Article pubs.acs.org/JPCA
Charge-Dependent Trends in Structures and Vibrational Frequencies of [CO-Au‑O2]q (q = −1, 0, +1) Complexes: Evidence for Cooperative Interactions Angela N. Smith and David T. Moore* Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Charge-dependent trends in the structures, vibrational frequencies, and binding energies of binary [AuCO]q and [AuO2]q and ternary [O2AuCO]q complexes (q = −1,0,+1), have been investigated using density functional theory calculations. Three different geometrical motifs, given descriptive names of “separated”, “pre-reactive”, and “longrange”, are identified for the ternary complexes. For the binary systems, the general trend is that the complexes become more diffuse as the charge becomes more negative, having longer intermolecular bond distances and weaker binding energies. The trends shown by the ternary complexes are more complicated, and are different for the various geometrical motifs. However, a general trend is that there is a cooperative interaction involving both the CO and O2 with the Au center, which becomes more pronounced as the negative charge on the complexes increases from cationic to neutral to anionic. This cooperative interaction leads to increased electron density on the O2 moiety, as is reflected in the bond-lengths and vibrational frequencies. Furthermore, it is found that for the pre-reactive complexes, the role of the Au center is to stabilize the formation of a conjugated π-system between the CO and O2 molecules.
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Wallace and Whetten conducted room temperature flow reactor studies of the binding of CO and O2 to gold cluster anions (AuN, N = 2−20) which showed an odd−even oscillation based on cluster size, with even clusters adsorbing one O2 molecule, and odd clusters failing to adsorb any.38 They also observed strong evidence of a cooperative binding effect, where the affinity of either reactant (CO or O2) for the cluster was enhanced by pre-adsorbing the other reactant.38 Subsequent cooled ion trap studies by the Woste group on the Au2− cluster were able to determine a complete catalytic cycle for CO2 formation, with an accompanying density functional theory (DFT) computational study that proposed two possible mechanisms involving CO3 “carbonate” and OCOO “peroxyformate” intermediates with low (5 Å, suggesting that the interaction is pure van der Waals, which is not well-predicted by the B3LYP functional. The small energy separation between these states leads to a large spin-contamination of the doublet wave function (⟨S2⟩ = 1.374), as has been reported previously,24,27,44 making the computational results (particularly the small binding energy) somewhat suspect. However, we note that the geometry and binding energy obtained here are at least consistent with those predicted in the comprehensive study of 42 DFT functionals by Fang et al.44 Those authors further found that the spin contamination was persistent in all single-reference methods up to CCSD(T), and furthermore that even multireference MRCI calculations predicted a qualitatively similar structure (although with a larger Au−O distance of 3.58 Å), with
