Bernd Wehrle

CPMAS Polarization Transfer Methods for Superposed Chemical Exchange and Spin Diffusion in Organic Solids

Abstract The question of how CPMAS polarization-transfer experiments (CP, cross polarization; MAS, magic-angle spinning) should be conducted in order to distinguish between slow chemical exchange and spin diffusion in the solid state has been studied. Both contributions can be separated by performing different types of polarization-transfer experiments in the laboratory and the rotating frame, since dynamics of spin diffusion but not chemical exchange differs from one experiment to the next. Generally, if both processes are present, polarization transfer is expected to be nonexponential and chemical-exchange as well as spin-diffusion rate constants can be obtained in one series of experiments. If the exchange is symmetric, however, polarization transfer is single exponential and a combination of different pulse experiments is required for obtaining rate constants of both processes. The results for 15N CPMAS NMR polarization-transfer experiments on crystalline meso-tetratolylporphin-15N4 (TTP) are presented. Experiments in the laboratory frame show that spin diffusion between the 15N atoms of TTP is characterized by a temperature-independent rate constant. The nature of this process was established by 1H decoupling during the mixing time, which results in quenching of the polarization transfer. Thus, the role of the 1H spin reservoir for laboratory-frame spin diffusion among chemically inequivalent 15N spins in 15N-enriched material is confirmed. At higher temperatures, polarization transfer in the laboratory and the rotating frame is observed due to a symmetric exchange of the nitrogen atoms arising from a double proton transfer which has been previously established. The double proton transfer rates observed with the different polarization-transfer methods agree well with the values predicted from high-temperature lineshape analysis and are found to be very close to the solution data.

15N NMR study of proton localization and proton transfer thermodynamics and kinetics in polycrystalline porphycene

Using high-resolution solid-state 15N cross-polarization magic angle spinning NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 15N-labeled porphycene were studied. Whereas at room temperature only a single 15N resonance is observed, indicating an equivalence of all nitrogen atoms arising from a quasi-degenerate fast proton transfer, four signals are observed at low temperatures, exhibiting temperature-dependent line positions. Their analysis is consistent with the presence of either (i) two different molecules A and B in the asymmetric unit, each of which is subject to a quasi-degenerate correlated double proton transfer, or (ii) a single molecule exhibiting all four possible near-degenerate tautomeric states, two trans- and two cis-tautomers, interconverting by fast single proton transfers. The average rate constants of the proton transfer processes are found to be in the nanosecond time-scale. These constants were obtained between 228 and 355K by analysis of the longitudinal 9.12 MHz 15N T1 relaxation times, which exhibit a minimum around 280 K. The relaxation analysis was performed in terms of a quasi-degenerate two-state proton transfer process which modulates the heteronuclear 1H–15N dipole–dipole interaction. From the value of T1 in the minimum, the crystallographic NN distance of 2.63 A and the hydrogen bond correlation for N—H···N hydrogen bonded systems, the two N···H distances of 1.10 and 1.60 A were obtained, i.e. a hydrogen bond angle of 152°, which are significantly different from the corresponding values of 1.03 and 2.28 A and 116° found for porphyrin. The analysis of the temperature dependence of the rate constants indicates tunneling as a major reaction pathway, involving a barrier of about 32 kJ mol−1. The finding of a larger NH distance and a smaller barrier for proton transfer as compared with porphyrin is rationalized in terms of the stronger intramolecular hydrogen bonds in porphycene. A strong coupling between these bonds would indicate that the proton tautomerism in porphycene corresponds to a correlated double proton transfer, in contrast to the stepwise transfer in porphyrin. Finally, a relation between the intrinsic 15N chemical shifts of porphyrinoids and the N···H distance was found, which might be useful for estimating geometries of porphyrinoids. Copyright

A novel is N chemical-shift NMR thermometer for magic angle spinning experiments

Variable temperature CPMAS NMR spectroscopy (cross polarization magic angle spinning) has become a valuable tool for elucidating molecular structure and dynamics in the solid state ( 1, 2). Precise temperature measurement is of prime importance for the success of this method. Generally, it is assumed that the sample temperature is equal to the temperature of the inlet sample driving and bearing gas and that there is no temperature gradient across the sample. This assumption is justified when small rotors are employed and when the thermometer is placed very close to the sample. In other cases this procedure may, however, lead to temperature calibration errors (3). Therefore, the indirect measurement of temperature via the observation of temperature-induced spectral changes due to solid-solid phase transitions or temperaturedependent chemical shifts is often more reliable. Whereas the use of a phase-transition thermometer is suitable for calibrating a conventional thermometer at a given phase-transition temperature (4)) chemical-shift NMR thermometers can be employed in a large temperature range. For 13C CPMAS NMR measurements a thermometer based on the Curie law behavior of the 13C chemical shifts of paramagnetic samarium acetate tetrahydrate (SAT) has been proposed (5, 6). However, an unfavorable ratio, for this material, of temperature-induced line shifts to the signal linewidth, especially in and above room temperature, make the development of improved chemical-shift thermometers for MAS studies desirable. We present here a “N CPMAS chemical-shift thermometer with a resolution of about +2 K, based on the tautomerism of an organic dye molecule TTAA, ’ shown in Fig. 1. The reaction network of Fig. 1 has been established previously ( 7, 8). Because of the low natural abundance and sensitivity of the 15N nucleus, TTAA was artifically enriched to about 95% with 15N, as has been described recently (8). The “N CPMAS spectra were measured using an electromagnet with a magnetic field of 2.1 T corresponding to a “N Larmor frequency of 9.12 MHz and a 5 mm Doty MAS probe (9) together with a homebuilt heat exchanger (10). The corresponding temperatures were measured with a Pt resistance thermometer (dimensions: 2.3 X 2.1 mm, Degussa), which was positioned approximately 3 mm from RF coil in the

A strategem for low-temperature magic-angle spinning using nitrogen spinning gas

Although the lowest temperatures in magic-angle-spinning (MAS) experiments have been reached with helium as the spinning gas (I), it is clear that, between ambient temperature and 77 K, nitrogen gas is preferable in terms of both cost and availability. Since the pressure of the spinning gas must be greater than 1 atm, condensation of gaseous nitrogen inside a liquid-nitrogen-cooled heat exchanger is unavoidable when the liquid nitrogen is kept at atmospheric pressure (2). This can lead to instabilities of the gas flow and unstable sample spinning. Condensation can, however, be avoided if the liquid-nitrogen bath is operated at an equilibrium pressure greater than that of the spinning gas. As shown in Fig. 1, this idea can be realized simply by incorporating the heat exchanger (H) into a pressurized liquid-nitrogen Dewar (G). The bath is first allowed to equilibrate at the pressure built up by a liquid-nitrogen fill, and this pressure is determined and maintained by means of a safety valve (E). To prevent ice formation on this valve, and also to conserve liquid nitrogen, a counterflow heat exchanger (F), in which the incoming gas is precooled by the exhaust gas, has been incorporated. A bypass (B) was added for mixing lowand ambient-temperature gas for intermediate temperature work. A conventional control system incorporating a thermocouple and heater was employed for fine temperature control. This method has worked successfully with a variety of magic-angle spinners (3). The spinning gas temperature which can be achieved may be calculated from the variation of the vapor pressure of liquid nitrogen in the vicinity of 77 K (2) e.g., 84 K at a bath pressure of 2 atm. Depending on heat losses in the gas transfer line and probe, the sample temperature will be somewhat higher. This is especially true if there is not enough space available for adequate thermal insulation. Using a 5 mm doublebearing MAS probe (36) in a 2.2 cm gap electromagnet, stable operation for several hours at 153 K sample temperature was achieved with spinning rates of 2 kHz. The apparatus can operate at 153 K for 24 h with 100 liter liquid-nitrogen capacity.

NMR STUDY OF ENVIRONMENT MODULATED PROTON TAUTOMERISM IN CRYSTALLINE AND AMORPHOUS PHTHALOCYANINE

Abstract Using high resolution 15 N and 13 C CPMAS NMR spectroscopy (CP - cross polarization, MAS - magic angle spinning) we have detected fast thermally activated proton transfer processes in solid 15 N enriched phthalocyanine (Pc). The synthesis of the latter is described. NMR experiments were performed on the crystalline α- and β-modifications, as well as on a novel amorphous modification (am-Pc). In order to extract thermodynamic and kinetic data from the NMR spectra an appropriate lineshape theory of bistable molecules in ordered and disordered matrices is developed. Bistable molecules are subject to exchange between at least two molecular states. The lineshape theory includes the possibility of site dependent perturbations of the rate and equilibrium constants of state exchange, as well as of exchange between different sites. For disordered environments bi-Gaussian distributions of the reaction enthalpies of the state exchange and of the enthalpies of activation are proposed. Different possibilities, including Marcus theory, of reducing this two-dimensional site distribution function to a one-dimensional distribution are discussed. The analysis of the NMR spectra gave the following results. Whereas the proton potential in α-Pc is quasisymmetric, the degeneracy of the tautomers in the β modification is lifted because of a subtle interplay between inter- and intramolecular interactions. The amorphous modification is characterized by a broad distribution of differently perturbed asymmetric double-minimum potentials, as expected for a disordered environment. Rotation of Pc in either of these modifications can be excluded. Proton transfer in the β phase is faster than in the α phase due to smaller energy of activation. This finding is interpreted with a different geometric arrangement of the inner nitrogen atoms in both phases. In addition, the proton transfer in the β phase is characterized by a smaller pre-exponential factor than in the α phase. This effect indicates substantial but different tunnel contributions to the reaction rates in both phases. The implications of the environment modulated proton transfer processes in Pc for the mechanism of hydrogen transfer reactions in liquids is discussed.

SOLID-STATE 15N CPMAS NMR STUDY OF THE STRUCTURE OF POLYPYRROLE

Abstract The chemical structure of polypyrrole (PPy) has been studied by 15 N CPMAS NMR experiments on the 15 N-labeled material. Only one broad line is found at about 120 ppm (external reference 15 NH 4 Cl) which indicates that all nitrogen atoms of PPy are protonated. Thus, PPy is best described as a chain of linked pyrrole molecules. Two-dimensional NMR experiments show that the 15 N line is inhomogeneously broadened. The width of the line is substantially larger in positively charged PPy.

NMR studies of elementary steps of hydrogen transfer in condensed phases

In this paper recent developments and results of dynamic high resolution NMR spectroscopy in the study of elementary steps of proton transfers in liquids and solids are reviewed.

15N CPMAS NMR study of the structure and reactions of polypyrrylenemethine and polyfurylenepyrrylenemethine

Abstract The chemical structure and reactions of the recently prepared conducting polymers polypyrrylenemethine (PPM) (H. Braunling and R. Becker, Ger. Offen. Patent Applic. No. 3 710 657 (Mar. 31, 1987)) and polyfurylenepyrrylenemethine (PFPM) have been studied using high-resolution solid-state 15 N CPMAS NMR spectroscopy of the 15 N-labeled compounds. The results show the presence of azomethine nitrogen atoms which can be protonated with acids and deprotonated again with bases as free base porphyrins. Thus, these polymers have to be regarded, at least partially, as polymer analogs of the corresponding free base porphyrins.

NMR and IR studies of novel multiple proton transfers in liquids, crystals, and organic glasses

Abstract Using high resolution dynamic NMR spectroscopy of liquids and solids we have detected a number of novel symmetrical intra- and intermolecular neutral double proton transfers between nitrogen atoms in 15 N enriched compounds such as porphines, porphycen, phthalocyanine, tetraazaannulenes, azophenine, oxalamidines, formamidines, etc. For some of these reactions full kinetic HH/HD/DD isotope effects have been measured from which information on the elementary steps of these reactions is obtained. Further information on these processes arises from the study of these compounds in the crystalline state or in solid disordered solid solution using variable temperature 15 N CPMAS NMR spectroscopy (CP=cross polarization, MAS=“magic” angle spinning). Generally, we observe that the gas-phase degeneracy of tautomers is lifted in the crystalline state. Whereas in the crystalline state each molecule is located in the same “site” we find a broad distribution of different sites when the molecules are imbedded in an organic glass. Different sites are characterized by a different distortion of the double minimum potential of the proton motion by the environment. Comparative IR studies of azophenine in solution and the solid state confirm this molecular picture.