BONDING, ELECTRONIC, AND VIBRATIONAL ANALYSIS OF THE AL-C2H4 COMPLEX USING DENSITY-FUNCTIONAL THEORY AND TOPOLOGICAL METHOD (ELF)

The density functional theory (DFT) has been used to reexamine the Al- C2H4 complex because of discrepancies between the results of post-Hart ree-Fock methods concerning the binding energy and the ordering of the metal-ligand stretching frequencies. In this study, equilibrium geome try, binding energy, and harmonic frequencies have been calculated usi ng the 6-311G(2d,2p) basis set, It is shown that the Al-C2H4 complex h as a C-2v symmetry equilibrium structure and a B-2(2) ground electroni c state, which is strongly bound by -13.3 kcal/mol after BSSE correcti on (to be compared to the -16 kcal/mol experimental value). The bondin g in the Al-C2H4 complex has been investigated by the electron localiz ation function (ELF). The aluminum-ethylene bonding is found to be mos tly electrostatic. The degree of weakening of the C=C double bond and the ordering of the two metal-carbon stretching modes have been discus sed using a harmonic vibrational and force constant analysis and compa red to the experimental results. Furthermore, a comparison of the shif ting between the two wagging modes for complexed and free ethylene has allowed us, on the basis of isotopic substitutions, to reassign the s ymmetry of the only observed wagging mode (B-2 instead of A(1)). We ha ve also suggested the reassignment of the experimental band reported a t 781 cm(-1) from the B-1 rocking mode to the A(1) symmetric wagging o ne.