Proton NMR study on the molecular dynamics of solid porphine-d2

Abstract

pulsed NMR techniques were used to investigate the solid-state dynamics of porphine molecules in which the central protons have been replaced by deuterons. Changes observed in the line width of the *H NMR signal suggest that, as happened to be the case with the fully protonated derivative, molecules of deuterated porphine rotate in the crystal. Variable-temperature measurements of the proton relaxation times in the rotating frame provided the activation parameters for this motion, which were found to be slightly higher than those observed for the nondeuterated compound. This effect can be related to a combined motion that has been proposed for reconciling the CPMAS NMR and the X-ray results available for porphine, according to which the solid-state tautomerism of the central hydrogens might produce a reorientation of the molecules in the crystals about their main molecular axes. Upon replacement of the central protons by deuterons this model predicts that kinetic isotope effects known to characterize the N-H tautomerism of porphyrins will slow the rates of macrocycle reorientations in the crystal, a dynamic behavior which is reflected in the experimental results.

Variable-Temperature Solid-State 15N Cross-Polarization Magic-Angle Spinning Nuclear Magnetic Resonance Analysis of Octamethylporphyrin

“…Free-base porphyrins possess two central hydrogens that may bind to any of the four inner nitrogens in the molecule. Exchange among the different possible arrangements results in a tautomeric process (Figure ) that has been extensively studied by variable-temperature solution − and solid-state − NMR. The latter analysis showed that crystal packing forces may disturb the equilibrium constant of the tautomerism, yielding exchange processes between nondegenerate states even in the case of symmetrically substituted molecules.…”

Variable-Temperature Solid-State 13C- and 15N-CPMAS NMR Analyses of Alkyl-Substituted Porphycenes

“…In free-base porphyrins the two inner hydrogens can migrate between the four central nitrogens, giving rise to two possible tautomers and therefore to a hydrogen migration process that has been extensively studied by means of solution − and solid-state NMR. − The latter studies revealed the presence of dynamic processes for structures that X-ray diffraction studies presented as motionless solids, a fact that can be attributed in part to the difficulties that X-ray diffraction has for localizing hydrogens . In porphycene 1 the porphyrinoid structure can also give rise to two tautomeric structures since it allows the presence of the NH exchange process 1a ⇌ 1b (Figure ).…”

Non‐uniform sampling: post‐Fourier era of NMR data collection and processing

“…Nuclear magnetic resonance experiments can be accelerated by designing new pulse sequences or even spectrometer hardware. Examples include spatial encoding of spectral dimensions in single‐scan spectroscopy, reducing the individual measurement time by speeding up the magnetization recovery after each scan, and observing a small fraction of the frequency space as in the Hadamard spectroscopy . The approach called covariance spectroscopy produces high‐resolution spectra by transferring the high resolution in the direct dimension to the indirect dimensions, and thus reducing the sampling demands in the indirect dimensions and shortening the experiment.…”