Vibrational spectra of cobalt(II), nickel(II), copper(II), zinc(II) Etioporphyrins-II, MN4C32H36

Abstract

The structure and spectral characteristics of metalloporphyrins furnish the information to understand the specific role of these compounds in the functioning of living systems. The IR spectra of Co(II), Ni(II), Cu(II) and Zn(II) etioporphyrins- II (MEP-II) were measured in KBr pellets which were pressed from finely ground KVr mixed with the sample. IR spectra were recorded on Avatar 360-FT-IR ESP spectrometer in the 400-4000 cm-1 frequency range. Molecular geometries and vibrational spectra were calculated by Gaussian 03 program package using PBE and B3LYP functionals. The basis set cc-pVTZ (20s16p8d2f1g/7s6p4d2f1g) on metal atoms and pVTZ on the other atoms (C, N - (10s6p1d/5s3p1d); H - (5s1p/3s1p)) were used. The properties were obtained for the singlet electronic state with closed shells (zinc and nickel etioporphyrins-II) and for the doublet state (copper and cobalt etioporphyrins-II). Differences of structural parameters, calculated by PBE and B3LYP methods, are within 0.011 Å and 0.6° for internuclear distances and valence angles, respectively. The structure of the macroheterocyclic ligand is close to be planar. Slight distortion from planarity is caused by the effect of ethyl groups. Five conformers have been studied for each compound. These conformers differ in the ethyl group position relative to the macroheterocycle plane. Isomerization energies of these conformers do not exceed 0.4 kJ/mol. Rotation barriers of ethyl group are 15.4-15.7 and 20.6-22.6 kJ/mol (B3LYP). For CoEP-II, CuEP-II and ZnEP-II compounds PBE and B3LYP approaches lead to the same conclusion about the geometry of the porphyrine cycle (quasiplanar). In case of NiEP-II, according to PBE calculation, quasiplanar structure was shown to be a saddle point at the potential energy surface and the ruffling distortion configuration (χ(Cα-N···N-Cα) ≈ 22.4°) of the macroheterocyclic ligand corresponds to the minimum. Assignment of vibrational modes was performed by the potential energy distribution analysis using the SHRINK program. Comparison of experimental and calculated IR spectra shows that the most observed bands are the result of the some vibrational modes superposition and cannot be assigned to any single vibrational mode. Positions of bands in simulated spectra are overestimated in comparison with the experimental spectra. Correlation between the bands in the simulated and experimental spectra is close to linear (correlation factor - 0.9999). The analysis of the obtained spectra shows that the frequencies related with moving of the metal atom lie in the 100-200 cm-1 frequency range. But the nature of the metal atom effects on the position bands in the 500 - 650 cm-1 range. These bands correspond to vibrations of the macroheterocycle. Changing the spatial orientation of the ethyl groups has an effect on the position of some bands in the region below than 600 cm-1. The ruffling distortion in NiEP-II leads to the appearance of the band at 250 cm-1 and changes the profile of the band at 200 cm-1. 22 bands are located to be strongly depended on the macrocycle cavity size. Only 7 bands from 22 possess enough intensity in IR spectra to be identified as markers of the macrocycle cavity size. © ISUCT Publishing.