Computational Studies
of Nonstoichiometric Sodium
Auride Clusters
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Abstract
The molecular structures of low-lying isomers of anionic
and neutral
sodium auride clusters have been studied computationally at the second-order
Møller–Plesset perturbation theory level using quadruple-ζ
basis sets augmented with a double set of polarization functions.
The first vertical detachment energies were calculated at the Møller–Plesset
level as the energy difference between the cluster anion and the corresponding
neutral cluster. The photodetachment energies of higher-lying ionization
channels were calculated by adding electronic excitation energies
of the neutral clusters to the first vertical detachment energy. The
excitation energies were calculated at the linear response approximate
coupled-cluster singles and doubles level using the anionic cluster
structures. The obtained ionization energies for NaAu<sup>–</sup>, NaAu<sub>2</sub><sup>–</sup>, NaAu<sub>3</sub><sup>–</sup>, NaAu<sub>4</sub><sup>–</sup>, Na<sub>2</sub>Au<sub>2</sub><sup>–</sup>, Na<sub>2</sub>Au<sub>3</sub><sup>–</sup>, Na<sub>3</sub>Au<sub>3</sub><sup>–</sup>, and Na<sub>2</sub>Au<sub>4</sub><sup>–</sup> were compared
to values deduced from experimental photoelectron spectra. Comparison
of the calculated photoelectron spectra for a few energetically low-lying
isomers shows that the energetically lowest cluster structures obtained
in the calculations do not always correspond to the clusters produced
experimentally. Spin-component-scaled second-order Møller–Plesset
perturbation theory calculations shift the order of the isomers such
that the observed clusters more often correspond to the energetically
lowest structure, whereas the spin-component-scaled approach does
not improve the photodetachment energies of the sodium aurides. The
potential energy surface of the sodium aurides is very soft, with
several low-lying isomers requiring an accurate electron correlation
treatment. The calculations show that merely the energetic criterion
is not a reliable means to identify the structures of the observed
sodium auride clusters; other experimental information is needed to
ensure a correct assignment of the cluster structures. The cluster
structures of nonstoichiometric anionic sodium aurides have been determined
by comparing calculated ionization energies for low-lying structures
of the anionic clusters with experimental data