ABINITIO STUDIES OF OPEN-SHELL COMPLEXES OF CO-GASES( WITH RARE)

The open-shell complexes HeCO+, NeCO+, and ArCO+ have been studied usi ng ab initio methods in both the ground and first excited states. Calc ulations were performed using the 6-311G* and 6-311++G** basis sets w ith unrestricted Hartree-Fock and Moller-Plesset perturbation theory t o fourth order (UMP4). The results predict that all the complexes are bound but that only the Ar species in the ground state is rigid. The m ost stable equilibrium conformation is linear X-CO+ in the ground stat e and T-shaped or linear X-OC+ in the first excited state. Observation s of these complexes by electronic spectroscopy may be difficult becau se of the prediction of a geometry change on excitation. The results f rom several other methods, including restricted open-shell Hartree-Foc k with Moller-Plesset, restricted Moller-Plesset for open shells, and quadratic configuration interaction with singles, doubles, and triples , were compared to check that the UMP calculations give a reasonable d escription of these open-shell molecules.