27Al NMR spectroscopy of triethylaluminium. A direct method to the determination of the proportion of monomer in solution

Abstract

The sole 27Al NMR signal of triethylaluminum (TEA) is shifted significantly to lower field with: (1) decreasing concentration; (2) increasing temperature; and (3) increasing polarity of the solvent; that is, in each case with an increase in the amount of the monomeric form. 27Al NMR chemical shifts of Al 2Et 6 and AlEt 3 are estimated as 153 ± 2 and 265 ± 10 ppm, respectively. By use of these and observed values, thermodynamic data K d, Δ H d and Δ S d were calculated for the dissociation of Al 2Et 6. The dependence of the monomer—dimer equilibria on the concentration of TEA in the solvents used indicated the participation of an “intermolecular” process in the exchange of bridging and terminal ethyl groups not only in aromatic solvents but also, in contrast to previous reports, in aliphatic hydrocarbons.