The argon tetroxide molecule, ArO4, and the isoelectronically associated perchlorate, ClO4-, and sulfate, SO42-, ions are investigated on different levels of ab initio theory. The equilibrium structures, harmonic vibrational frequencies, and heats of formation are computed applying density functional theory, second order Moller-Plesset perturbation, singles and doubles coupled-cluster with triples corrections, and Bruekner’s doubles coupled-cluster with triples corrections methods in conjunction with various one-particle basis sets. The calculations demonstrate that the description of the bond characteristics in argon tetroxide is sensitive to the applied level of theory. A careful analysis of the global potential energy surface shows that a stationary point exists for the ArO4 complex corresponding to a local mininium. The calculated equilibrium Ar-O bond distance of 1.48 Angstrom for this structure is slightly longer than the corresponding bond length of the perchlorate ion. Harmonic frequencies for ArO4 obtained using Bruekner’s doubles coupled-cluster with triples corrections are found to have a similar pattern like those obtained for the isoelectronic series of ions SiO44-, PO43-, SO42-, ClO4-. Using the concept of an isodesmic reaction, the enthalpy of formation of ArO4 is determined to be endothermic by as much as 1236 kJ/mol. The present theoretically predicted strong endothermicity and the large Ar-O bond distance are in conflict with the monotonic trends obtained for the isoelectronic ions, but can be supported by other chemical extrapolation schemes.
1999. Vol. 103, no 41, 8295-8302 p.