Theoretical study of the NH tautomerism in free base porphyrin

NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused Structure Delays the Proton Transfer

We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H₂QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with ¹H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two nonfused pyrroles, and the NH tautomerism of 3 was evidenced by variable-temperature (VT) ¹H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (ΔS^‡ = 89 J mol^-1 K^-1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state, as evidenced by the crystal structure and NMR measurements. On the basis of the kinetic studies and density functional theory calculations, the stability of intermediates in the NH tautomerism of 3 and the transition states have been discussed in detail.

Profiling energetics and spectroscopic signatures in prototropic tautomers of asymmetric phthalocyanine analogues

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to explain discrepancies in UV-vis and MCD spectra of the metal-free tribenzo[b,g,l]thiopheno[3,4-q]porphyrazine (1), substituted tribenzo[b,g,l]porphyrazine (2), and 2,3-bis(methylcarboxyl)phthalocyanine (3). On the basis of gas-phase and polarized continuum solvation model (PCM) DFT and TDDFT calculations, it was suggested that both NH tautomers contribute to the spectroscopic signature of 1, whereas the Q-band region of 2 and 3 is dominated by a single NH tautomer. For all tested compounds, it was found that the combination of the BP86 exchange-correlation functional, 6-31G(d) basis set, and TDDFT-PCM approach provides the best accuracy in energies of the Q(x)- and Q(y)-bands of the individual NH tautomers as well as correctly describes their relative energy differences, which are important in understanding of experimental spectroscopy of the target systems.

Theoretical Studies of Nuclear Magnetic Resonance Parameters for the Proton-Exchange Pathways in Porphyrin and Porphycene

The nuclear magnetic resonance (NMR) parameters in porphyrin and porphycene have been calculated to investigate their changes during the process of proton exchange, using density-functional theory (DFT) for both the spin-spin coupling constants and the shielding constants. In addition, in calculations on the smaller 1,3-bis(arylimino)isoindoline molecule, we have tested the performance of our computational approach against experimental data. The calculated nuclear spin-spin coupling constants and shielding constants have been analyzed as functions of the progress of the proton transfer between two nitrogen atoms. The one-bond couplings between proton and nitrogen, dominated by the Fermi-contact term, decay steeply as the internuclear distance increases. The small changes in the intramolecular J(HH) coupling between two inner protons are mainly determined by the sum of relatively large spin-orbit terms. The isotropic shielding constant shows a strong deshielding of the nitrogen nuclei as the proton migrates away. Both the isotropic shielding of the exchanged protons and the shielding anisotropy exhibit a minimum close to the transition states.