A THEORETICAL INVESTIGATION OF THE ALUMINUM BENZENE COMPLEX

Five optimal structures for the aluminum-benzene complex have been determined theoretically: (1) Al centered (C6v), (2) Al-centered boat (C2v), (3) Al on-top (C(s)), (4) Al sigma-bridging (C(s)), and (5) Al pi-bridging (C(s)). Structures 4 and 2 are most likely the forms of the complex observed in low-temperature electron spin resonance (ESR) experiments. Both structures involve a boat distortion of benzene and Al sigma-bonds with the benzene out-of-plane para C atoms. We find little evidence for a stable Al pi-complex. The high-temperature Al-benzene ESR spectra can be explained by the low distortion energy for the benzene boat. Structure 3 with Al bonding on top of a single C atom is also interesting since it may be prototypical for the chemisorption interaction of an Al atom on graphite. The optimized structures were obtained from Hartree-Fock calculations using a Dunning-Hay basis set.