The ionization energy of and dissociation energies of KO2 and KO2+
Introduction
Recently, we have performed a number of calculations on alkali metal monoxide [1], [2], [3], [4], [5] and dioxide [6], [7] species, concentrating our efforts in obtaining accurate adiabatic ionization energies (AIEs). In the process of obtaining these quantities we also obtained equilibrium geometries of the neutrals and cations, and as a simple extension, the dissociation energies and heats of formation of the neutrals and cations. These results have been used, in the case of sodium, to model the appearance of sporadic sodium layers in the upper atmosphere [8], [9]. Potassium is also an important atmospheric metallic species [10], and sporadic potassium layers have been observed [11].
Since the MX species are generally largely ionic, they strongly resemble M+X− in the neutral ground state, which is quite strongly bound, because of the Coulomb attraction. The lowest energy ionization process corresponds, essentially, to removal of the negative charge from X−, and so the cation is fairly weakly bound, since now the Coulomb attraction is replaced by a charge/(induced-)dipole interaction.
For the alkali metal monoxides, the M+·O− structure of the neutral species leads to the presence of two low-lying states, corresponding to the two orientations of the `hole' in the occupancy of the 2p orbitals on O− (directed towards the closed-shell M+, or perpendicular to it): these are the and states, respectively. It is a well-known fact that the ground state of LiO and NaO is , but that of the heavier RbO, CsO and FrO is ; the ground electronic state of KO is still controversial (see Introductions of [1], [2], [3], [4], [5]).
For the alkali metal superoxides, MO2, again, the ionic nature of the ground state leads to a M+·O2− species, where again there is a `hole' in the occupancy of the 2pπ* orbitals, which can be either in-plane, or out-of plane, giving rise to or states, respectively. Partridge and co-workers [12], [13] and others [14], [15] have demonstrated that, in contrast to the monoxides, the ground electronic state is the state for LiO2–CsO2.
Previously [6] we employed the CCSD(T) and B3LYP methods, making use of large polarized valence basis sets in combination with effective core potentials (ECPs) to study KO2. Our most reliable geometry in that work indicated that KO2 had a bond angle of 32.3°, and was essentially a K+ cation interacting with an O2− moiety—this was in good agreement with the conclusions of Partridge et al. [12], who used a modified coupled-electron pair (MCPF) approach; and in fair agreement with some earlier UHF calculations by Plane et al. [16]. In the present work, we make use of our previously reported geometry [6] for KO2, and in addition calculate the equilibrium geometry of KO2+. We then use RCCSD(T) calculations with large basis sets to calculate the adiabatic ionization energy of KO2, and the dissociation energy of KO2+. We compare these calculated quantities with values derived previously in photoelectron studies [17], involving one of the authors.
Section snippets
Geometries
The geometry of KO2+ was optimized using both the (U)B3LYP and (U)QCISD methods (only the 1s orbitals on O was frozen) as implemented in the Gaussian suite of programs [18]. The basis set employed for K was basis set B of [6], which consists of the LANL2 ECP [19], augmented with further basis functions; overall, this basis set is denoted LANL2[8s8p4d]. The ECP describes the 1s22s22p6 electrons. For O, the standard 6-311+G(3d) basis set was employed. The results of the geometry optimizations and
Geometry and vibrational frequencies
To our knowledge, there have been no previous reports of the geometry nor vibrational frequencies of KO2+. The linear state obtained is consistent with our previous findings for NaO2+[7]. The bond length increases between KO2 and KO2+—notwithstanding the change in symmetry from C2v to C∞v—from 2.41 to 2.92 Å: this is exactly as one would expect as the Coulomb attraction between K+ and O2− is lost, to be replaced only by the charge/induced-dipole interaction. This is further demonstrated in
Conclusions
High-level ab initio calculations have allowed the accurate calculation of ionization energies for KO2 and dissociation energies for both KO2 and KO2+. The D0 value for the cation is very small, and suggests that upon ionization of KO2, most of the KO2+ will be formed above the dissociation limit, and so lead to K++O2. The D0 value for the neutral was in good agreement with the best previous theoretical calculations, and two previous experimental determinations; other experimental
Acknowledgements
The authors are grateful to the EPSRC for the award of computer time at the Rutherford Appleton Laboratories under the auspices of the Computational Chemistry Working Party (CCWP), which enabled these calculations to be performed. E.P.F.L. is grateful to the Research Grant Council (RGC) of the Hong Kong Special Administration Region for support. T.G.W. is grateful to the EPSRC for the award of an Advanced Fellowship.
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