Gold dimer

Perreault, D., Drouin, M., Michel, A., Miskowski, V.M., Schaefer, W.P. and Harvey, P.D. (1992) Silver and Gold Dimers. Crystal and Molecular Structures of Ag2(dmpm)2Br2 and [Au2(dmpm)2] (PFe)2 and Relation between Metal-Metal Force Constants and Metal-Metal Separations. Inorganic Chemistry, 31, 695-702. 

The Au(ll) oxidation state was also observed in cyclometalated gold dimers [386, 387] and in gold complexes of hA(diphenylphosphino)amine [388]. The specific feature of these Au(ll) complexes is that their 6 - A q relation is located midway between that of Au(l) and Au(lll) complexes. 

Fig. 10.4 (a) Enhancement factor (EF) distribution as a function of wavelength for the hot spots formed between the gold dimer and gold tip shown in Fig. 10.3. (b) Spatial distribution of enhancement factors for the dimer and tip. EF decreases strongly with distance and can be more than an order of magnitude smaller than the maximum only a few nanometers away (Figure reproduced from Ref [19])... 

The first term represehts the repulsive branch and the second term represents the attractive branch of the interaction potential between two atoms. By performing a non linear least square fit procedure the parameters (Al, ai, Ai A2, a2, A2) of the empirical pair potential are determined. In the fit procedure we have used the binding energy values of Au-dimer calculated at various interatomic distances by RDFT. The estimated points by RDFT and the fitted function are shown in Fig. 1. The potential parameters for the gold dimer interaction are determined as Ai = 1222.86345, A2 = -3.93623329, Ai = 2.94056151, A2 = -1.30223862, ai = 0.806351693, 2 = 0.216139972. In these parameters energy is in eV, and distance is in... 

As an example, the reactions of the negatively charged gold dimer and trimer cluster ions with carbon monoxide inside the rf-ion trap are presented in Fig. 1.36 [185]. While no reaction products of Au2 and Aus with carbon monoxide are detected at room temperature, cooling down leads first to the formation of mono-carbonyls and at the lowest temperatures around 100 K to a maximum adsorption of two CO molecules on Au2 and also on Au3 as can be seen from the mass spectra depicted in Fig. 1.36a. In order to deduce the reaction mechanism of the observed reactions, the reactant and product ion... 

Catalytic CO Oxidation by Free Au2. The potential catal3dic activity of Au2 in the CO combustion reaction was first predicted by Hakkinen and Land-man [382]. The subsequent experimental investigation employing an rf-ion trap indeed revealed the catalytic reaction of the gold dimer and, in conjunction with theory, a detailed reaction cycle could be formulated [33]. Also for particular larger gold cluster anions evidence for catalytic CO2 formation has... 

The solid lines in Fig. 1.64a are obtained by fitting this equilibrium mechanism to the experimental data. However, the extend in which the gold dimer is reformed increases with increasing CO concentration. Thus, the reaction mechanism must involve more intermediate steps representing the influence of CO. 

To reveal the complete reaction mechanism, the reaction was investigated at lower temperatures. The product ion mass spectrum recorded at 100 K with O2 and CO in the ion trap (Fig. 1.63b) shows the appearance of the coadsorption complex Au2(C0)02 discussed above. This complex represents a key intermediate in the reaction mechanism of the catalytic oxidation of CO to CO2 as has been predicted in the earlier theoretical study [382]. The experimental evidence obtained so far demonstrates that O2 adsorption is likely to be the first step in the observed reaction mechanism. Subsequent CO coadsorption yields the observed intermediate (Fig. 1.63b) and finally the bare gold dimer ion must be reformed. The further strategy to reveal the full reaction mechanism consists in varying the available experimental parameters, i.e., reaction temperature and reactant partial pressures. This procedure leads to a series of kinetic traces similar to the one shown in Fig. 1.64b and c [33]. The goal then is to find one reaction mechanism that is able to fit all experimental kinetic data obtained under the various reaction conditions. This kinetic... 

As discussed above, the postulation of an equilibrium in step (1.65b) is essential to the reaction mechanism. This in turn has implications on the possible structure of the intermediate species Au2(C0)02 in the catalytic cycle (1.65) which might be a simple coadsorption of the two molecules, e.g., on different sides of Au2 or already a reacted carbonate COs-like species adsorbed onto the gold dimer. Apparently, the observed intermediate can... 

Fig. 1.67. Schematic representation of the gas-phase catalytic cycle for oxidation of carbon monoxide by gold dimer anions based on the reaction mechanism determined by kinetic measurements in conjunction with first-principles simulations. The numbers denote calculated energy barriers in eV. Also displayed are geometric structures of reactants and intermediate products according to the calculations (large, grey spheres, Au small gray spheres, C dark spheres, O) [33]...

There is still a notable discrepancy between experimental data and density functional results, which consistently overestimate (by 5 pm) the bond length in gold dimer. This discrepancy arises from the dispersion (van der Waals) forces which are relatively strong between... 

Unpublished results for the electric field effect on the stability of the no-pair copper and gold dimers. For example, in a field of 0.015 a.u., Cu is stabilized compared to Cu by 7kcal mol". ... 

Such gold nanoparticles were also employed for the examination of the enantiomeric excess of histidine. " Chiral gold dimers were used by Xu et al. to detect cysteine down to a concentration of 20 pM. The chiral signal may also be of use for the identification of the concentration of toxins in environmental samples. In work by several groups,... 

Another prototypical molecule, which has been extensively investigated in numerous studies with relativistic electronic structure methods is the gold dimer Au2 [607,653,707,710,752,754,1023,1024,1096-1116]. Spin-orbit coupling may be neglected owing to its closed-shell electronic structure. Electron-correlation effects, however, play a decisive role for obtaining reliable spectroscopic parameters. Table 16.3 provides results obtained with various methods. 

As an example of a pseudo-relativistic all-electron DFT method. Table 16.3 lists results from van Wiillen s scalar-relativistic ZORA-DFT study [707]. Leaving the comparatively large error in coe aside, we note that the remaining spectroscopic constants are well reproduced. A subsequent relativistic DFT study on the gold dimer employed a four-component Dirac-Kohn-Sham method and the scalar-relativistic ZORA approach [1114]. Table 16.3 also shows that the scalar-relativistic ZORA approach yields good results compared to the four-component reference. Recall that the accuracy of such DFT calculations is determined by the approximate nature of the exchange-correlation functional employed (see section 8.8). 

S. Knecht, J. M. Ugalde, M. D. Morse, I. Infante. An interpretation of the absorption and emission spectra of the gold dimer using modem theoretical tools. Phys. Chem. Chem. Phys, 14 (2012) 8732-8741. 

Table S Calculated Equilibrium Bond Length (A) and Dissociation Energy (eV) for the Gold Dimer ...

Mixed complexes containing gold and a second transition metal ion are also emissive. As an example, a mixed iridium-gold dimer has been synthesized that is emissive at low temperature. This compound [AuIrCl(CO)0u-dppm)2] has an Au-Ir separation of 2.986(1) A. The complex shows a da pa transition at 440 nm (e = 18,600 cm" ), along with a weaker band at 518 nm that may be the... 

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