ISOMERIZATION PATHWAY OF THE ALUMINUM MONOCARBONYL ISOCARBONYL PAIR, ALCO/ALOC - EVIDENCE OF A CYCLIC MINIMUM/

The isomerization pathway between AlOC and AlCO has been explored at t he self-consistent field, configuration interaction, and coupled-clust er levels of theory. Five stationary points on the Al+CO potential ene rgy surface were located and show that the path of Al migration from t he isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barri er to the AlCO isomer. A quantitative analysis of the relative stabili ties of the isomers as well as the ZPVE-corrected isomerization barrie rs are presented and compared to the boron carbonyl analogs. At the co upled-cluster level with single, double, and perturbatively applied co nnected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the c yclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 k cal/mol more stable than AlOC. The barriers from AlOC to the cyclic is omer and to the dissociation products P-2 Al and X (1)Sigma(+) CO are only 3.5 and 1.0 kcal/mol, respectively, and leave the tentative exper imental observation of AlOC in doubt. On the other hand, the cyclic st ructure 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 cyc lic isomer is estimated to be near 2.5 kcal/mol. The C-O harmonic stre tching frequency of the cyclic isomer at this level is predicted to be 1605 cm(-1) and provides a guide for the possible experimental observ ation of this species. (C) 1998 American Institute of Physics.