Isomerization pathway of the aluminum monocarbonyl/isocarbonyl pair, AlCO/AlOC: Evidence of a cyclic minimum

Abstract

The isomerization pathway between AlOC and AlCO has been explored at the self-consistent field, configuration interaction, and coupled-cluster levels of theory. Five stationary points on the Al+CO potential energy surface were located and show that the path of Al migration from the isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barrier to the AlCO isomer. A quantitative analysis of the relative stabilities of the isomers as well as the ZPVE-corrected isomerization barriers are presented and compared to the boron carbonyl analogs. At the coupled-cluster level with single, double, and perturbatively applied connected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the cyclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 kcal/mol more stable than AlOC. The barriers from AlOC to the cyclic isomer and to the dissociation products P2 Al and X 1Σ+ CO are only 3.5 and 1.0 kcal/mol, respectively, and leave the tentative experimental observation of AlOC in doubt. On the other hand, the cyclic structure lies in a substantial well with barriers of 19.4 and 14.9 kcal/mol to AlCO and AlOC, respectively. The barrier to Al+CO from the cyclic isomer is estimated to be near 2.5 kcal/mol. The C–O harmonic stretching frequency of the cyclic isomer at this level is predicted to be 1605 cm−1 and provides a guide for the possible experimental observation of this species.