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

Solid state effects on the electronic structure of H2OEP

“…The molecule was distorted in the spirit of ref. 46, namely, one of the two diagonals of the N-atoms square was stretched by 0.1. All the atoms were rigidly shifted accordingly, so that finally only four C-C bonds within the macrocycle were stretched.…”

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

“…Although the 1 H variable-temperature solution NMR spectra of porphycene did not allow detection of the presence of each of the tautomers, an 15 N−CPMAS NMR study of porphycene 15 showed that although the hydrogen migration process was too fast to be quantified by the changes introduced in the spectral line shapes, the potential energy profile of the reaction in the solid possessed two minima. Moreover, as also happens to be the case for porphyrins, − it was found that the symmetry of the double-minimum potential that can be expected in the cases of symmetrically substituted porphycenes was broken when the molecules go from solution to the solid phase. An explanation for the abnormally high rate of proton migration in porphycenes as compared to porphyrins can be obtained from the X-ray crystal structure of porphycene, according to which in 1 the presence of a short N (1) −N (2‘) distance strongly favors the N (1) −H···N (2‘) interaction.…”

“…In other words, the tautomerism rate is faster than the time scale of 13 C NMR. However, several factors could affect the 13 C−CPMAS spectra in porphyrinoids and influence the spectra, as was the case with porphyrins. , These are the ring current effects due to effective crystal packing, and the presence of 14 N nuclei where the eigenstates of the nuclei are not in general eigenstates of the Zeeman Hamiltonian and therefore preclude the averaging of the dipolar coupling between neighboring 13 C− 14 N nuclei by MAS. This residual coupling may affect the shape of the α-carbon resonances .…”

“…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 ).…”

NMR Spectroscopic Study of Tautomerism in Solution and in the Solid State

“…Solid-state 13 C and 15 N CP-MAS NMR spectra of porphyrin derivatives were studied at different temperatures [125,126]. The N-H exchange could be analyzed from slow to fast exchange, and equilibrium constants K T of N-H tautomerism were calculated [125].…”