Hybrid density functional theory, Gaussian, and complete basis set ab initio studies of the stability of aluminum monocarbonyl and aluminum isocarbonyl

Hybrid Density Functional Theory (DFT) computational studies of the complexation between aluminum and carbon monoxide was performed in order to determine its stability for experimental detection and accuracy of DFT computed values with some highly reliable ab initio methods. Four highly accurate ab initio correlation calculations (G1, G2, G2(MP2), and CBS-Q) agree that AlCO is more stable than its isomer AlOC, with a Al-CO bond dissociation energy of about 10.0 kcal/mol. Its energy is also 22.6 kcal/mol lower than the energy of AlOC. The hybrid DFT methods (B3LYP, B3P86, and B3PW91) agree quite well with the CBS-Q ab initio calculations for AlCO, but they are unable to find a minimum for the AlOC complex on the potential energy curve for the Al-OC dissociation. This failure is attributed to the known problem of DFT methods to compute reaction barriers for reactions with low reaction barriers, which is the case for the AI-OC bond dissociation. The CBS-Q estimated the barrier to be 6.2 kcal/mol. There is also a substantial problem inherent in the G1, G2, G2(MP2), and CBS-Q ab initio correlational approaches to study complexes for which HF calculations cannot find a minimum such as for AlCO. It is also interesting to point out that MP2 is incapable of bringing Al and CO together into a complex with an similar to 2.1 Angstrom Al-CO bond distance since they are separated by 3.6 Angstrom. Nevertheless, both ab initio and DFT results agree that AlCO should be experimentally observable.