Carbon-13 nuclear magnetic resonance studies of tricyclopropylaluminum, -gallium, and -indium

Article abstract of DOI:10.1021/om00064a001

¹³C NMR studies have been carried out on tricyclopropylaluminium, -gallium, and -indium. The concentration and temperature dependence of the NMR spectra of the aluminum derivative have been interpreted in terms of a singlely bridged intramolecular bridge-terminal exchange process with activation parameters of E_a^? + 12,4 (2)kcal/mol, ΔH? + 11,7 (2)kcal/mol, and ΔS? -13,1 (1.2) eu. Investigation of the ¹³C spin-lattice relxation times at low temperature and calculation of the relative spinning rates, ρ, of the cyclopropyl groups shows that the bridging cyclopropyl groups and the terminal cyclopropyl groups lying on the same side of the A1-C-A1-C rings rotate at the same rats, while the terminal groups on the opposite side rotate at 5-8 times this rate. This confirms the proposal that the low-temperature averaging process observed is a result of the variations in the rotation rate of the bridging cyclopropyl group and not from freezing the terminal cyclopropyl groups in different configurations. The study of the gallium and indium derivatives shows that their ¹³C chemical shifts are highly dapendent on temperature, concentration, and solvent. The measurement of the temperature and concentration dependence of these chemical shifts leads to the conclusion that these species are involved in an equilibrium between monomer and dimer and that the shift in the relative concentration of these two species leads to the observed changes in chemical shift. On this basis, equilibrium constants and enthalpies of dissociation have been calculated. The enthalpies of dissociation obtained for toluene solutions are 5.6 and 4.0 kcal/mol for the gallium and indium compounds, respectively.