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

Citation
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
Citations number
63
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
ISSN journal
14639076 → ACNP
Volume
3
Issue
8
Year of publication
2001
Pages
1446 - 1458
Database
ISI
SICI code
1463-9076(2001)3:8<1446:NAHNRA>2.0.ZU;2-G
Abstract
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).