J. Phys. Chem. 1992, 96, 6914-6978
6974
A Theoretical Approach to the Molecular Structure and Vlbratlonal Spectrum of the AICpH,, Complex from CASSCF and UHF Second-Order Perturbation Calculations Javier Femhdez Sanz,* Antonio Mlrquez, and Julio Anguiano Department of Physical Chemistry, Faculty of Chemistry, University of Sevilla, E-4101 2 Sevilla, Spain (Received: March 2, 1992)
Ab initio quantum mechanical calculations have been carried out on the molecular structure and vibrational spectra of the AIC2H4complex. Equilibrium geometries, harmonic force fields, and frequencies have been calculated by using a double-f basis set with polarization functions on the heavy atoms at CASSCF and UHF second-order Moller-Plesset levels. The preferred type of the aluminum-ethylene bonding, r-bonded (C,) or a-bonded (C8),depends dramatically on the wave function being correlated or not, and our calculations show that the effect of the electron correlation is of nondynamical character. e a l s the weakness of the AI-C bond while the lowering of the force constant The low value found for the force constant FAH:m associated with C-C stretching shows that the ethylenic C-C bond is considerably weakened upon complexation. The calculated frequencies both for AICzH4and its isotopic derivatives agree reasonably well with the experiment.
1. Introduction
Elementary complexes formed from one organic ligand and one metal, such as AlCzH4, constitute an exciting field nowadays since they are real models of organometallic bonds. Since it was first prepared by Kasai and McLeod’ in 1975, the electronic and molecular structure of AlCzH4 has been the subject of considerable effort, both theoretical and experimental. Following Kasais’s analysis,2 the Al-ethylene adduct is formed by the interaction between an unpaired electron residing in an Al py atomic orbital and the s* antibonding molecular orbital of ethylene. This scheme leads to a complex (2B2electronic state) in which the A1 atom binds symmetrically to the ethylene molecule and accounts for the observed properties of its ESR spectrum.
1
Recent experimental work has shown this complex to be stable in hydrocarbon matrices at room t e m p e r a t ~ r e ”and ~ to have a binding energy of about 16 kcal/mol? More recently, Manceron and Andrew# (MA) recorded the infrared spectrum of AlCzH4 in solid argon. The vibrational analysis carried out was in agreement with a C , structure. Preliminary theoretical studies were not successful at interpreting either the stability of this complex or its geometric structure. Thus, in the r-adduct described above, ab initio calculations predicted the A1 atom to be weakly bonded to the ethylene (
