Theoretical study of the formation and isomerization of Al2H2

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The lowest singlet and triplet potential energy surfaces of Al2H2 have been characterized using a full-valence complete active space self-consistent field (CASSCF) wavefunction. The CASSCF geometries of minima on the singlet potential energy surface are compared to those previously reported using density functional theory (DFT) and coupled cluster (CCSD(T)) methods. Energies at the CASSCF geometries are corrected for dynamic correlation effects using multi-reference second-order perturbation theory (MRMP2) and CCSD(T). Relative energies calculated at the MRMP2//CASSCF level are comparable to those evaluated at CCSD(T) optimized geometries. This approach to characterizing stationary points and calculating relative energies is utilized to describe isomerization pathways between minima on the lowest singlet and triplet surfaces and to characterize pathways leading to formation of bound Al2H2 species from various fragments (e.g., AlH, AlH2, Al2, and H2). The results presented here confirm that the global minimum is the singlet dibridged isomer, with other singlet isomers lying slightly higher in energy. Though not previously analysed, most triplet structures were found to be less than 20 kcal mol−1 higher in energy than their singlet counterparts. A purely attractive singlet reaction channel, involving the insertion of H2 directly into the Al–Al bond of an excited Al2 species, was located and found to be exothermic by about 40 kcal mol−1. Based on energy and frequency analyses of the singlet and triplet surfaces, previous conclusions that only the singlet dibridged and monobridged isomers have been observed in matrix isolation experiments are analysed further.