| Краткое содержание: | The finding of extraordinary catalytic activities of gold nanoparticles
has aroused renewed interest in gold chemistry.1,2 Considerable effort
to characterize the chemical interactions between gold clusters and a
variety of molecules has been made.3-12 An earlier study of the
interaction between gold clusters and O2 by Cox et al.13 showed that
Aun
+ cation clusters can react with O2 only for n ) 10, while gold
anion clusters exhibit an odd-even effect toward O2. This observation
was later confirmed by Whetten and co-workers.14 Cox et al. also found
that Au+ is reactive toward CH4 but Au- is not.13 Recently, Zhai et
al.15 used photoelectron spectroscopy (PES) to elucidate the structure
of AuO2
- and found that it adopts a linear OAuO- structure. The
Au-H2O complex has been studied extensively. Hrusa´k et al.,16
Hertwig et al.,17 and Feller et al.18 independently studied the structures
of Au+(H2O)n (n ) 1-4) using various high-level post-Hartree-Fock
methods. Zheng et al.12 recently carried out a PES experiment on
Au(H2O)n
- (n ) 1, 2). Enhancement of CO oxidation on a supported
gold nanocluster by water was reported by Bongio
o and Landman,19
suggesting significant interactions between gold clusters and H2O.
Moreover, the bonding between a noble-gas (NG) atom and Au+ has
attracted growing attention.4,20-22
In this communication, we report a joint experimental and theoretical
study of the interactions between gold anion, Au-, and an NG atom
(NG ) Ne, Ar, Kr, Xe) or a molecule of O2, CH4, or H2O. Except for
the Au- · · ·H2O interaction, which is comparable to strong hydrogen
bonding, all of these are weak charge-induced intermolecular interactions.
The observation of a weakly bound Au(O2)- complex shows
the inertness of Au- toward O2, in line with the previous observation
of the odd-even effect in the reactions of Aun
- clusters and O2. By
comparing with results of high-level ab initio calculations, we
demonstrate that anion PES is a good technique for probing weak
charge-induced intermolecular interactions.
Weak intermolecular interactions are difficult to measure in a
quantitative fashion. We have previously observed weakly bonded CO
in Aux(CO)y
- complexes for large y beyond a saturation limit.7b,c
Recently, we were able to produce very cold anion clusters to form
complexes of Aun
- clusters with O2 and Ar,23-25 allowing us to
investigate physisorption using PES. The current experiment was
carried out with a magnetic-bottle PES apparatus equipped with a laser
vaporization cluster source, details of which can be found in the
Supporting Information (SI).
Figure 1 displays the 193 nm spectra for AuAr-, Au(H2O)-, and
AuO2
- compared with that of Au-. The spectrum of AuAr- (Figure
1b) is identical to that of Au- (Figure 1a) except for a disce
ible
blue shift (∼35 meV) due to the weak interaction between Au- and
Ar. The spectral features of Au(H2O)- (Figure 1c) are also similar to
those of Au-, but there is a much larger blue shift (0.47 eV) as a
result of the much stronger interaction between Au- and H2O. Notably,
Zheng et al.12 reported the PES spectrum of Au(H2O)- at 355 nm,
allowing only the first band (X) to be observed (it was also vibrationally
resolved). The spectrum of the AuO2
- (Figure 1d) species is more
complicated. Previously, we reported the PES spectra of pure OAuOusing
N2O as a carrier gas.15 Features due to the gold dioxide anion
are clearly present in Figure 1d (labeled as X′ and A′-E′). In addition,
we observed features similar to those of bare Au-, which are clearly
derived from a Au(O2)- complex. In previous studies, we have reported
Aux(O2)-andAuxAry
-complexesundercoldexperimentalconditions.23-25
In the current study, we found that the AuO2
- spectrum was strongly
dependent on our source conditions: the relative intensities of the
Au(O2)- features increased as colder clusters were produced. We also
measured the PES spectrum of AuO2
- at 355 nm (3.496 eV) and
observed a slight blue shift (∼25 meV) of the first band of Au(O2)-
relative to that of Au- (Figure S1), suggesting the weak nature of the
bonding between Au- and O2. As we suggested previously,15 Audoes
not react with O2, and the formation of the OAuO- dioxide
species in our cluster source is from the reactions of Au- with O atoms.
We also carried out ab initio calculations to elucidate the structures
and binding energies of the AuM- complexes. We used the CCSD(T)
method and the augmented Dunning correlation-consistent basis sets
† University of Nebraska-Lincoln.
‡ Washington State University and Pacific Northwest National Laboratory.
§ Easte
Oregon University.
Figure 1. Photoelectron spectra of (a) Au-, (b) AuAr-, (c) Au(H2O)-,
and (d) AuO2
- at 193 nm. Notably, the spectrum in (d) contains contributions
from a physisorbed Au(O2)- complex (X, A, B) and the OAuO- dioxide
(X′, A′-E′).15
10.1021/ja903043d CCC: $40.75 XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A
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Published on June 19, 2009 on http://pubs.acs.org | doi: 10.1021/ja903043d
(aug-cc-pVTZ and aug-cc-pVQZ) for H, C, O, Ne, Ar, and Kr and
the same basis sets with pseudopotentials (aug-cc-pVTZ-PP and augcc-
pVQZ-PP)26 for Au and Xe. Geometry optimization was performed
at the MP2/aug-cc-pVTZ(PP) level of theory. The basis-set superposition
error (BSSE) was corrected using the counterpoise method. The
equation used to evaluate the CCSD(T) complete-basis-set (CBS) limit
and other computational details are given in the SI.
The calculated Au--M binding energies and average M polarizabilities
are given in Table 1, and the corresponding structures are
displayed in Figure 2. For Au(O2)-, the binding energy between Auand
O2 is only 0.78 kcal/mol, which is 0.11 kcal/mol smaller than that
of AuAr-, consistent with the smaller PES spectral shift. The interaction
between Au- and H2O is much stronger than the weak intermolecular
interactions in the other species and comparable to a strong hydrogen
bond (>10 kcal/mol). Mulliken charge analysis suggests that the charges
of Au- and the H atom closest to Au- are -1.06e and 0.45e,
respectively. Hence, electrostatic interactions between Au- and H2O
play an important role, inducing the large blue shift in the PES spectrum
(Figure 1c).
Since our theoretical results reproduced the experimental trend of
the weak intermolecular interactions in Au(O2)-, AuAr-, and
Au(H2O)-, we extended our calculations to the interactions in AuM-
(M ) Ne, Kr, Xe, CH4). As Table 1 shows, the binding energies
calculated using two different basis sets show a consistent trend. In
particular, the CCSD(T)/aug-cc-pVQZ results are very close to those
in the CBS limit, indicating that the calculated binding energies are
converged. Furthermore, the trend in the binding energies of these
Au--M species is correlated with the average polarizability of M
(except in the case of H2O). Notably, the binding energies of the
corresponding neutral complexes at the anion geometries (Table S1)
are significantly less than those of the optimized anion species (Table
1), which implies that electrical induction plays a major role in these
anion complexes (especially in Au– · · ·H2O, for which the binding
energy increases by more than an order of magnitude compared with
that of the neutral Au· · ·H2O complex). It should also be noted that
the trend of the calculated vertical detachment energies (VDEs) of
Au-, Au- · · ·O2, Au- · · · Ar, and Au- · · ·H2O is consistent with the
trends of the measured VDEs and binding energies (Table 1). The
only exception is Au- · · · Xe, which has a larger binding energy but a
slightly smaller VDE compared with Au-· · ·CH4. This exception might
be due to the use of pesudopotential basis sets for Xe.
In summary, we have shown that PES can be a very sensitive tool
for probing weak intermolecular interactions between Au- (or gold
clusters) and gas atoms (or molecules). High-level ab initio calculations
confirm the trend in the relative interactions in various Au--M
complexes revealed by the PES results. Surprisingly, Au- has stronger
interactions with Ar than with O2. The ability to form weakly bonded
complexes has recently been exploited to probe the exact gold cluster
sizes at which the 2D-to-3D24 and cage-to-pyramid25 structural
transitions occur.
Acknowledgment. The experimental work was supported by NSF
(CHE-0749496) and performed at the EMSL, a national scientific user
facility sponsored by the DOE Office of Biological and Environmental
Research. The theoretical work was supported by grants from NSF
(CHE-0427746, DMR-0820521) and the Nebraska Research Initiative.
Supporting Information Available: Experimental and computational
methods, binding energies, and PES spectra. This material is
available free of charge via the Inte
et at http://pubs.acs.org.
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