The optimized geometries, infrared spectra and chemical thermodynamics of (HAlNH)~n(n=1～6) clusters in gas phase were investigated using ab initio calculations at HF/6-31G level and density function theory (DFT) at B3LYP/6-31G level. The processes of oligomerization (HAlNH)~n were discussed. The ground state geometric structures of (HAlNH)~n (n=1～6) clusters are C~s(n=1), D~2~h(n=2), D~3~h(n=3), T~d(n=4), C~s(n=5) and D~3~d(n=6) symmetry, respectively. The a(C~s) is the single state with an Al-N triple bond. The Al-n double bonds exist in b(D~2~h), c(D~3~h) and e(D~5~h) structures. For cube e(T~d) and hexagonal f(D~3~d) structures, the Al-N is a single bond. The vibrational spectra data indicated that the structures a～f are equilibrium. The order of stability is as follows: f > d > e > c > b > a. In addition, the structures and thermodynamics of (ClAlNH)~n and (HAlNH)~n clusters were compared.

Key words: GEOMETRICAL ISOMERISM, ALUMINIUM COMPOUNDS, NITROGEN COMPOUNDS