The calculated harmonic frequencies (RMP2(fc)/6-31G*) of the three isomeric alanes Me(3)Al . NH3, Me(3)-Al (NH3)-N-15, and Me(3)Al . ND3 are assigned to the measured IR absorptions of these compounds, matrix isolated in argon at 10 K. The results are compared with the known literature data. The irradiations of the matrix-isolated amine adducts yield aminodimethylalane. The experimental IR data of the three isotopomers Me(2)AlNH(2), Me(2)Al(15)-NH2, and Me(2)AlND(2) agree well with the calculated harmonic frequencies. Elucidated from ab initio calculations, Me(2)AlNH(2) is a C-2v-symmetrical molecule with an AIN bond length of 1.790 Angstrom. This AIN distance is 15% shorter than that of the starting compound Me(3)Al . NH3 (2.106 Angstrom). The predicted AIN rotation barrier of 40.7 kJ mol(-1) is much lower than the known barriers of the isoelectronic silaalkenes and the lighter congeners, aminoboranes and alkenes, respectively.