Ab initio MO studies on the stabilities and binding energies in the complexes of the ions and CO or N 2 molecules; ion = Li +, Na +, Be 2+, and Mg 2+

Abstract

Stable conformations of the ions (Li +, Na +, Be 2+ and Mg 2+) and two CO (or N 2) molecules were surveyed using the Hartree—Fock (HF) MO method with the 3-21G basis set. Relative stability among the conformations was computed by the HF MO and the third-order Møller—Plesset perturbation (MP3) methods with the 6-31G* basis set. An asymmetrical structure (OCXCO), which was predicted to be most stable in the H +(CO) 2 cluster, does not result in a potential minimum and changes to a symmetrical D ∞h conformation. At the highest level of theory in this work (MP3/6-31G*) conformation a (OCXCO), in which both carbon atoms of the CO molecules bond to the ion, is the most stable. The HF MO method failed to predict the most stable conformation; it predicted conformation c, (COXOC), where both oxygen atoms bond to the ion. Stable conformations of the ions (Li + and Be 2+) and three CO (or N 2) molecules were also surveyed by the HF/3-21G method following the HF/6-31G* energy computations. T-shaped structures, which are most stable in the H +(CO) 3 and H +(N 2) 3 clusters, do not show potential minima; symmetrical D 3h conformations were predicted to be the most stable. Thermochemical informations of the clustering reactions at 298.15 K were computed. Enthalpy changes computed with the electron correlation effect were found to be good candidates for the theoretical values since, with this method, the zero-point vibrational-energy corrections are small and nearly cancel the corrections due to the changes of the translational energies, rotational energies and PV work term. Enthalpy changes show a small break after Δ H 2,1 in the clustering reactions of mono-cations (Li + and Na +) and CO (or N 2) molecules, although no distinct shell structures were found.