Concerning the Crystal Structure of Porphine: A Proton Pulsed and 13C CPMAS NMR Study

Article abstract of DOI:10.1021/ja00200a016

Solid-state NMR techniques were employed in order to study the structure and the dynamics of prophine.The changes observed in the line width of the 1H NMR signal between 173 and 443 K suggest that the porphine macrocycles rotate in the crystals.This was confirmed by recording the 13C CPMAS NMR spectra at different temperatures which showed, in addition to expected coalescence of signals due to the central hydrogens tautomerism, a broadening of the resonances due to overall molecular rotation.These studies, coupled to measurements at different temperatures and fieldsof the relaxation times of the 1H magnetization in the rotating frame, allowed us to obtain activation parametres for the motion which are, within experimental error, equal to those made available by CPMAS NMR for the tautomerism of the central hydrogens.These results suggest an explanation for what seems to be a contradiction between the structure of phorphine observed by X-ray, according to which the central hydrogens are localized in opposite pairs of nitrogens, and the structure observed by CPMAS in which the hydrogen migrate between the four central nitrogens.If it is assumed that the migration of the central hydrogens is coupled to a 90 deg rotation of the molecules, the translational symmetry of the crystal will not be changed by the tautomerism, and an X-ray analysis would always detect a single tautomer.