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Wavenurnbers Jam ]

compared directly with the ones in the temperature-programmed reaction spectrum but are lower limits. We have also observed an IR absorption band at 1300 cm"1, which we attribute to superoxo/peroxo-type O2. In order to better disentangle the vibrational band of 13Co adsorbed on Au8 deposited on defect-poor films (B) from that of the CO weakly bound to the support material, the sample was annealed to 120 K. In this way, the 13CO frequency from MgO-adsorbed CO disappears, otherwise observed at 2127 cm"1.

(27) [see also (18)], given in Table 1 for the isolated Au8/O2/13CO complex (which we present first in order to allow a clear distinction between charging and other support-related effects), reveal that the 13CO stretch vibration shifts to lower frequency in a manner that is correlated with increased charging of the complex (given by Qc in Table 1), as well as with the estimated increase in the population of the antibonding state [given by

SQ(CO) in Table 1]. For a neutral free complex with zero spin [Qc = 0 and S = 0 in Table 1; see the corresponding atomic configuration shown in the inset of Fig. 2], a vibrational frequency of 2009 cmj1 was obtained for the adsorbed 13CO molecule. We attribute the calculated decrease in frequency (61 cmj1) in comparison to the value calculated for the free molecule (2070 cmj1) to a net excess charge [SQ(CO) = 0.28e, where e is the electron charge] on the adsorbed molecule. The excess charge on the molecule results from back-donation into the CO(2p*) antibonding state.

As expected, such donation of charge from occupied orbitals of the metal to unoccupied states of the adsorbed molecule is even more pronounced (0.88e) for the more electronegative O2 molecule. Upon charging the complex with up to 1.0e, the net excess charge on the 13CO molecule increases to 0.41e, and the CO stretch redshifts by 89 cmj1 to 1920 cm"1. The increased degree of charging of the metal cluster with back-donation, and the consequent decrease of v(CO), increase the C-O bond length from 1.148 A for Qc = 0 to 1.158 A for Qc = 1.0e. Similar changes were seen starting from the triplet state of the neutral complex (Table 1).

We next focused on the adsorption of CO to gold complexes surface-supported on perfect or defective magnesia [with an oxygen molecule preadsorbed at the interface between the cluster periphery and the MgO surface (Fig. 2B)]; that is, Au8/O2/MgO or Au8/O2/MgO(FC). Three energy-optimized deposited cluster configurations pertinent to the experimental work are displayed in the top row of Fig. 2, A to C. The bare adsorbed Au8 cluster (Fig. 2A) exhibits only a slight distortion from the structure of the corresponding gas-phase neutral cluster (4, 5), consisting mainly of a displacement of the gold atom of the cluster closest to the surface oxygen vacancy. The cluster is anchored strongly to the defective MgO surface (with a binding energy of 3.44 eV) compared to a significantly weaker binding to the defect-free surface (1.22 eV). From examination of the charge-difference isosurfaces displayed in Fig. 2, we observe that bonding of the Au8 cluster to the MgO(FC) surface is accompanied by a significantly larger degree of charge transfer from the magnesia surface to the gold cluster (Fig. 2E) as compared to the case of adsorption on an F-center-free surface (Fig. 2D).

Optimal geometries with a single adsorbed CO molecule, Au8/O2/CO/MgO(FC), and at saturation coverage of the cluster [that is, with three adsorbed CO molecules, Au8/O2/ (CO)3/MgO(FC)] are shown in Fig. 2, B and C, respectively. Comparison between the bare-cluster geometry in Fig. 2A with those shown in Fig. 2, B and C, reveals a significant change

Fig. 2. Optimized configurations of (A) a bare Au8 cluster (yellow spheres) adsorbed on an F center of a Mg0(001) surface (O atoms are in red and Mg atoms in green) and (B) a surface-supported gold octa-mer with O2 adsorbed at the interface between the Au8 cluster and the magnesia surface and a CO molecule adsorbed on the top triangular facet (the C atom is depicted in gray). There is a significant change in the geometry of Au8 compared to the one shown in (A). The inset between (A) and (B) shows a local-energy-minimum structure of the free Aus cluster in the three-dimensional (3D) isomeric form with coadsorbed O2 and CO molecules. Although in the global ground state of the free Au8/O2/CO complex the gold octamer is characterized by a higher degree of 2D character (74), we chose to display here an isomer that closely resembles the structure of the surface-adsorbed complex: Compare the structure in the inset with that shown in (B). (C) Au8 on the magnesia surface [MgO(FC)] with three CO molecules adsorbed on the top facet of the cluster and an O2 molecule preadsorbed at the interface between the cluster and the magnesia surface. The molecule marked CO(2) lies parallel to the surface and is thus not IR-active in the experimental configuration used here. The C-O bond length d(CO(/)), the charge transferred to the CO(/) molecule AQ(/), and the calculated C-O vibrational frequency v(/) (/ =1, 2, and 3), as well as the corresponding values for the O2 molecule, are as follows: d(CO(/)) [A] = 1.151, 1.158, 1.153;d(O2) = 1.422;8Q(/) [e] = 0.31, 035, 0.32;8Q(O2) [e] = 1.12;v(/) [cmj1] = 1993, 1896, 1979. Isosurfaces of charge differences are as follows: (D) Au8 cluster adsorbed on defect-free MgO, 8p = Ptot - (pAu8 + PMgO), where p = p[Au8/MgO]; (E) Au8 cluster anchored to a surface F center of MgO, 8P = Ptot - (Pau8 + rMEO^)^ where Ptot = P[Au8/MgO(FC)]; (F) same as (E) but with O2 and CO molecules adsorbed on the gold cluster, 8p = Ptot - (Pau8 + Po2 + Pco + Pmeo(fc)), where Ptot = P[Au8/O2/CO/MgO(FC)]. Pink isosurfaces represent 8p < 0 (depletion) and blue ones correspond to 8p > 0 (excess). All of the isosurfaces are plotted for the same (absolute) value of the density difference (8p) in order to allow direct comparison between the different cases.

Charge Transfer Charge Difference

Table 1. Results for free Au8/O2/13CO complexes as a function of the amount of excess electron charging Qc, shown for various values of the spin. For the neutral cluster (Qc = 0), we show the triplet (S = 1) and singlet (S = 0) states. The quantities that we display are: v(13CO), the 13CO vibrational frequency; the calculated excess electronic charge on the adsorbed molecules SQ(CO) and SQ(O2), with the procedure used for achieving these estimates described in (78); and the bond distances d(CO) and d(O2). The calculated values for the isolated molecules are d(CO) = 1.14 A and d(O2) = 1.24 A, compared to the gas-phase experimental values of 1.13 and 1.21 A, respectively. The calculated vibrational frequency of gas-phase 13CO is 2070 cm"1, which is 25 cm"1 smaller than the experimental value vexp(13CO) = 2095 cm"1.

Table 1. Results for free Au8/O2/13CO complexes as a function of the amount of excess electron charging Qc, shown for various values of the spin. For the neutral cluster (Qc = 0), we show the triplet (S = 1) and singlet (S = 0) states. The quantities that we display are: v(13CO), the 13CO vibrational frequency; the calculated excess electronic charge on the adsorbed molecules SQ(CO) and SQ(O2), with the procedure used for achieving these estimates described in (78); and the bond distances d(CO) and d(O2). The calculated values for the isolated molecules are d(CO) = 1.14 A and d(O2) = 1.24 A, compared to the gas-phase experimental values of 1.13 and 1.21 A, respectively. The calculated vibrational frequency of gas-phase 13CO is 2070 cm"1, which is 25 cm"1 smaller than the experimental value vexp(13CO) = 2095 cm"1.

Qc(e)

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