ALUMINUM-NITROGEN MULTIPLE BONDS IN SMALL ALNH MOLECULES - STRUCTURES AND VIBRATIONAL FREQUENCIES OF ALNH2, ALNH3, AND ALNH4

Ab initio molecular electronic structure methods have been used to study aluminum-nitrogen analogues of the simplest alkyne and alkene molecules and their isomers and the experimentally studied AlNH3. Geometries, relative energies, and harmonic vibrational frequencies of isomers of formulas AlNH2, AlNH3, and AlNH4 were obtained at the self-consistent field (SCF), single and double excitation configuration interaction (CSSD), and coupled cluster including all single and double substitutions (CCSD) levels of theory. The lowest energy isomers were Al-NH2, HAl-NH2, and H2Al-NH2. The HAl-NH isomer is quasi-linear and has a theoretical bond length indicative of a triple bond. It, however, is very high energy, as evidenced by the very large negative heat of hydrogenation, -62 kcal/mol, and the lower energy of the Al-NH2 isomer. The double bond (sigma bond plus N to Al pi donation) is much more favorable. The strength of the Al-N bond depends strongly on the number of hydrogens on the aluminum atom because the hydrogens increase the Lewis acid strength of the aluminum atom. The addition and insertion products of aluminum atoms and ammonia are fully characterized and compared to previous experimental and theoretical studies.