Porphine force field: in-plane normal modes of free-base porphine; comparison with metalloporphines and structural implications

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Abstract

A valence force field for free-base porphine (H₂P) is developed from a recently reported Ni-porphine (NiP) force field (Li, X.-Y.; et al. J. Phys. Chem. 1990, 94, 31-47). It allows for force constant adjustments related to bond length changes indicated by X-ray diffraction studies. All fundamental frequencies of natural abundance H₂P and its d₂ (N-H/D), d₄ (meso H/D), d₈ (pyrrole C_β-H/D) and d₁₂ (meso + pyrrole H/D) isotopomers are calculated and are in very good agreement with experimental data Calculations for isotopomers with 15N substitution at two pairs of opposite pyrroles are carried out to examine the coordinate mixing between two types of pyrroles. It is found that the nature of the modes calculated for metalloporphine (MP) is well preseved in H₂P, so that the local-coordinate description of the normal modes, as proposed for MP, is also applicable to H₂P. However, certain concerted pyrrole vibrations are localized on only one type of pyrrole, either protonated (pyrrole-like) or deprotonated (pyrrolenine-like). This differentiation disappears in MP due to the geometrical "squaring-up" effect upon metal coordination, as evidenced by X-ray diffraction studies. The in-plane N-H deformation coordinate is not localized on only one band in either vibronic or IR spectrum, a conclusion consistent with the observation. A comparison of vibrational fundamentals of the ground electronic state is made between several metalloporphines and free-base porphine to explore the effect of metal coordination on the force field of the π skeleton. There are primarily three types of metal-sensitive H₂P modes: (a) those that are sensitive to the metal size; (b) those that are sensitive to the metal-ring d-π* back bonding; (c) those that are sensitive to the strength of M-N bonds. The structural origin of so called marker modes is discussed in light of the present and previously reported work. Finally, we remark on the difference between the force field presented in this work and force fields reported in the literature.

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