Calculated and experimental geometries and infrared spectra of metal tris-acetylacetonates: Vibrational spectroscopy as a probe of molecular structure for ionic complexes. Part I

The geometries and infrared spectra of the trivalent metal trisacetylaceton ate complexes (M[O2C5H7](3)) (M = Sc, Ti, VI Cr, Mn, Fe, Co, Al) have been calculated using nonlocal hybrid density functional theory (DFT) with a spl it-valence plus polarization basis for the ligand and valence triple-zeta f or the metal. These molecules are uncharged, which facilitates the calculat ions, but at the same time are fairly ionic, resembling biologically import ant metal complexes with "hard" ligands (O, N). DFT has been widely used to model such complexes, but very few rigorous comparisons have been per-form ed for experimentally well-characterized model compounds. Vibrational spect ra are very sensitive to molecular structure and thus constitute an adequat e test of the theory. After a mild scaling correction, the calculated frequ encies are in excellent agreement with the experimental fundamentals, and t he predicted infrared intensities are qualitatively correct. The results al low an unambiguous assignment of the observed infrared spectra; some earlie r assignments have been revised. Our results show that current routine theo retical techniques can predict accurate vibrational spectra for this class of compounds. In part I we focus on Fe, Cr, Sc, and Al tris-acetylacetonate s; these are high-spin Dg complexes that are expected to present no Jahn-Te ller distortion. (Ti, V, Mn, and Co tris-acetylacetonates are treated in pa rt II.) Correlating calculated infrared spectra with experiment should lead to firm structural predictions in these difficult systems.