U. Langer et al., N-15 and H-2 NMR relaxation and kinetics of stepwise double proton and deuteron transfer in polycrystalline tetraaza[14]annulene, PHYS CHEM P, 3(8), 2001, pp. 1446-1458
We have studied the dynamics of nano- to picosecond proton transfer process
es 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 measurem
ents 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 sa
mple of polycrystalline doubly deuterated TTAA-d(2). By an analysis of the
temperature dependent isotropic N-15 chemical shifts of the four inequivale
nt N-15 atoms in TTAA evidence was obtained for a network of proton transfe
r steps between two trans-tautomers 1 and 2 and two cis-tautomers 3 and 4 w
hich interconvert by single proton transfers. However, in the temperature r
ange 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 ni
trogen atoms was observed, whereas the two deuterons in TTAA-N-15(4) give r
ise 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 arr
ow 3 reversible arrow 2 was developed and used to convert the relaxation da
ta into the rate constants including the H/D isotope effects of the two rea
ction steps. Some N-15 relaxation data obtained for TTAA at natural N-15 ab
undance allowed us to assign a larger barrier to the reaction step 1 revers
ible arrow 3 and a smaller barrier to the step 3 reversible arrow 2 which d
ominates the longitudinal N-15 and H-2 relaxation. The Arrhenius diagram in
cluding the kinetic isotope effects showed that tunneling is operative at l
ow temperatures. The results are discussed in comparison to those obtained
previously for related intramolecular proton transfers in porphyrin, porphy
cene 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).