Using high-resolution solid-state 15 N cross-polarization magic angle spinning NMR techniques, the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 15 N-labeled porphycene were studied. Whereas at room temperature only a single 15 N 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 15 N T 1 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 1 H-15 N dipole-dipole interaction. From the value of T 1 in the minimum, the crystallographic NN distance of 2.63 Å and the hydrogen bond correlation for N-HÁÁÁN hydrogen bonded systems, the two NÁÁÁH distances of 1.10 and 1.60 Å were obtained, i.e. a hydrogen bond angle of 152°, which are significantly different from the corresponding values of 1.03 and 2.28 Å 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 À1 . 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 15 N chemical shifts of porphyrinoids and the NÁÁÁH distance was found, which might be useful for estimating geometries of porphyrinoids.
Solid state, variable temperature 15N‐NMR studies make it possible to predict the behavior of the four N atoms and the two internal H atoms in the isomeric porphine and porphycene. The spectra of porphine indicate a statistical disorder of the internal H atoms. In the case of porphycene it can be concluded that there are two non‐equivalent unsymmetric proton transfer systems. Since the NH…N distances are very short, the energy barrier for the rearrangement is very small. Therefore, other than in the case of porphine, in porphycene the interconversion of the tautomers is so rapid that the rate constants for the proton transfer could not be determined by the 15N‐CPMAS‐NMR method.
The liquid and solid state tautomerism of 5,10,15,20‐tetraphenylporphyrin‐15N4 (meso‐tetraphenylporphyrin, TPP) has been studied by dynamic NMR spectroscopy and by NIR spectroscopy. The kinetic HH/HD/DD isotope effects on the liquid state tautomerism were reinvestigated in the temperature region between 200 K and 300 K. It was confirmed that the kinetic HH/HD isotope effects are large and the HD/DD isotope effects small. At 298 K the values 9.7 and 1.8 were obtained. The Arrhenius curve of the HH reaction is non‐linear indicating that the proton transfer takes place by thermally activated tunneling at low temperatures. The kinetic isotope effects are consistent with a stepwise proton transfer pathway involving intermediate tautomeric states in which protons are bound to adjacent nitrogen atoms. Solid state NMR experiments were performed on triclinic TPP, where the reaction barrier is higher compared to TPP in the liquid state. In addition, the degeneracy of the tautomerism in the solid state is lost due to intermolecular interactions. The latter conclusion is supported by the finding that for tetragonal mixed crystals of [(TPP—14N4—Ni)0.1(TPP—15N4—H2)0.9] the degeneracy of the tautomerism and its barrier is equal to the values found for the liquid solution. The NIR measurements show the presence of NH‐stretching overtones in the region of about 6450 cm−1. The anharmonicity of this vibration is of the usual order, which indicates that the reaction coordinate of proton transfer is not identical with the normal modes of the NH‐stretching vibrations, but rather a combination of various vibrational modes including the stretching, bending and skeletal modes. As a consequence, there will be a large number of vibrational states from which tunneling might occur.
N M R 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 TI of heteronuclei located in such a way that their dipolar interaction to the mobile protons is modulated by the transfer process.The T1 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 TI values. Heteronuclear longitudinal relaxation in the presence of proton transfer is described in the theoretical section for the cases of (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 I , 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 casee relaxation is quasi-monoexponential and governed in very good approximation by the same TI values as in case iii. Therefore, rate constants of ultrafast proton transfers can be obtained from CPMAS T1 measurements as long as other relaxation mechanisms are not operative. As an example, a dynamic 15N CPMAS NMR study of polycrystalline dimethyldibenzotetraaza[ 14lannulene (DTAA: 1,8dihydr0-6,13-dimethyldibenzo[b,i]-~~N~-( 1,4,8,1 1)-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 15N 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 lSN T I values of DTAA are dominated by proton-transfer-induced dipolar relaxation.A TI minimum of protonated DTAA was observed around 350 K. Using the theory of case iii, this observation allowed us to convert the TI values measured into rate constants of proton transfer in the milli-to nanosecond time scale and to determine the IH-l5N 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 T1 arising from a proton-transfer-induced modulation of the chemical shift anisotropy (CSA) had to be taken into...
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