Theoretical Comparative Study of Free Base Porphyrin, Chlorin, Bacteriochlorin, and Isobacteriochlorin: Evaluation of the Potential Roles of ab Initio Hartree−Fock and Density Functional Theories in Hydroporphyrin Chemistry

Abstract

Local density functional (LDF) and ab initio Hartree-Fock (HF) calculations are reported for free base porphyrin, chlorin, bacteriochlorin, and isobacteriochlorin. The LDF optimized geometries provide a detailed picture of the structural effects of peripheral hydrogenation on the Cα−Cβ and Cβ−Cβ bond distances, the Cα−Cβ−Cβ and Cα−N−Cα angles, and the size of the central metal-binding cavity of the macrocycles. The optimized structures of the isobacteriochlorin tautomers are of particular interest. The structure of the cis tautomer bears a close resemblance to cis-porphyrin, an intermediate in the double-proton migration of free base porphyrins. Since the minimum-energy points of tetrapyrrole potential energy surfaces at the HF level correspond to unrealistic frozen resonance forms with alternating single and double bonds, the delocalized LDF optimized geometry of the low-symmetry trans-isobacteriochlorin provides a good example of the advantages of density functional theory (DFT) over HF theory in tetrapyrrole geometry optimizations. The latter structure also provides a good illustration of the structural effects of Cβ−Cβ hydrogenation, especially on the Cα−N−Cα angles. The LDF valence ionization potentials (IPs) of the hydroporphyrins, whose absolute values are expected to be in near-quantitative agreement with true experimental values, can serve as a valuable substitute for nonexistent photoelectron spectroscopic data. The lowest LDF and HF IPs of the tetrapyrroles, which decrease with increasing peripheral saturation, correlate well with electrochemical data. LDF calculations on cationic states of different symmetries indicate that the two lowest IPs of hydroporphyrins should lie within approximately 0.5 eV of each other and the third IP should exceed the second by 1 eV or more. Thus, the four-orbital model of porphyrin electronic structure applies reasonably well to hydroporphyrin IPs and photoelectron spectra. LDF equivalent-core calculations provide a detailed picture of molecular charge distributions of hydroporphyrins, key aspects of which are in excellent agreement with available X-ray photoelectron spectroscopic (XPS) data. In contrast, HF theory, which has provided a good description of the XPS of a number of porphyrinic molecules, yields rather poor molecular charge distributions for hydroporphyrins. Overall, in quantitative studies of both structural and electronic properties of hydroporphyrins, HF theory performs rather poorly or even unacceptably. In contrast, the performance of DFT was excellent in every way.