The Uncertain Bond Energy of the NaAu Molecule: Experimental Redetermination and Coupled Cluster Calculations A. Ciccioli* and G. Gigli* Dipartimento di Chimica, Sapienza Universita ̀ di Roma, Roma, Italy * S Supporting Information ABSTRACT: The dissociation energy of the intermetallic molecule NaAu, for which two largely at variance experimental values are available in the literature, has been redetermined by the Knudsen effusion mass spectrometry method. The molecule has been produced in the vapor phase by a specially designed experimental setting inspired by the double oven technique. The equilibrium of dissociation to atoms as well as the exchange equilibrium with the gold dimer were monitored mass- spectrometrically over about a 600 K temperature range. The third-law analysis of the equilibrium data provides the dissociation energy D 0 ° (NaAu, g) = 245.3 ± 6.8 kJ/mol, corresponding to a formation enthalpy at 298 K of 228.3 ± 7.5 kJ/mol. The NaAu species was also studied computationally at the CCSD(T) level with basis sets of increasing zeta quality thus allowing to evaluate the molecular parameters and the dissociation energy at the complete basis set limit. 1. INTRODUCTION In the framework of our recent efforts to provide reliable experimental determinations of the dissociation energy of new diatomics, 1-6 particular attention has been devoted to gold- containing intermetallics, 1,2,6 which represent a case of special interest. The rich chemistry of gold can be traced back to its high electronegativity, which allows different kinds of chemical bond to be formed. In addition, relativity and electron correlation effects do play a significant role in the energetics of the gold-containing molecules. We here report a contribution aimed at clarifying the bond energy of the NaAu diatomic molecule. In the last 15 years, the alkali-gold diatomics (MAu) have been studied computationally by a variety of methods: four- component density functional theory (DFT) for the entire series, 7,8 as well as for KAu to FrAu; 9 DFT and CCSD(T) for LiAu through KAu; 10 Dirac-Fock, 11 four-component DFT, and CCSD(T) 12 for CsAu; RI-MP2 13 and RI-MP2, SCS-MP2, and RI-CC2 14 for NaAu; DFT for NaAu. 15,16 These results indicate a distorted W trend of the dissociation energy along the alkali group, with minima at Na and Rb. Contrarily to the computational studies, only a few experimental investigations were reported for the MAu diatomics. Dissociation energies have been determined by Knudsen effusion mass spectrometry (KEMS) for LiAu, 17 NaAu, 18 RbAu, and CsAu. 19 Furthermore, in a resonant two- photon ionization spectroscopy study, 20 values for the dissociation energies of NaAu and KAu have been proposed. For the NaAu diatomic, the two reported bond energies (D 0 °) are in large disagreement. The KEMS value, 18 based on a few data points, is 212.1 ± 12.6 kJ/mol, whereas the value proposed in the spectroscopic study 20 is 254.7 ± 19.3 kJ/mol. Moreover, the theoretical results are spread over the ample range 167-266 kJ/mol. This discrepancy, together with the rather large uncertainty associated with the spectroscopic value, prompted us to undertake new KEMS experiments aimed at determining a reliable thermochemical value of the dissociation energy of the NaAu molecule. We here report our experimental results complemented with a computational study at the coupled cluster CCSD(T) level with basis sets of increasing zeta quality, including the extrapolation to the complete basis set (CBS) limit. 2. EXPERIMENTAL AND COMPUTATIONAL METHODS The well established Knudsen effusion mass spectrometric (KEMS) technique 21 has been employed. In this method, the molecular beam generated with the vapors effusing from a high temperature Knudsen source is monitored with a mass spectrometer (a single focusing 90° PATCO instrument in our experiments). Ions were produced by electron impact with an emission current regulated at 1.0 mA and an energy continuously variable up to 100 eV, thus allowing to measure the ionization efficiency curves (IEC) for each species, that is, the ion intensity vs the energy of the electron beam. In order to favor the formation of the NaAu species inside the Knudsen molecular source, the partial pressures of both sodium and gold should be as large as possible, but still within the limit imposed by the molecular effusion conditions (typically 10 -3 bar), because only in that flow regime the measured ion current can be reliably related to the partial pressure inside the cell. These constraints are demanding when, as in the case of sodium and gold, the fugacities of the pure Received: March 8, 2013 Revised: May 16, 2013 Article pubs.acs.org/JPCA © XXXX American Chemical Society A dx.doi.org/10.1021/jp402374t | J. Phys. Chem. A XXXX, XXX, XXX-XXX