HIGH-RESOLUTION SOLID-STATE NMR RELAXOMETRY AS A KINETIC TOOL FOR THE STUDY OF ULTRAFAST PROTON TRANSFERS IN CRYSTALLINE POWERS - DIMETHYLDIBENZOTETRAAZA[14]ANNULENE

NMR relaxometry combined with high-resolution solid-state NMR techniques has been explored as a kinetic tool for the study of ultrafast proton transfers in solids. Rate constants of proton transfer are obtained in the milli- to nanosecond time scale by analysis of the longitudinal spin-lattice relaxation times T-1 of heteronuclei located in such a way that their dipolar interaction to the mobile protons is modulated by the transfer process. The T-1 measurements are facilitated by proton cross-polarization (CP), magic angle spinning (MAS), and proton decoupling during the detection period. In contrast to the study of static powders, the CPMAS method also provides the equilibrium constants of proton transfer necessary to obtain the rate constants from the T-1 values. Heteronuclear longitudinal relaxation in the presence of proton transfer is described in the theoretical section for the cases (i) static powders, (ii) powders rotating at the magic angle, and (iii) powders where longitudinal relaxation is isotropically averaged by magnetization transfer. In case i relaxation is multiexponential and difficult to evaluate. In case iii relaxation is truly exponential and characterized by a single longitudinal relaxation time T-1, related in a straightforward way to the dipolar interaction and the equilibrium and rate constants of proton transfer. This case is, however, difficult to realize experimentally, by contrast to the MAS case ii. As shown theoretically, in this relaxation is quasi-monoexponential and governed in very good approximation by the same T-1 values as in case iii. Therefore, rate constants of ultrafast proton transfers can be obtained from CPMAS T-1 measurements as long as other relaxation mechanisms are not operative. As an example, a dynamic N-15 CPMAS NMR study of polycrystalline dimethyldibenzotetraaza[14]annulene (DTAA: 1,8-dihydro-6,13-dimethyldibenzo[b,i]-N-15(4)-(1,4,8,11)-tetraazacyclotetra-deca-4,6,11,13-tetraene) is presented. As shown previously, DTAA is subject to an intramolecular double proton transfer between two tautomers which are degenerate in the gas phase but inequivalent in the crystalline solid. Longitudinal N-15 relaxation of DTAA under MAS conditions has been monitored at 2.1 and 7 T in a large temperature range and was found to be monoexponential. Deuteration in the mobile proton sites drastically reduced the relaxation rates, proving that the N-15 T-1 values of DTAA are dominated by proton-transfer-induced dipolar relaxation. A T-1 minimum of protonated DTAA was observed around 350 K. Using the theory of case iii, this observation allowed us to convert the T-1 values measured into rate constants of proton transfer in the milli- to nanosecond time scale and to determine the H-1-N-15 distances. The validity of this approach was verified by additional experiments performed on static powders and complete data analyses in terms of cases i and ii. At 7 T a small contribution to T-1 arising from a proton-transfer-induced modulation of the chemical shift anisotropy (CSA) had to be taken into account. In the milli- to microsecond time scale the rate constants obtained by N-15 T-1 analysis agree very well with those obtained by N-15 CPMAS line shape analysis. The resulting Arrhenius curve shows a small deviation from linearity. In conclusion, relaxometry of powdered crystalline or of disordered solids under MAS conditions constitutes a reliable technique for obtaining rate constants of ultrafast proton transfers in organic solids.