Dimerization versus complexation of triethylaluminum and diethylaluminum chloride:: an ab initio determination of structures, energies of formation, and vibrational spectra

Ab initio Hartree-Fock, density functional and Moller-Plesset schemes are adopted to characterize the geometrical structures, energies of formation and infrared spectra of dimers of diethylaluminum chlorine (DEAC) and triethylaluminum (TEAL), and their complexes formed with dimethylether. This is the first theoretical approach which shows that ethyl-bridged aluminum compounds are stable with respect to dissociation; electron correlation effects being large and necessary for assessing the stability of the ethyl bridges. For the dimer of TEAL, the MP2/6-31G* estimate, corrected for the BSSE via the counterpoise scheme and including the ZPVE correction, is in close agreement with experimental data. This study shows that pure TEAL and DEAC are present under the dimeric form but they react with ethers to form more stable co-catalyst/ether complexes. The new infrared bands appearing on the formation of dimers and complexes are discussed. It turns out that combining far-infrared spectroscopy and quantum-chemical calculations can help in identifying uniquely the structure of alkylaluminum chloride dimers and also their possible complexation by ethers. (C) 1998 Elsevier Science B.V. All rights reserved.