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High resolution HI and B11 magnetic resonance spectra are presented for liquid ALB3H12 and highly deuterium-substituted derivatives. In the case of H1 spectra the technique of double resonance is also employed, with saturation of either the B11 or A127 resonance. It has been found, using such spectra for identification, that at moderate temperatures AlB3H12 undergoes reversible dissociation into B2H6 and a hitherto unrecognized compound, described as A12B4H18. The equilibrium is characterized by an extraordinarily large standard entropy change. The nuclear magnetic resonance spectra of the new substance have also been studied.It is concluded that AlB3H12 is characterized by a bridge bond structure, analogous to that of B2H6, but that a dynamic process renders bridge and terminal protons indistinguishable. Evidence is offered in support of the view that a quantum-mechanical tunnel effect is involved. The structure assigned to A12B4H18 has similar features, but it is concluded that proton tunnelling and actual rotation of borohydride groups are in operation.

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