Electron correlation in tetrapyrroles: ab initio calculations on porphyrin and the tautomers of chlorin

Abstract

Ab initio calculations including geometry optimization at the Hartree-Fock, MP2, and LDF levels have been carried out for free-base porphyrin and for the tautomers of free-base chlorin, using polarized basis sets. TheHartree-Fock approximation artificially favors frozen resonance structures with alternating single and double bonds for these compounds. This incorrect behavior is completely reversed when correlation effects are accounted for. Correct, delocalized structures of tetrapyrroles are obtained with the MP2 and LDF levels of approximation.

Conformational control of cofactors in nature – the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

“…It has long been known that some form of treatment of the correlation energy is necessary to provide adequate description of the first-row transition metal complexes due to the near degeneracy of the 4s and 4p orbitals as well as amongst the 3d orbitals. 58 Moreover, larger errors typically result from uncorrelated calculations upon metals with incompletely filled d-shells to the right of the d-block (i.e., FeCu) 59 and porphyrins also display significant electronic correlation, 60 as a result of the near degeneracies of the frontier orbitals and the many electrons that are present in the delocalized π-system. For example, uncorrelated HF optimizations of porphyrin were shown to yield structures with severe framework bond length alternation or "frozen resonance forms" of C 2v symmetry, whereas inclusion of electron correlation via MP2 or LDF optimization provided the correct D 2h structure; notably, the severity of the bond-length alternation in the HF converged structure increased with larger basis sets.…”

“…For example, uncorrelated HF optimizations of porphyrin were shown to yield structures with severe framework bond length alternation or "frozen resonance forms" of C 2v symmetry, whereas inclusion of electron correlation via MP2 or LDF optimization provided the correct D 2h structure; notably, the severity of the bond-length alternation in the HF converged structure increased with larger basis sets. 60 The comparisons were also relevant to chlorin and moreover, vibrational calculations of chlorin implied that it is more flexible with respect to out-of-plane distortions than porphyrin, with lowenergy vibrational energies for the sad and ruf modes plus an even softer mode involving a localized torsion of the saturated C b -C b bond. 60 Since then, Density Functional Theory (DFT) has found significant application in the modelling of biologically relevant porphyrin and related compounds, 61 and the efficient way in which DFT explicitly incorporates electron correlation within its theoretical formalism is certainly a major contributor to this success.…”

“…60 The comparisons were also relevant to chlorin and moreover, vibrational calculations of chlorin implied that it is more flexible with respect to out-of-plane distortions than porphyrin, with lowenergy vibrational energies for the sad and ruf modes plus an even softer mode involving a localized torsion of the saturated C b -C b bond. 60 Since then, Density Functional Theory (DFT) has found significant application in the modelling of biologically relevant porphyrin and related compounds, 61 and the efficient way in which DFT explicitly incorporates electron correlation within its theoretical formalism is certainly a major contributor to this success.…”

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Tetraoxaporphyrin dication, the oxygen analogue of porphyrin, has the highly symmetrical D 4h structure [272,273] which may be modulated through five redox stages [274]. The structural parameters of the three molecular species have been the object of a large number of computational studies all of which indicate that the stable forms of porphyrin and tetraoxaporphyrin have D 2h and D 4h geometries, respectively, when electron correlation is taken into account [275][276][277][278][279][280][281][282]. The experimental [272,283,284] and calculated [275] bondlengths of the three macrocycles are reported in Figure 45.…”

Structure and binding energies of the porphine dimer

“…It has D 2h symmetry [6] which has been confirmed with theoretical methods including electron correlation treatment. Hartree-Fock calculations predict a C 2v symmetric structure [7,8]. It is found less often in nature than its metalated derivatives, e.g.…”