'H NMR Study on the Molecular Dynamics of Solid Porphine-d,

IH 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techniques were used to investigate the solid-state dynamics of porphine molecules in which the central protons have been replaced by deuterons.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. Variabletemperature 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