Density functional calculations on transition metal compounds: From metal tris-acetylacetonates to the active site of cytochrome P450cam.

摘要

The geometries and infrared spectra of the trivalent metal acetylacetonate complexes, M[O₂C₅H₇] ₃, (M = Sc, Ti, V, Cr, Mn, Fe, Co, Al) have been calculated using non-local hybrid density functional theory (DFT) with a split-valence plus polarization basis for the ligand and valence triple-zeta for the metal. These molecules are uncharged, which facilitates the calculations, but at the same time are fairly ionic, resembling biologically important metal complexes with “hard” ligands (O, N). DFT has been widely used to model such complexes, but very few rigorous comparisons have been performed for experimentally well-characterized model compounds. Vibrational spectra are very sensitive to molecular structure and thus constitute an adequate test of the theory. After a mild scaling correction, the calculated frequencies are in excellent agreement with the experimental fundamentals while the predicted infrared intensities are qualitatively correct. The results allow an unambiguous assignment of the observed infrared spectra; some earlier assignments have been revised. Our results show that current routine techniques can predict accurate vibrational spectra for this class of compounds. Fe-, Cr-, Sc- and Al-complexes are high-spin D₃ complexes that are expected to present no Jahn-Teller distortion. Ti-, V-, Mn-, and Co-trisacetylacetonates have ground states that may distort from D₃ symmetry. Fundamentals are assigned and one experimental band is further investigated. Correlating predicted infrared spectra with experiment should lead to firm structural predictions in these difficult systems; To extend the application of DFT to study larger systems of biological interest, the active site of the ferrous dioxygen species and the putative transient reduced ferrous dioxygen species in Cytochrome P450cam are modeled. The structures, energies, and vibrational frequencies of the feasible spin states of these models have been investigated using B3LYP method with a good quality basis set. For the ferrous dioxygen species, theoretical predictions are compared with available experimental Resonance Raman spectra and X-ray Crystallography data as well as with previous theoretical results. The ground state of the ferrous dioxygen species is found, experimentally, to be the diamagnetic singlet. As this state has a strong open shell character, its energy was obtained by projecting out the triplet component from the Unrestricted DFT description of the open-shell singlet (antiferromagnetic) wavefunction. There have been no X-ray structures reported for the reduced ferrous dioxygen species. Because of its transient nature, the effect of the second electron reduction of the ferrous dioxygen heme site on the structure and spin state of the resulting reduced oxyferrous species, is unknown. The calculated ground state of the reduced ferrous dioxygen species is a low-spin doublet state, which agrees with reported ESR experiments. The structural effects of the second electron addition are discussed. The effects of the trans (axial) ligand on the Fe-O distance were further investigated on a slightly smaller model for which five- and six-coordinated iron (II) porphyrin complexes with CO, NO, and O₂ are revisited. There is good agreement found with the experimental data for the cases when an adequate basis set is used and the right spin states are calculated.