PREDICTION OF 4 EXPERIMENTALLY DISTINGUISHABLE ISOMERS ON THE POTENTIAL-ENERGY SURFACE OF [AL,C-2,H-2](+)

Ab initio molecular orbital theory has been used to investigate the ga s-phase reaction of Al+(S-1) with acetylene((1) Sigma(g)(+)). The ener getics obtained with the refined GAUSSIAN-2 procedure reveal the exist ence of at least four experimentally distinguishable [Al,C-2,H-2](+) i somers on the singlet potential energy surface (PES) with the relative G2 energies in kcal/mol: Al+(HCCH) (1), 0.0; Al+(C(H)CH) (2), 25.9; H -Al+-CCH (3), 8.4; Al+(CCH2) (4), 28.8. Frequency analysis characteriz es them unambiguously to be minimum structures. Additionally, the tran sition-state structures connecting the various minima have been determ ined. A comparison is made with regard to results of matrix studies on the neutral [Al,C-2,H-2] species and related ab initio calculations o n the doublet PES. The differences of the acetylene((1) Sigma(g)(+)) - -> vinylidene((1)A(1)) rearrangement in the presence of Al.(P-2), Al+( S-1) and in the metal-free case are discussed, and it is observed that analogous to the neutral aluminum, the cation catalyzes the isomeriza tion process vinylidene((1)A(1)) --> acetylene((1) Sigma(g)(+)). Addit ionally, oxidative addition of acetylene to the Al+ center was found t o be exothermic by only 6.6 kcal/mol, but it proceeds over a substanti al barrier of at least 59.1 kcal/mol.