Calculated and experimental geometries and infrared spectra of metal tris-acetylacetonates: vibrational spectroscopy as a probe of molecular structure for ionic complexes. Part II Irina Diaz-Acosta, Jon Baker , James F. Hinton, Peter Pulay Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701-1201, USA Received 6 March 2002; received in revised form 24 April 2002; accepted 24 April 2002 Abstract Following on from our previous work on Sc, Fe, Cr, and Al (Part I; see J. Phys. Chem. A, 105 (2001) 238), the geometries and infrared spectra of the trivalent metal tris-acetylacetonate complexes (M[O 2 C 5 H 7 ] 3 ;M/Ti, V, Mn, Co) have been studied both experimentally and theoretically using nonlocal hybrid density functional theory with a split- valence plus polarization basis for the ligand and valence triple-j for the metal. Unlike the D 3 complexes studied in Part I, those of Ti, V and Mn are candidates for Jahn-Teller distortion due to fractional d-shell occupancy. Using scale factors transferred from Part I, our calculated frequencies are in very good agreement with experimentally observed fundamentals. Our investigation shows that the V and Mn complexes distort to C 2 ground states, but D 3 Ti tris- acetylacetonate is stable. Further investigation of the weak band observed around 800 cm 1 in the Fe complex (and present in almost all studied first-row transition metal tris-acetylacetonates), which we were unable to assign theoretically in Part I, supports the argument that this band is not a fundamental but is due to Fermi resonance. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Metal tris-acetylacetonates; Vibrational spectra; Scaled quantum mechanical force field; Density functional theory; Jahn- Teller distortion 1. Introduction Metal atoms, especially transition metals, are present at the active catalytic center in almost a third of all known enzymes. Iron and zinc are especially important, and occur in relatively high concentration in the human body (5.4 and 2.4 g per 70 kg, respectively); most of this is involved in enzyme function [1]. Mn, Co, Cu and Mo are also important. These metals are involved in a number of processes, including binding of substrates to orient them properly for subsequent reaction, mediating oxidation /reduction and electron trans- port reactions through reversible changes in the metal oxidation state, and electrostatically stabi- lizing negative charges. Metal (III) tris-acetylace- tonate complexes are particular accessible species for studying bonding, ligand coordination and  Corresponding author. Tel.:  /1-501-575-5080; fax:  /1- 501-575-4049 E-mail address: baker@comp.uark.edu (J. Baker). Spectrochimica Acta Part A 59 (2003) 363  /377 www.elsevier.com/locate/saa 1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S1386-1425(02)00166-X