the two deuterons in TTAA-15N4 give rise to two different 2H T1 relaxation times. An appropriate 15N and 2H relaxation theory in the presence of the reaction sequence 1⇄3⇄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 15N relaxation data obtained for TTAA at natural 15N abundance allowed us to assign a larger barrier to the reaction step 1⇄3 and a smaller barrier to the step 3⇄2 which dominates the longitudinal 15N and 2H 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)-15N4-(1,4,8,11)-tetraazacyclotetra-deca-4,6,11,13- tetraene).