Striking similarities between elementary silicon and aluminum compounds: monobridged, dibridged, trans-bent, and vinylidene isomers of aluminum hydride (Al2H2)

Abstract

Ab initio quantum mechanical methods have been used to study the singlet potential energy surface of A12H2. Optimum geometries and harmonic vibrational frequencies were obtained for four geometrical isomers using the self-consistent-field (SCF), configuration interaction (Cl), and coupled cluster (CC) methods. Both correlation methods including single and double excitations (CISD, CCSD) were employed, and all wave functions were determined with both DZP and TZ2P basis sets. Final energy predictions are obtained using large atomic natural orbital basis sets, and including the effects of triple excitations perturbatively using CCSD(T) methods. We found the planar dibridged structure to be the global minimum, as predicted earlier by Baird. However, our analysis also predicts the existence of a remarkable monobridged minimum, which has recently been observed experimentally for Si2H2. Two additional low-lying minima are found, corresponding to trans-bent and vinylidene-like structures. The dibridged, monobridged, and trans-bent structures can be understood as resulting from the three possible ways of coordinating the two electron-rich sites of diatomic A1H to the electron-deficient aluminum centers. The energy of these structures with respect to dissociation to two A1H monomers is quite low and is related to the large difference between the first and second Al-H bond dissociation energies of the parent A1H3 compound.

Unusual Structures of the Parent Molecules Diarsene, Distibene, and Dibismuthene: Toward Their Observation

“…Moreover, monobridged equilibrium structures are not restricted to Si 2 H 2 , Ge 2 H 2 , Sn 2 H 2 , and Pb 2 H 2 . Al 2 H 2 has a monobridged equilibrium geometry at 9 kcal mol −1 above its global minimum [18] . In 1997 it was shown [19] from theoretical vibrational frequencies and infrared intensities that four years earlier Chertihin and Andrews [20] had unknowingly observed this Al 2 H 2 monobridged structure.…”

A Stable Neutral Compound with an Aluminum–Aluminum Double Bond

“…The Al–Al bond length in 4 is 2.3943(16) Å and thus shorter than those theoretically calculated for the NHC-stabilized parent dihydro- and dichloro-dialumenes (2.444 and 2.494 Å, respectively) due to the more σ-electron-donating effect of the silyl groups. It is also notably shorter than the bonds in the theoretically predicted parent dialumene Al 2 H 2 (2.613 Å) and the structurally characterized one-electron π-bonded monoanionic dialuminum compounds A and B (Chart , 2.470(2) and 2.53(1) Å, respectively), and even shorter than in the dianionic compound (Na 2 [Ar′AlAlAr′]) ( C ) (Chart , 2.428(1) Å) . Therefore, compound 4 represents the shortest Al–Al bond reported to date.…”

Homonuclear multiple bonding in heavier main group elements

“…In the lighter species (AlH) 2 and (GaH) 2 the trans-bent form is also calculated to be lower in energy than the linear forms. 143,144 The Ga᎐Ga distance, 2.656 Å is ca. 0.14 Å longer than the normal Ga᎐Ga single bonds given in Table 3, whereas the calculated Al᎐Al bond length is very similar to that of a single bond.…”