The vibrational spectrum of isolated AlH4−: An infrared matrix isolation and ab initio study

Abstract

An aluminium tetrahydride species has been formed after electronic excitation at about 5 eV of AlH2(H2) van der Waals aggregates isolated in argon matrices. Isotopic studies characterized the five isotopic species (corresponding to −H4, −H3D, −H2D2, −HD3 and −D4 containing molecules) and demonstrated that this species has four equivalent Al–H bonds with a tetrahedral or very nearly tetrahedral structure. Ab initio studies using the coupled-cluster singles and doubles [CCSD (T,E4T)] and density functional theory (DFT) methods confirmed the highly unstable character of an hypothetical neutral AlH4 species as previously established by other workers, which would spontaneously dissociate back into AlH2(H2). On the other hand, the calculated vibrational spectrum of the tetrahydroaluminate anion, AlH−4, would perfectly fit that of the observed species. Identification of the isolated AlH−4 is thus proposed.