Precious metal-molecular oxygen complexes: Neon matrix infrared spectra and density functional calculations for M(O2), M(O2)2 (M = Pd, Pt, Ag, Au)

Article abstract of DOI:10.1021/jp010058f

Laser-ablated palladium, platinum, silver, and gold atoms react with molecular oxygen in excess neon during condensation at 4 K. Reaction products, M-(η²-OO), M-(η²-OO)₂ (M = Pd, Pt), PtO, PtO₂, AuO₂,

Full text of DOI:10.1021/jp010058f

5812
J. Phys. Chem. A 2001, 105, 5812-5822
Precious Metal-Molecular Oxygen Complexes: Neon Matrix Infrared Spectra and Density
Functional Calculations for M(O2), M(O2)2 (M ) Pd, Pt, Ag, Au)
Xuefeng Wang and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, McCormick Road, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319
ReceiVed: January 7, 2001; In Final Form: March 29, 2001
Laser-ablated palladium, platinum, silver, and gold atoms react with molecular oxygen in excess neon during
condensation at 4 K. Reaction products, M-(η2-OO), M-(η2-OO)2 (M ) Pd, Pt), PtO, PtO2, AuO2, M-(η1-
-
OO) (M ) Ag, Au), and anions Ag-(η1-OO)-, Au-(η1-OO)2 , are identified from isotopic shifts and splittings
in their matrix infrared spectra. Density functional theory (BPW91 and B3LYP) calculations are performed
on the product molecules for comparison of calculated frequencies and 16O/18O isotopic ratios. Doping with
CCl4 to serve as an electron trap eliminates the anion bands, which further supports the anion identifications.
The reaction mechanism and comparison with argon matrix experiments are discussed. The AuOO complex
exhibits an unusually large neon-argon matrix shift, which suggests increased charge transfer in the more
polarizable argon matrix.
Introduction
platinum (from crucibles), silver (99.9%), and palladium (99.9%)
(Johnson-Matthey) metals were used as targets; the ablation laser
energy was varied from 5 to 20 mJ/pulse. Laser-ablated metal
atoms were co-deposited with molecular oxygen (0.2%-0.05%)
in excess neon onto a 4 K CsI window at 1-2 mmol/h for 30
min. Oxygen (Matheson), isotopic 18O2 (97.8% 18O, Yeda),
scrambled sample (16O2/16O18O/18O2 ) 1:2:1), and mixed
isotopic sample (16O2/18O2) were used in different experiments.
Infrared spectra were recorded at 0.5 cm-1 resolution on a
Nicolet 750 with 0.1 cm-1 accuracy using an HgCdTe detector.
Matrix samples were annealed at different temperatures, and
selected samples were subjected to broad-band photolysis by a
medium-pressure mercury lamp (Philips, 175 W) with the globe
removed.
Complexes formed between metals and molecular oxygen
have received extensive investigations, because they play
important roles in fields of catalysis,1,2 synthesis, materials,
surface science,3-6 and biochemistry.7-10 The formation of
M(O2)n complexes (M ) Cu, Ag, Au; n ) 1, 2) have been
investigated by infrared spectra in solid argon.11 Kasai and
Jones12 reported ESR spectra of mono- and bis(dioxygen)
complexes of Cu, Ag, and Au in solid argon and suggested ion
pairs of the form M+(O2)-, with Cu(O2) and Ag(O2) in bent
end-on structures and Au(O2) in a symmetric side-on structure.12
However, both Ag(O2) and Au(O2) were suggested to be side-
on complexes from glassy matrix ESR spectra.13 The Pd(O2)
and Pt(O2) complexes appear to have the side-on structures
based on matrix infrared isotopic spectra.14,15
Results
Recently, laser-ablated transition-metal atom reactions with
molecular oxygen have been studied extensively in this group.
Various metal oxides, dioxides, superoxides, and peroxides as
well as cations and anions have been identified from matrix
infrared spectra.15-25 However, most of the previous work has
been done in a condensing argon matrix. In this paper, we report
the reactions of laser-ablated Pd, Pt, Ag, and Au atoms with
molecular oxygen in excess neon during condensation at 4 K
because weakly bound complex products may be susceptible
to different matrix interactions. Several neutral and negative
metal-oxygen complexes are produced in the low-temperature
matrix and identified by isotopic substitution and density
functional theory (DFT) calculations.
Infrared spectra of laser-ablated Pd, Pt, Ag, and Au atom
reaction products with O2 trapped in excess neon at 4 K are
presented in Tables 1, 2 and Figures 1-8. Absorptions common
in the experiments, namely O3 (1039.9 cm-1), O4- (972.9 cm-1),
+
+
trans-O4 (1164.3 cm-1), cyclo-O4 (1320.2 cm-1), and O6
+
(1435.0 cm-1), have been reported previously25,28,29 and are not
discussed here. Spectra after annealing and broad-band pho-
tolysis are also shown in the figures. Infrared spectra obtained
using 18O2 samples, mixed 16O2 + 18O2 samples, and scrambled
16O2 + 16O18O + 18O2 samples are shown in Figures 2, 4, 6,
and 8. In addition, several experiments were done with 0.02%-
0.04% CCl4 added to the 0.2% O2 sample to serve as an electron
trap, and the absorptions due to anions were eliminated from
the spectra of the deposited samples.30,31
Experimental Section
The density functional theoretical calculations of metal oxides
and metal-oxygen complexes are given in Tables 3-8 for
comparison. The Gaussian 94 program32 was employed to
calculate the structures and frequencies of expected metal-
oxygen complexes, anions, and cations, using the BPW91 and
B3LYP functionals.33,34 The 6-311+G(d) basis set for oxygen
and Los Alamos ECP plus DZ for metal atoms were used.35,36
All the geometrical parameters were fully optimized, and the
The experimental methods for reactions of laser-ablated metal
atoms with small molecules during condensation in excess argon
have been previously described in detail.26,27 The Nd:YAG laser
fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse
width) was focused onto a rotating metal target. Gold and
* Corresponding author: lsa@virginia.edu.
10.1021/jp010058f CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/25/2001

Precious Metal-Molecular Oxygen Complexes
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5813
Figure 1. Infrared spectra in the 1200-950 cm-1 region for laser-ablated Pd atoms co-deposited with 0.1% O2 in neon at 4-5 K. (a) Sample
deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min, and (e) after annealing to 10 K.
TABLE 1: Infrared Absorptions (cm-1) from Co-deposition of Laser-Ablated Pd and Pt Atoms with O2 in Excess Neon at 4 K
16O2
18O2
16O2 + 16O18O + 18O2
R(16O/18O)
assignment
Pd + O2
1113.7
1112.5
1032.6
1030.1
973.0
1051.1
1050.0
site
1112.5, 1093.6, 1081.8, 1068.1, 1062.0, 1050.0
1030.2, 1001.9, 972.6
1.0595
1.0591
Pd-(η2-OO)2
site
972.6
Pd-(η2-OO)
Pt + O2
1057.1
1056.0
958.7
930.0
839.7
site
997.0
912.0
878.6
793.9
1055.9, 1036.6, 1027.4, 1010.3, 1006.4, 997.0
958.5, 944.2, 912.0, 867.7
929.9, 904.8, 878.6
1.0592
1.0512
1.0585
1.0577
Pt-(η2-OO)2
OPtO
Pt-(η2-OO)
PtO
839.7, 793.9
TABLE 2: Infrared Absorptions (cm-1) from Co-deposition of Laser-Ablated Ag and Au Atoms with O2 in Excess Neon at 4 K
16O2
18O2
16O2 + 18O2
16O2 + 16O18O + 18O2
R(16O/18O)
assignment
Ag + O2 2109.7 1995.2
1.0574
1.0578
1.0603
1.0599
1.0593
1.0595
1.0594
1.0602
1.0585
O3
2107.2 1992.1 2107.2, 2063.5, 2023.3, 1992.9
2074.9 1957.0 2073.9, 1957.0
2052.5 1936.4 2052.4, 1936.5
1102.5 1040.8
O3 (site)
2074.6, 2020.7, 2013.3, 1957.6
2052.8, 1999.8, 1991.6, 1936.7
1102.4, 1076.9, 1069.6, 1040.8
1942.0, 1015.3, 1010.5, 982.6
1037.7, 1005.0, 999.9, 972.4
1029.8, 1004.2, 999.0, 971.5
796.1, 786.7, 776.3, 771.8, 762.5, 751.8
(Ag)x-(η1-OO)
Ag-(η1-OO)-
Ag-(η1-OO)
1042.1
1030.7
1030.0
796.1
983.6 1041.9, 984.0
972.9 1030.8, 972.9
971.5 1030.0, 971.5
(Ag)x-(η1-OO)
Ag-(η1-OO)-
Ag-(η1-OO)- (site)
Ag+O3
-
752.1
796.0, 786.5, 762.5, 752.1
Au + O2 1213.6 1144.8 1213.8, 1144.7
1212.9, 1180.3, 1144.4
1.0601
1.0591
1.0589
1.0510
Au-(η1-OO)
1205.6 1138.3 1205.2, 1140.0
Au-(η1-OO) (site)
Au-(η1-OO)2
OAuO
-
911.9
824.2
861.2
784.2
911.9, 886.2, 863.0
824.2, 808.4, 784.2
824.1, 808.4, 784.0
harmonic vibrational frequencies were obtained analytically at
the optimized structures. The computed geometric parameters,
relative energies, dipole moments, frequencies, intensities, and
isotopic frequency ratios are summarized in Tables 3-8. We
find the lowest open OPdO structure to be a bent triplet state
using the 6-311+G(d) basis set, in contrast to earlier calculations
with the smaller D95* basis,15 and to be higher in energy than
the Pd(O2) complex. Platinum is different: OPtO is a linear
singlet state and is more stable than the Pt(O2) complex. The
Pd insertion into O2 is sufficiently endothermic (calculated
+27.0 kcal/mol) to prevent this reaction, but in contrast, the
exothermic (calculated -56.4 kcal/mol) reaction to form OPtO
is observed here.
Discussion
The new absorptions will be identified from isotopic shifts,
splittings, and DFT frequency calculations.
Pd-(η2-OO). The absorption at 1030.1 cm-1 in Pd + O2/Ne
experiments appeared on deposition, increased on annealing to
6 and 8 K, decreased on photolysis, and increased further on
annealing to 10 K. This band shifted to 972.6 cm-1 in the 18O2
experiments and gave the 1.0591 18O/16O isotopic ratio. The
scrambled 16O2 + 16O18O + 18O2 sample gave triplets with 1:2:1
intensity profile at 1030.2, 1001.9, and 972.6 cm-1 indicating
that two equivalent O atoms are involved in this vibration. The
band at 1030.1 cm-1 can be assigned to the Pd-(η2-OO)
complex. (To simplify notation, the band is labeled Pd(O2) in

5814 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Wang and Andrews
Figure 2. Infrared spectra in the 1200-950 cm-1 region for laser-ablated Pd atoms co-deposited with 0.05% (16O2 + 16O18O + 18O2) in neon. (a)
Sample deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min, and (e) after annealing
to 10 K.
Figure 3. Infrared spectra in the 1100-820 cm-1 region for laser-ablated Pt atoms co-deposited with 0.1% O2 in neon at 4-5 K. (a) Sample
deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min, and (e) after annealing to 10 K.
Figure 1.) This band was also observed at 1023.4 cm-1 in our
previous argon matrix experiments15 and in the early work of
Huber et al.,14 which identified a side-bonded Pd(O2) complex.
Pd-(η2-OO)2. The sharp band at 1112.5 cm-1 and matrix site
at 1113.7 cm-1 are assigned to Pd-(η2-OO)2. This band was
observed after deposition, increased greatly on annealing to 6
and 8 K, but decreased on broad-band photolysis. The 18O2
DFT calculations further support this assignment. The
counterpart for the 1112.5 cm-1 band appeared at 1050.0 cm-1
,
calculations (Tables 3 and 4) show that the Pd-(η2-OO) complex
giving the 16O/18O isotopic ratio 1.0595, which is very close to
the value 1.0592 for Pd-(η2-OO) and the 1110.1/1047.6 )
1.0596 ratio for the argon matrix counterparts.20 The sextets
observed in scrambled 16O2 + 16O18O + 18O2 isotopic spectra
(Figure 2) identify two equivalent OO subunits with equivalent
atoms involved in an O-O stretching mode with side-bonded
O2, as found in early argon matrix work.14
1
has C2V symmetry with a A1 ground state. The O-O bond
lengths are predicted to be 1.348 Å (BPW91) and 1.320 Å
(B3LYP), which are much longer than that of free O2. The
calculated O-O stretching frequency at the BPW91 level is
1077.1 cm-1 with 1.0606 16O/18O isotopic ratio, which is close
to the observed neon matrix values (1030.1 cm-1 16O/18O )
,
1.0591). The calculated isotopic ratio for this mode at the
B3LYP level is the same, but the predicted frequency (1160.3
cm-1) is substantially higher, even higher than expected.37 Scale
factors (observed/calculated) are 0.956 (BPW91) and 0.853
(B3LYP). Clearly, this metal-molecular oxygen complex is
modeled better by the BPW91 functional.
Similar DFT calculations were done for Pd-(η2-OO)2, and
results are listed in Tables 3 and 4. The predicted O-O bond
lengths for the D2h structure are 1.312 Å (BPW91) and 1.298
Å (B3LYP), which are slightly shorter than that in Pd-(η2-OO),
indicating the O-O bond is more reduced in Pd-(η2-OO) than

Precious Metal-Molecular Oxygen Complexes
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5815
Figure 4. Infrared spectra in the 1090-770 cm-1 region for laser-ablated Pt atoms co-deposited with 0.1% O2 in neon at 4-5 K. (a) Sample
deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min, and (e) after annealing to 10 K.
Figure 5. Infrared spectra in the 1150-750 cm-1 region for laser-ablated Ag atoms co-deposited with 0.2% O2 in neon at 4-5 K. (a) Sample
deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min, and (e) after annealing to 10 K.
in Pd-(η2-OO)2. The calculated frequency of the O-O stretching
mode at the BPW91 level is 1107.9 cm-1, only 4.6 cm-1 lower
than the 1112.5 cm-1 experimental value, and at the B3LYP
level is 1145.6 cm-1, only 33.1 cm-1 higher than the experi-
mental value. The predicted 16O/18O isotopic ratios at both levels
reproduce the experimental value very well.
The Pd-(η2-OO)2 structure with D2d symmetry calculated at
the B3LYP level is 2.2 kcal/mol higher in energy than the D2h
structure. The predicted frequency of the O-O stretching mode
is 1194.4 cm-1, which is 81.9 cm-1 higher than the experimental
value. However, another important feature of the D2d structure
is the small imaginary 140.7 cm-1 frequency due to the
degenerate (e symmetry) torsional mode around the C2 axis,
indicating that the D2d structure is a second-order saddle point.
Hence, we believe that the Pd-(η2-OO)2 complex, like the Ni
analogue,17 has the D2h structure rather than the D2d geometry
first deduced.14
PtO and PtO2. A sharp band was observed at 839.7 cm-1
after initial deposition, which showed a slight decrease on early
annealing and a considerable decrease on broad-band photolysis
(Figure 3). The 18O2 counterpart for this band at 793.9 cm-1
gave a 1.0577 16O/18O isotopic ratio. In the scrambled isotopic
experiment, a doublet was observed, indicating only one oxygen
atom. This band is assigned to the PtO molecule. The observed
neon matrix frequency is very close to the 841.1 cm-1
fundamental deduced from the emission spectrum38,39 and the
828.0 cm-1 argon matrix value.15 The observed frequency also
agrees with DFT calculations at 844.8 cm-1 (BPW91) and 843.5
cm-1 (B3LYP). The 3Σ- ground state is found by both BPW91
and B3LYP functional calculations.
The neon matrix absorption at 958.7 cm-1 is assigned to the
PtO2 molecule, which is slightly higher than our argon matrix
observation of this species at 953.3 cm-1 15
.
In the scrambled
isotopic experiment, a triplet 1:2:1 profile at 958.5, 944.2, and

5816 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Wang and Andrews
Figure 6. Infrared spectra in the 2130-1900 and 1180-960 cm-1 regions for laser-ablated Ag atoms co-deposited with 0.2% 16O2 + 0.2% 18O2
in neon at 4-5 K. (a) Sample deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 8 K, (d) after full-arc photolysis for 20 min,
and (e) after annealing to 10 K.
Figure 7. Infrared spectra in the 1250-750 cm-1 region for laser-ablated Au atoms co-deposited with 0.05% O2 in excess neon at 4-5 K. (a)
Sample deposited for 60 min, (b) after annealing to 6 K, (c) after annealing to 10 K, (d) after full-arc photolysis for 15 min, and (e) after annealing
to 11 K.
912.0 cm-1 was observed, indicating that two equivalent oxygen
atoms are involved in this molecule. The 1.0512 16O/18O isotopic
frequency ratio is much lower than the ratio observed for PtO
and Pt-(η2-OO), suggesting an antisymmetric O-Pt-O mode.
In the scrambled isotopic experiment, a weak, new band at 867.7
cm-1 tracks with the 944.2 cm-1 band, and is due to the
symmetric-stretching mode of 16OPt18O. This observation agrees
with our DFT calculations: both BPW91 and B3LYP func-
tionals predict the PtO2 molecule to have a Σg ground state
with the linear OPtO structure and an antisymmetric stretching
mode at 962.2 cm-1 (BPW91) and 1013.6 cm-1 (B3LYP).
Furthermore, the predicted separations of antisymmetric and
symmetric modes of 16OPt18O are 69.5 cm-1 (BPW91) and 77.3
cm-1 (B3LYP), which agree with the experimental value (76.5
cm-1). A bent triplet state is 25 kcal/mol higher in energy and
has inappropriate frequencies.
Pt-(η2-OO). The band at 930.0 cm-1 is assigned to the
symmetric O-O stretching mode of Pt-(η2-OO) in solid neon.
This band shifts to 878.6 cm-1 in the 18O2 experiment (16O/18O
isotopic ratio 1.0585). The 1:2:1 triplet in the scrambled isotopic
experiment indicates that two equivalent oxygen atoms are
involved. This observed frequency agrees with the 928.0 cm-1
argon matrix observation14,15 and DFT calculations. The O-O
stretching vibration was predicted at 966.6 cm-1 using the
BPW91 functional, while a B3LYP calculation gave 1032.0
cm-1, slightly higher than the BPW91 value. The calculated
16O/18O ratios, 1.0605 (BPW91) and 1.0605 (B3LYP), are
slightly higher than the experimental ratio, 1.0585, owing to
anharmonicity.
1
+
Pt-(η2-OO)2. The strong, sharp band at 1056.0 cm-1 is
assigned to the O-O stretching frequency of Pt-(η2-OO)2, first
observed at 1051 cm-1 in solid argon.14,15 This band increased

Precious Metal-Molecular Oxygen Complexes
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5817
TABLE 3: Relative Energies and Geometries Calculated at BPW91 and B3LYP Levels for MO2, M-(η2-OO), and M-(η2-OO)2
(M ) Pd, Pt)
relative energy
(kcal/mol)
molecule
state
µa
geometry (Å, degree)
BPW91/6-311+G(d)/Lanl2DZ
Pd-(η2-OO)
Pd-(η2-OO)
OPdO
1A1
3B1
3A2
1Ag
1A1
1Σ+
3A2
1Ag
0.0
+34.0
+9.7
0.0
4.47
PdO, 2.070; OO, 1.348; OPdO, 38.0
PdO, 2.255; OO, 1.261; OPdO, 32.5
PdO, 1.787; OPdO, 134.7
PdO, 2.067; OO, 1.312; OPdO, 37.0
PtO, 2.005; OO, 1.403; OPtO, 41.0
PtO, 1.728; OPtO, 180.0
2.87
2.39
0.0
4.13
0.0
Pd-(η2-OO)2(D2h)
Pt-(η2-OO)
OPtO
0.0
-58.1
-33.3
0.0
OPtO
2.03
0.0
PtO, 1.776; OPtO, 135.2
PtO, 2.034; OO, 1.341; OPtO, 38.8
Pt-(η2-OO)2(D2h)
B3LYP/6-311+G(d)/Lanl2DZ
3.92
PdO
3Σ-
1A1
3A2
1Ag
1A1
3Σ-
1A1
1Σ+
3A2
1Ag
1A1
PdO, 1.863
Pd-(η2-OO)
OPdO
0.0
+27.8
0.0
4.48
2.88
0.0
PdO, 2.072; OO, 1.320; OPdO, 37.1
PdO, 1.778; OPdO, 131.1
PdO, 2.043; OO, 1.298; OPdO, 37.1
PdO, 2.062; OO, 1.313; OPdO, 37.1
PtO, 1.755
PtO, 2.007; OO, 1.375; OPtO, 40.1
PtO, 1.714; OPtO, 180.0
PtO, 1.769; OPtO, 134.6
PtO, 2.011; OO, 1.328; OPtO, 38.6
PtO, 2.001; OO, 1.367; OPtO, 40.0
Pd-(η2-OO)2(D2h)
Pd-(η2-OO)2(D2d)
PtO
+2.2
0.0
3.03
4.45
0.0
2.31
0.0
Pt-(η2-OO)
OPtO
0.0
-40.6
-20.7
0.0
OPtO
Pt-(η2-OO)2(D2h)
Pt-(η2-OO)2(D2d)
+2.9
0.0
a Dipole moment (Debye).
TABLE 4: Bond-Stretching Isotopic Frequencies (cm-1),
Intensities (km/mol), and Frequency Ratios Calculated at the
BPW91 and B3LYP Levels for the Structures Described in
Table 3
molecule
state
16O
18O
R(16O/18O)
BPW91/6-311+G(d)/Lanl2DZ
PdO
3Σ-
1A1 1077.1(40)
621.9(13.7)
591.0(12.4)
1015.6(36)
1.0523
1.0606
1.0607
1.4965
1.0606
1.0605
1.0557
1.0516
1.0527
1.0605
Pd-(η2-OO)
Pd-(η2-OO)
OPdO
3B1 1339.4(228) 1262.7(202)
3A2
771.6(54)
735.1(49)
Pd-(η2-OO)2(D2h) 1Ag 1107.9(551) 1044.6(490)
Pt-(η2-OO)
PtO
OPtO
1A1
3Σ-
1Σ+
3A2
966.6(9)
911.5(8)
844.8(28)
962.2(115)
797.3(45)
800.2((22)
915.0(104)
757.4(40)
975.2(362)
OPtO
Pt-(η2-OO)2(D2h)
1Ag 1034.2(408)
B3LYP/6-311+G(d)/Lanl2DZ
PdO
3Σ-
1A1 1160.3(77)
598.7(3.7)
569.0(3.4)
1094.0(68)
1.0522
1.0606
1.0605
1.0603
1.0605
1.0558
1.0516
1.0528
1.0604
1.0604
Pd-(η2-OO)
Figure 8. Infrared spectra in the 1250-750 cm-1 region for laser-
ablated Au atoms co-deposited with 0.05% 16O2 + 16O18O + 18O2 in
excess neon at 4-5 K. (a) Sample deposited for 60 min, (b) after
annealing to 6 K, (c) after annealing to 10 K, and (d) after annealing
to 10 K.
Pd-(η2-OO)2(D2h) 1Ag 1145.6(832) 1080.2(738)
Pd-(η2-OO)2(D2d) 1A1 1194.4(149) 1126.5(131)
Pt-(η2-OO)
PtO
OPtO
1A1 1032.0(26)
973.1(24)
798.9(30)
963.9(136)
757.7(33)
3Σ-
1Σ+ 1013.6(150)
3A2
797.3(36)
843.5(33)
on annealing to 6 and 8 K, decreased on photolysis, and
increased greatly on further annealing to 11 K. The 18O2
counterpart for this band at 997.0 cm-1 gives a 16O/18O isotopic
ratio of 1.0592, which is slightly higher than the ratio for Pt-
(η2-OO) and appropriate for the antisymmetric O-O stretching
mode in the Pt-(η2-OO)2 molecule. The sextet in the 16O2 +
16O18O + 18O2 experiment suggests that two equivalent OO
subunits are involved in this molecule. This assignment is
confirmed by DFT calculations: both BPW91 and B3LYP
functionals predicted that Pt-(η2-OO)2 has the 1Ag ground state.
The O-O stretching frequency was predicted at 1034.2 cm-1
at the BPW91 level, while the B3LYP functional gave 1064.8
cm-1, which is slightly higher than the BPW91 value but very
close to the experimental determination at 1056.0 cm-1. The
D2d structure of Pt-(η2-OO)2 is 2.9 kcal/mol higher in energy
than the D2h structure calculated at the B3LYP level. Although
the predicted frequency of the O-O stretching mode at 1084.0
cm-1 is close to the experimental observation, two equivalent,
small negative frequencies at -51.7 cm-1 indicate a second-
OPtO
Pt-(η2-OO)2(D2h)
Pt-(η2-OO)2(D2d)
1Ag 1064.8(625) 1004.2(555)
1A1 1084.0(70)
1022.3(62)
order saddle point. Similar conclusions have been reached for
Ni-(η2-OO)2.17 Our calculations disagree with the original
proposal of D2d structures14 for these M(O2)2 molecules and
provide strong evidence that the structures are D2h.
Ag-(η1-OO). The thermal Ag experiments produced a 1078.9
cm-1 argon matrix band, which was assigned to Ag(O2),11c and
our laser-ablation investigation gave this band at 1075.7 cm-1
with an associated overtone at 2133.2 cm-1 24
However, with
.
16O2 + 16O18O + 18O2, both of these features give quartet
patterns (2133.2, 2076.1, 2073.1, 2014.6 cm-1 and 1075.7,
1046.6, 1045.1, 1015.2 cm-1), which suggest two inequivalent
oxygen atoms.
A weak, broad 1102.5 cm-1 band appeared in neon matrix
experiments on deposition, increased slightly on annealing, and
decreased slightly on photolysis (Figure 5). This band shifted

5818 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Wang and Andrews
TABLE 5: States, Relative Energies (kcal/mol), and Geometries Calculated at BPW91 and B3LYP Levels for Silver Oxygen
Complexes and Oxides
molecule
state
rel energy
µa
geometry (Å, deg)
BPW91/6-311+G(d)/Lanl2DZ
Ag-(η1-OO)b
2A′′
4A′′
2A′
3A′′
1A′
5A′′
3A′
4B1
0.0
2.83
0.04
5.56
1.59
3.99
5.56
1.98
0.0
0.80
0.87
1.93
5.31
AgO, 2.311; OO, 1.266; AgOO, 118.8
AgO, 4.359; OO, 1.221; AgOO, 122.4
AgO, 2.158; OO, 1.314; AgOO, 115.9
AgO, 2.640; OO, 1.295; AgOO, 123.0
AgO, 2.251; OO, 1.339; AgOO, 120.3
AgO, 4.311; OO, 1.221; AgOO, 115.2
AgO, 2.469; OO, 1.218; AgOO, 130.6
AgO, 2.032; OAgO, 180.0
AgO, 2.257; OO, 1.264; AgOO, 120.3; OAgO, 179.7
AgO, 2.243; OO, 1.267; AgOO, 120.2; OAgO, 179.8
AgO, 2.189; OO, 1.333; AgOO, 114.9; OAgO, 178.1
AgO, 2.329; OO, 1.361; OOO, 113.7; OAgO, 58.6
7.6
21.6
-34.6
-24.1
-22.8
180.2
52.4
Ag-(η1-OO)-
Ag-(η1-OO)+
OAgO
Ag-(η1-OO)2 (C2V)
4A1
2A1
3B2
0.0
9.4
-53.6
Ag-(η1-OO)2- (C2V)
-
Ag+O3
2B2
B3LYP/6-311+G(d)/Lanl2DZ
Ag-(η1-OO)c
2A′′
4A′′
2A′
0.0
2.61
0.07
6.60
2.16
4.13
5.02
0.0
1.03
1.35
2.73
5.84
AgO, 2.369; OO, 1.245; AgOO, 119.3
AgO, 3.894; OO, 1.206; AgOO, 122.0
AgO, 2.136; OO, 1.321; AgOO, 114.0
AgO, 2.499; OO, 1.293; AgOO, 120.3
AgO, 2.268; OO, 1.321; AgOO, 120.2
AgO, 2.469; OO, 1.204; AgOO, 136.1
AgO, 1.916; OAgO, 180.0
AgO, 2.168; OO, 1.268; AgOO, 121.1; OAgO, 178.8
AgO, 2.174; OO,1.271; AgOO, 111.9; OAgO, 180.0
AgO, 2.099; OO, 1.345; AgOO, 111.5; OAgO, 177.8
AgO, 2.325; OO, 1.343; OOO, 113.4; OAgO, 57.7
3.9
19.4
-36.8
-20.9
176.2
82.6
Ag-(η1-OO)-
3A′′
1A′
Ag-(η1-OO)+
OAgO
3A′′
+
4Σg
Ag-(η1-OO)2 (C2V)
4B1
2B2
3A2
2B1
0.0
4.6
Ag-(η1-OO)2- (C2V)
-40.5
Ag+O3
-
a Dipole moment (Debye). b The geometry of AgOO with BPW91 and SCI-PCM model (neon ꢀ ) 1.24) AgO, 2.229; OO, 1.310, AgOO, 118.6°.
c The geometry of AgOO with B3LYP and SCI-PCM model (neon ꢀ ) 1.24) AgO, 2.203; OO, 1.543; AgOO, 111.1°.
TABLE 6: Bond-Stretching Isotopic Frequencies (cm-1),
Intensities (km/mol), and Frequency Ratios Calculated at
BPW91 and B3LYP Levels for the Structures Described in
Table 5
complex as it is the only new absorption at higher wavenumbers
than the 1075.7 cm-1 argon matrix band. Although the sym-
metric stretching mode of ozone is expected in this region,40
the broad 1102.5 cm-1 band is too strong for the ozone present.
With Pt and Pd, a very weak, sharp 1103.5 cm-1 band is
observed with about 25% of the ozone band intensity at 700.0
cm-1. In our Ag experiments, the broad 1102.5 cm-1 band is,
however, much stronger than the sharp 700.0 cm-1 ozone
absorption, and most of this absorption is due to AgOO.
molecule
state
16O
18O
R (16O/18O)
BPW91/6-311+G(d)/Lanl2DZ
+
OAgO
4Σg
449.3(0.3)
429.7(0.3)
1.0456
1.0609
1.0608
1.0608
1.0608
1.0608
1.0608
1.0607
1.0608
1.0608
1.0608
1.0609
1.0608
Ag-(η1-OO)a
Ag-(η1-OO)-
Ag-(η1-OO)+
2A′′ 1267.3(668) 1194.6(594)
4A′′ 1540.7(0.1)
1452.4(0.1)
2A′ 1092.4(456) 1029.8(405)
3A′′ 1174.3(877) 1107.0(779)
2
2
Two doublet electronic states, A′′ and A′, were calculated
for Ag-(η1-OO), and the latter is about 20 kcal/mol higher.41
The calculated O-O stretching frequencies of the 2A′ state are
1092.4 cm-1 (BPW91) and 1078.8 cm-1 (B3LYP), respectively,
which are in excellent agreement with the 1102.5 cm-1 neon
matrix observation. However, the calculated O-O vibrations
1A′ 1054.6(633)
5A′′ 1534.8(0.9)
3A′ 1515.9(15)
994.2(562)
1446.8(0.8)
1429.1(13)
1A′′ 1503.1(174) 1416.9(154)
Ag-(η1-OO)2(C2V) 4A1 1263.3(1426) 1190.9(1268)
2A1 1251.8(1444) 1180.1(1332)
2
of the A′′ state are 1267.3 cm-1 (BPW91) and 1325.2 cm-1
Ag-(η1-OO)2-(C2V) 3B2 1106.9(273) 1043.4(243)
Ag+O3-(C2V)
2B2
780.5(226)
735.8(201)
(B3LYP), which are much higher than the experimental value,
although 2A′′ was predicted to be the ground state. Here,
polarization-induced charge transfer must be considered. The
self-consistent isodensity-polarized continuum model (SCI-
PCM) in the Gaussian 98 program42 was used to describe the
Ag-(η1-OO) complex isolated in the low-temperature neon
matrix. At both BPW91 and B3LYP levels, only the A′′
electronic state is located for Ag-(η1-OO), and the calculated
frequencies (1100.5 and 1153.2 cm-1, Table 6) are very close
to the experimental value. It is concluded that partial charge
transfer occurs when the Ag-(η1-OO) complex is trapped in a
solid matrix. This result agrees with the matrix ESR spectrum,
which characterized AgOO as ionic.13
Ag-(η1-OO)-. The 1030.7 cm-1 band and its site at 1030.0
cm-1, observed on sample deposition in Ag + O2/Ne experi-
ments, increased slightly on annealing, but decreased on full
arc photolysis. Additional annealing failed to reproduce these
bands, and they were eliminated with 0.02% CCl4 added to the
sample, strongly suggesting an anion species.21,29-31 With the
0.2% 18O2/Ne sample, the two bands shifted to 972.9 and 971.5
cm-1, giving 16O/18O ratios 1.0594 and 1.0602, which is the
B3LYP/6-311+G(d)/Lanl2DZ
+
OAgO
4Σg
641.7(0.2)
613.6(0.2)
1.0458
1.0608
1.0608
1.0607
1.0609
1.0608
1.0608
1.0608
1.0607
1.0608
1.0607
Ag-(η1-OO)b
2A′′ 1325.2(1323) 1249.3(1175)
4A′′ 1631.7(0.1)
1538.2(0.1)
2A′ 1078.8(391) 1017.0(347)
3A′′ 1159.1(1355) 1092.6(1203)
1A′ 1096.5(1011) 1033.7(899)
Ag-(η1-OO)-
Ag-(η1-OO)+
3A′′ 1621.7(5)
1528.8(5)
Ag-(η1-OO)2(C2V) 4B1 1163.8(3652) 1097.1(3245)
2B2 1114.5(5691) 1050.7(5053)
Ag-(η1-OO)2-(C2V) 3A2 1122.0(34)
1057.7(30)
812.3(293)
Ag+O3-(C2V)
2B1
861.6(329)
a The calculated O-O frequency with BPW91 and SCI-PCM Model
(neon ꢀ ) 1.24) 1100.5 cm-1 (1321.6 km/mol). Calculation failed to
converge using argon. b The calculated O-O vibration with B3LYP
and SCI-PCM Model (neon ꢀ ) 1.24) 1153.2 cm-1 (5.2). Calculation
failed to converge using argon.
to 1040.8 cm-1 with 18O2 and gave a 1.0593 16O/18O ratio,
suggesting an O-O stretching vibration. The 16O2 + 16O18O +
18O2 sample gave a broad 1070 cm-1 intermediate feature.
Accordingly, this band must be considered for the AgOO

Precious Metal-Molecular Oxygen Complexes
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5819
TABLE 7: States, Relative Energies (kcal/mol), and Geometries Calculated at BPW91 and B3LYP Levels for Gold Oxygen
Complexes and Oxides
molecule
state
rel energy
BPW91/6-311+ G(d)/Lanl2DZ
µa
geometry (Å, deg)
Au-(η1-OO)
2A′′
4Σ
0.0
1.23
0.04
1.64
4.07
2.81
0.45
0.00
0.0
0.45
0.31
0.38
3.26
AuO, 2.197; OO, 1.249; AuOO, 119.0
AuO, 4.348; OO, 1.221; AuOO, 180.0
AuO, 2.945; OO, 1.263; AuOO, 134.9
AuO, 2.220; OO, 1.320; AuOO, 118.9
AuO, 3.792; OO, 1.225; AuOO, 123.7
AuO, 2.286; OO, 1.215; AuOO, 122.3
AuO, 1.813; OAuO, 180.0
AuO, 1.895; OAuO, 180.0
AuO, 1.893; OAuO, 172.0
AuO, 2.181; OO, 1.255; AuOO, 119.3; OAuO, 178.5
AuO, 2.165; OO, 1.257; AuOO, 119.4; OAuO, 178.8
AuO, 2.231; OO, 1.266; AuOO, 113.5; OAuO, 85.4
AuO, 2.053; OO, 1.330; AuOO, 119.1; OAuO, 176.2
6.8
-54.7
-42.6
22.6
212.5
31.8
Au- (η1-OO)-
3A′′
1A′
5A′′
3A′′
2Πg
Au-(η1-OO)+
OAuO
OAuO-
3Σg
-63.0
-40.0
0.0
-
1A1
4A1
2A1
2A2
1A1
Au-(η1-OO)2(C2V)
9.4
Au-cyc(OO)2(C2V)
Au-(η1-OO)2-(C2V)
19.8
-54.3
B3LYP/6-311+G (d)/Lanl2DZ
Au-(η1-OO)
2A′′
4Σ
0.0
0.52
0.07
1.37
3.90
0.0
0.00
0.66
0.32
0.54
1.84
AuO, 2.469; OO, 1.220; AuOO, 119.7
AuO, 3.815; OO, 1.206; AuOO, 180.0
AuO, 2.944; OO, 1.245; AuOO, 123.4
AuO, 2.225; OO, 1.304; AuOO, 118.6
AuO, 1.798; OAuO, 180.0
0.9
-55.7
-39.6
50.8
Au-(η1-OO)-
3A′′
1A′
2Πg
OAuO
OAuO-
3Σg
-55.8
-30.7
0.0
AuO, 1.881; OAuO, 180.0
-
1A1
4A1
2B1
1A1
AuO, 1.877; OAuO, 169.8
Au-(η1-OO)2
AuO, 2.232; OO, 1.235; AuOO, 119.6; OAuO, 179.2
AuO, 2.041; OO, 1.262; AuOO, 119.3; OAuO,174.6
AuO, 2.102; OO, 1.318; AuOO, 115.9; OAuO, 175.4
29.5
-
Au-(η1-OO)2
-72.7
a Dipole moment (Debye).
TABLE 8: Bond-Stretching Isotopic Frequencies (cm-1),
Intensities (km/mol), and Frequency Ratios Calculated at
BPW91 and B3LYP Levels for the Structures Described in
Table 7
and disappeared with CCl4 added, suggesting different modes
of the same molecule. The 1.0599 16O/18O isotopic ratio of this
band is very close to the ratio for the 1030.7 cm-1 band. Similar
doublets with 1:1 shape in the mixed 16O2 + 18O2 experiment
and quartets with 1:1:1:1 distribution in the 16O2 + 16O18O +
18O2 experiment were also observed. This frequency is too high
for an O-O stretching fundamental, so a combination or
overtone band must be considered. The 2052.5 cm-1 band is
8.9 cm-1 less than 1030.7 × 2 ) 2061.4 cm-1, which is
appropriate for anharmonicity and the overtone assignment.
The Ag-(η1-OO)- anion assignment is further supported by
theoretical calculations. Although the 3A′′ state is calculated to
be 12-16 kcal/mol lower than the 1A′ state, the O-O stretching
frequencies 1054.6 cm-1 (BPW91, 16O/18O ) 1.0608) and
1096.3 cm-1 (B3LYP, 16O/18O ) 1.0609) calculated for the 1A′
state are in very good agreement with the observed 1030.7 cm-1
molecule
state
16O
18O
R (16O/18O)
BPW91/6-311+G(d)/Lanl2DZ
OAuO
2Πg
769.9(24)
635.5(62)
644.9(101)
732.1(21)
604.3(55)
613.2(89)
1.0517
1.0517
1.0517
1.0608
1.0608
1.0609
1.0608
1.0608
1.0609
1.0607
1.0608
-
OAuO-
3Σg
1A1
Au-(η1-OO)
Au- (η1-OO)-
Au- (η1-OO)+
2A′′ 1316.5(437) 1241.0(389)
4Σ
1541.9(0.1) 1453.5(0.1)
3A′′ 1295.2(770) 1220.9(684)
1A′ 1106.5(424) 1043.1(377)
3A′′ 1462.1(143) 1378.3(127)
Au-(η1-OO)2(C2V) 4A1 1289.6(1020) 1215.6(906)
2A1 1275.2(1087) 1202.2(966)
Au-cyc(OO)2(C2V) 2A2
1170.3(561) 1103.2(498)
1268.8(128) 1196.1(114)
3
Au-(η1-OO)2-(C2V) 1A1 1051.3(912)
991.1(810)
1.0607
value (16O/18O isotopic ratio 1.0594), but the calculated A′′
frequencies are 63-120 cm-1 higher. This suggests that 1A′ is
actually the ground state.
B3LYP/6-311+G(d)/Lanl2 DZ
-
OAuO
2Πg
800.8(23)
675.9(53)
690.0(103)
761.4(21)
642.7(46)
656.1(92)
1.0517
1.0517
1.0517
1.0608
1.0608
1.0608
1.0608
1.0608
1.0608
1.0607
-
OAuO-
3Σg
Ag+O3-. In the Ag + O2/Ne experiment, a band observed
after deposition at 796.1 cm-1 increased greatly on higher
temperature annealing. This band shifted to 752.1 cm-1 with a
1.0585 16O/18O isotopic ratio. Quartets in the mixed 16O2 + 18O2
experiment and sextets with the scrambled 16O2 + 16O18O +
18O2 sample were observed, and the distributions of both quartets
and sextets are analogues to isotopic spectra of the O3 in the
same sample. This band is assigned to the AgO3 complex. A
similar band was observed at 790 cm-1 in analogous argon
matrix spectra24 and at 791.8 cm-1 by Tevault et al.43 The
assignment is strongly supported by DFT calculations. The
ν3(O3) mode frequency in the AgO3 complex is predicted at
780.5 cm-1 with a 1.0608 16O/18O isotopic ratio in BPW91
calculation, which agrees with the observed value, and a
reasonable frequency at 861.6 cm-1 and a 1.0607 ratio is given
by B3LYP calculation. The position of this frequency in the
1A1
Au-(η1-OO)
Au-(η1-OO)-
Au-(η1-OO)2
2A′′ 1476.6(441) 1392.0(392)
4Σ
1631.5(0.2) 1538.0(0.1)
3A′′ 1342.8(1541) 1265.8(1370)
1A′ 1150.9(673) 1084.9(578)
4A1 1337.8(2424) 1261.1(2154)
2B1 1138.9(2890) 1073.6(2779)
1A1 1057.1(1388) 996.6(1234)
Au-(η1-OO)2
-
O-O stretching vibrational ratio. In the mixed 16O2 + 18O2
experiment, doublet bands at 1030.8 and 972.9 cm-1, as well
as 1030.0 and 971.5 cm-1, were produced, which indicates that
only one OO subunit is involved in this molecule. In addition,
quartets with approximately 1:1:1:1 distributions were observed
with the scrambled 16O2 + 16O18O + 18O2 sample, suggesting
one oxygen molecule coordinated to silver with the end-on
structure.
range of alkali metal ozonides44 suggests a charge-transfer
-
The weak band centered at 2052.5 cm-1 has exactly the same
behavior as the 1030.7 cm-1 absorption. This band appeared
on deposition, increased on annealing, decreased on photolysis,
complex Ag+O3
.
(Ag)x-(η1-OO). The absorption at 1042.1 cm-1 increased
markedly on annealing, disappeared on photolysis, and re-

5820 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Wang and Andrews
matrix but does not account for the experimental differences.
Although the more ionic 2A′ state for AuOO would not converge
with DFT, a more ionic low-lying electronic state is suggested
to be trapped in solid argon. Gold is capable of strong chemical
bonds, and the weak Au-OO interaction will require still more
sophisticated theoretical treatments.45
The mono(dioxygen) complexes of Cu, Ag, and Au have been
studied by ESR spectroscopy. Howard et al.13 reported that the
two oxygen atoms in Ag(O2) and Au(O2) complexes are
equivalent on the basis of the hyperfine structure from 17O
nuclei. However, a side-on structure of gold mono(dioxygen)
complex was suggested in an analysis of the matrix isolation
quadrupole interaction, although the ESR spectrum of Au(O2)
was unusually complicated.12 We conclude that two inequivalent
oxygen atoms are involved in our neon matrix isolation spectra,
and the end-on Au-(η1-OO) structure is strongly suggested.
Figure 9. Infrared spectra in the 1250-750 cm-1 region for laser-
ablated Au atoms co-deposited with 0.05% O2 in excess noble gas. (a)
0.2% O2 in neon co-deposited at 4-5 K, (b) after annealing to 8 K, (c)
after full-arc photolysis, (d) 1% O2 in argon co-deposited at 7-8 K,
(e) after full-arc photolysis, and (f) after annealing to 40 K.
Au-(η1-OO)2-. A broad band centered at 911.9 cm-1
exhibited sensitive annealing and photolysis behavior (Figure
8). This band appeared on annealing to 6 K, increased greatly
on annealing to 10 K, but decreased on photolysis. The band
shifted to 861.2 cm-1 with 18O2 and gave a 1.0589 16O/18O ratio,
which characterizes an O-O stretching mode. Mixed 16O2 +
18O2 revealed an isotopic triplet, indicating the involvement of
two OO subunits in this mode. Unfortunately, with the
scrambled 16O2 + 16O18O + 18O2 sample, the isotopic shape is
not resolved due to the broad absorption. The 911.9 cm-1 band
is eliminated by doping with CCl4 in the sample to trap electrons
during deposition, which strongly suggests an anion identifi-
appeared on further annealing. In the experiments with CCl4
added, the band was also observed, suggesting a neutral
species. This band shifted to 983.6 cm-1 with 18O2 and gave a
1.0595 16O/18O ratio. The doublets with 1:1 relative intensities
in mixed 16O2 + 18O2 experiments (Figure 6) and quartets with
approximate 1:1:1:1 shape suggest a single O-O subunit
analogous to the Ag-(η1-OO) complex. The marked growth on
annealing and appearance about 60 cm-1 lower suggests a higher
complex, and (Ag)x-(η1-OO) is proposed for this band. The
associated 2074.9 cm-1 band shifted to 1957.0 cm-1 with 18O2
(1.0603 16O/18O isotopic ratio), which is appropriate for O-O
stretching vibrational ratios. This band tracked with the 1042.1
cm-1 band very well, and an overtone band of 1042.1 cm-1
must be considered; the 9.3 cm-1 difference between the
observed 2074.9 cm-1 and 1042.1 × 2 ) 2084.2 cm-1 is
appropriate for anharmonicity.
cation.21,29-31 Assignment of this band to Au-(η1-OO)2 is
-
1
supported by theoretical calculations. The A1 ground state is
predicted for Au-(η1-OO)2-, and the O-O antisymmetric
stretching frequency at 1051.3 cm-1 (BPW91) and 1057.1 cm-1
(B3LYP) are 13% higher than the observed 911.9 cm-1 band.
The calculated 16O/18O isotopic ratio agrees with the experi-
mental value. The symmetric vibrational mode was calculated
to be much weaker and is not observed here.
Au-(η1-OO). The broad band centered at 1213.6 cm-1 is
assigned to the Au-(η1-OO) complex. This band appeared on
deposition, increased on annealing to 6 and 10 K, decreased on
photolysis, and increased again on further annealing to 11 K
(Figure 7). The 1213.6 cm-1 band shifts to 1144.8 cm-1 in the
18O2 experiment with a 1.0601 16O2/18O2 isotopic ratio and
shows the 1:1 doublet shape with mixed 16O2 + 18O2 sample.
However, the scrambled 16O2 + 16O18O + 18O2 sample gave a
triplet centered at 1212.9, 1180.3, and 1144.4 cm-1, suggesting
that one OO subunit is involved in this molecule, but the
attachment of OO to gold is uncertain because the broad isotopic
bands cannot give structure information. However, our DFT
calculations suggest that Au-(η1-OO) has end-on coordination
with a bent structure, which is analogous to the Ag-(η1-OO)
complex. The calculations predicted Au-(η1-OO) to have a 2A′′
ground state and O-O stretching vibration at 1316.5 cm-1 with
a 1.0608 16O/18O ratio (BPW91) and at 1476.6 cm-1 with a
1.0608 16O/18O ratio (B3LYP), which are in excellent agreement
with the observed isotopic ratio, but the frequency is overesti-
mated by 100.8 cm-1 (BPW91) and 260.9 cm-1 (B3LYP).
Recall that a 1093.8/1091.7 cm-1 band has been assigned to
AuO2 in solid argon.11c,24 The 1213.6 cm-1 neon matrix band
appears to be counterpart to the 1093.8 cm-1 argon matrix band.
This comparison (Figure 9) suggests that AuOO is substantially
more ionic in the more polarizable argon matrix. A similar SCI-
AuO2. The sharp band at 824.2 cm-1 is assigned to AuO2,
which was observed in argon matrix laser-ablation experiments
at 817.9 cm-1 24
. This reaction product was produced on sample
deposition, increased 10% on annealing, and increased 100%
on photolysis. The 1:2:1 triplet in the scrambled isotopic
experiment (Figure 8) indicates two equivalent oxygen atoms
in this molecule. The 824.2 cm-1 band exhibits a 1.0510 16O/
18O isotopic ratio, which is much lower than the ratio in
molecular oxygen complexes and appropriate for a ν3 funda-
mental in a linear O-Au-O molecule. The 824.2 cm-1 band
appeared with about two-thirds of the absorbance in the CCl4
doped experiment, so the anion identification is highly unlikely.
Similarly, the 1% O2 argon matrix experiment was repeated with
0.1% CCl4, and the 817.9 cm-1 band was observed with 70%
of the yield as in an identical control experiment without CCl4.
Our B3LYP functional calculation gave linear OAuO with an
intense band at 800.8 cm-1 and a 1.0518 16O/18O isotopic ratio,
which matches the observed values very well. The BPW91 level
calculation provided a reasonable frequency at 769.9 cm-1 and
a 1.0516 16O/18O isotopic ratio.46 In contrast, the OAuO- anion
calculations gave much lower frequencies (Table 8), which are
not compatible with the observed spectrum.
Comparison of Metal-Oxygen Complexes and Matrices.
The bonding of Pd, Pt, Ag, and Au atoms with molecular oxygen
is quite different. In the Pd and Pt experiments, side-on M-(η2-
OO) (M ) Pd, Pt) complexes are observed; however, end-on
M-(η1-OO) (M ) Ag, Au) complexes are found in the Ag and
2
PCM calculation (BPW91) performed for A′′ AuOO gave a
1310.7 cm-1 fundamental in neon and a 1302.8 cm-1 funda-
mental in argon (ꢀ ) 1.63), which again shows the trend that
the O-O stretching frequency decreases in the more polarizable

Precious Metal-Molecular Oxygen Complexes
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5821
TABLE 9: Atomic Charge Distributions Based on Mulliken and Natural Bond Orbital Analysis (BPW91) and Observed O-O
Stretching Frequencies for MO2 Molecules
charges
Mulliken
1O
natural bond orbital
1O
observed freq (cm-1
)
metal
2O
metal
2O
neon
argon
Au-(η1-OO) (2A′′)
Ag-(η1-OO) (2A′′)
Ag-(η1-OO) (2A′)
Pt-(η2-OO) (C2V)
0.064
0.278
0.473
0.464
0.404
0.439
-0.210
-0.277
-0.378
-0.232
-0.202
-0.220
0.146
-0.001
-0.095
-0.232
-0.202
-0.220
0.225
0.340
0.620
0.574
0.488
0.910
-0.154
-0.223
-0.418
-0.287
-0.244
-0.455
-0.071
-0.117
-0.202
-0.287d
-0.244f
-0.455
1213.6a
1102.5a
1102.5
930.0a
1093.8c
1075.7c
1075.7
928.0e
Pd-(η2-OO) (C2V)
1030.1a
1093.8b
1023.0e
1096.9g
Li+(O2
)
-
a This work. b Unpublished result in this group. c Reference 24. d Orbital breakdown: Pt[core] 6s0.46 5d8.95 6p0.10; O[core] 2s1.87 2p4.39
15. f Orbital breakdown: Pd[core] 5s0.19 4d9.27; O[core] 2s1.86 2p4.35 g Reference 49.
.
e Reference
.
Au investigations. The Mulliken and natural bond orbital (NBO)
charge distributions and experimental O-O stretching frequen-
cies are listed in Table 9; the NBO distributions are expected
to be more reliable.47,48 The NBO charges on Pd and Pt in side-
on M-(η2-OO) are +0.57 and +0.49, but not near as high as
for Li in the molecule Li+(O2-). The observed O-O stretching
frequencies Pd(O2) and Pt(O2) are slightly lower than the O-O
vibration in Li+(O2-), but the bonding mechanism is very
different. The lithium superoxide molecule was first character-
ized as a highly ionic Li+(O2-) species,49 and theoretical studies
have verified the ionic model for the bonding.50 Comparing the
NBO electron configurations of Pd(O2) and Pt(O2), the 4d
population of Pd is higher than the 5d population of Pt, but the
2p oxygen population in Pd(O2) is slightly lower than that in
Pt(O2). These charges indicate that more back-donation from
the Pt 5d to O2 π* occurs. Accordingly, the O-O bond is
weakened in Pt(O2), and the insertion reaction can proceed with
little barrier.
OO) (reaction 1). The B3LYP energy will be used to discuss
the reaction mechanism because B3LYP calculations typically
give more reliable energies.
Pt + O2 annealing f Pt-(η2-OO)
∆E ) -2.3 kcal/mol (1)
This reaction is exothermic by 2.3 kcal/mol. In addition, OPtO
can be produced on broad-band photolysis at the expense of
Pt-(η2-OO):
Pt-(η2-OO) + photolysis f OPtO ∆E ) -40.6 kcal/mol (2)
Insertion reaction 2 is even more exothermic: this means that
the rearrangement of the molecular complex is thermodynami-
cally favorable but has an energy barrier. The behavior is
analogous to the reactions of platinum and molecular oxygen
in solid argon,14,15 and recent thermal Pt reactions form OPtO
only on photolysis.53
In the Au + O2 experiments, a weak OAuO band at 824.2
cm-1 was observed following deposition. This indicates that
gold atoms generated from laser ablation do not react with
molecular oxygen as effectively as ablated platinum atoms,
although ablated Au atoms also have excess translational and
metastable electronic energy. The Au-(η1-OO) complex is the
major product formed on sample annealing, which is exothermic
by 1.2 kcal/mol (reaction 3). However, the insertion reaction
from Au-(η1-OO) to OAuO is endothermic by 50.8 kcal/mol
(reaction 4), but the reaction is observed on broad-band UV
photolysis. The weak intermediate 16OAu18O band observed at
808.4 cm-1 with 16O2 + 18O2 suggests that some OAuO is made
from O atom reaction 5. Although AuO was not observed in
our neon matrix experiments, presumably due to reaction 5, AuO
was found at 619.2 cm-1 in solid argon.24
For end-on M-(η1-OO) (M ) Ag, Au) complexes, the Au
atom is essentially neutral, and the charge on Ag is much higher,
2
particularly in the A′ state. The O-O stretching frequency of
Au-(η1-OO) is much higher in solid neon than in solid argon,
and the latter approaches the O-O vibration of Li+(O2-).
The neon-argon matrix host difference observed for these
molecular oxygen complexes provides information about the
guest species. The Li+(O2-), Pd-(η2-OO), and Pt-(η2-OO)
species exhibit small differences (3, 7, and 2 cm-1, respectively)
between neon and argon matrixes (Table 9), which show that
the bonding and O-O frequency are affected little by a change
in the polarizability of the medium, which is typical for most
molecules.51 Note that the least ionic of these molecules Pd(O2)
exhibits the largest difference. This is much more pronounced
for Ag-(η1-OO) and Au-(η1-OO), where the O-O frequency is
clearly medium-dependent. The Ag-(η1-OO) and Au-(η1-OO)
complexes exhibit 27 and 120 cm-1 neon-argon differences,
respectively, suggesting that these two complexes increase
charge transfer in the more polarizable argon host. As a result,
the O-O stretching frequencies observed in the argon matrix
are very close to the same mode for Li+(O2-). In this regard,
ESR spectra suggest that these complexes are highly ionic in
solid argon, but AuOO was not examined in solid neon.12 The
difference between Ag and Au here is due in part to the greater
relativistic effect (and inertness) for gold.52
Au + O2 annealing f Au-(η1-OO) ∆E ) -1.2 kcal/mol (3)
Au-(η1-OO) f AuO2
∆E ) +50.8 kcal/mol
(4)
(5)
O + AuO f OAuO
It is interesting to note that Ag-(η1-OO)- was produced in
Ag + O2 experiments, but Au-(η1-OO)2 was observed in Au
-
+ O2 experiments. Here, the electron affinity may play an
important role in the formation of the anions. The ground-state
Ag-(η1-OO)- anion was calculated to be 36.8 kcal/mol lower
in energy than the neutral Ag-(η1-OO) complex, so the electron
affinity of the neutral Ag-(η1-OO) is estimated to be about 1.6
eV. The electron affinity of Ag-(η1-OO)2 is predicted to be 40.5
kcal/mol, which is close to the value of Ag-(η1-OO). However,
the Au-(η1-OO)2-anion is calculated to be 72.7 kcal/mol lower
than the neutral counterpart, indicating that the electron affinity
of Au-(η1-OO)2 is much higher than that of Ag-(η1-OO)2. As
Reaction Mechanisms. The metal dioxides, OPtO and
OAuO, were observed in our experiments, but the relative yields
are different. In the Pt + O2 experiment, the OPtO molecule is
a major product after sample deposition. It is concluded that
the platinum atoms generated by laser ablation have sufficient
excess energy to activate the O-O bond and give the OPtO
insertion product. However, further annealing cannot increase
PtO2 but does produce the oxygen molecular complex Pt-(η2-
-
expected, Ag-(η1-OO)- and Au-(η1-OO)2 were observed in

5822 J. Phys. Chem. A, Vol. 105, No. 24, 2001
Wang and Andrews
the experiments. The Au-(η1-OO)2 anion band increased on
-
(11) (a) Tevault, D. E. J. Chem. Phys. 1982, 76, 2859. (b) Ozin, G. A.;
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annealing after photolysis, suggesting a very high electron
affinity for Au(OO)2. Although Au(OO)2 was not observed here,
Au(OO)2- can be formed by reaction of Au(OO) and O2-; the
latter is involved in the formation of O4-, which is observed in
these experiments.28,29
(14) Huber, H.; Klotzbucher, W.; Ozin, G. A.; Vander Voet, A. Can J.
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Conclusions
(15) Bare, W. D.; Citra, A.; Chertihin, G. V.; Andrews, L. J. Phys. Chem.
A 1999, 103, 5456 (Pd, Pt + O2 in argon).
Laser-ablated palladium, platinum, silver, and gold atoms
react with molecular oxygen in excess neon during condensation
at 4 K. Further reactions on annealing and photolysis are
investigated. The reaction products were identified on the basis
of the isotopic shifts of 18O2 and isotopic multiplets for mixed
16O2 + 18O2 and scrambled 16O2 + 16O18O + 18O2 samples.
The side-on Pd-(η2-OO) and Pd-(η2-OO)2 complexes were
observed after sample deposition and further annealing, while
similar complexes, Pt-(η2-OO) and Pt-(η2-OO)2, were produced
in the reaction of platinum atoms with molecular oxygen. The
O-O stretching frequencies of M-(η2-OO) (M ) Pd, Pt) are
lower than the O-O vibration in the Li+(O2-) molecule, but
the bonding mechanisms are different: d orbitals are involved,
and Pt(O2) exhibits the effect of more d orbital back-bonding
than Pd(O2). In addition, the insertion product OPtO was formed
via the reaction of platinum atoms generated by laser ablation
with molecular oxygen and by photoisomerism of PtO2.
In the Ag, Au + O2 experiments, end-on M-(η1-OO) (M )
Ag, Au) complexes were produced, which represent different
bonding from side-on M-(η2-OO) (M ) Pd, Pt). The analogous
OAuO dioxide molecule was observed. In addition, the anions
(16) Chertihin, G. V.; Andrews, L. J. Phys. Chem. 1995, 99, 6356 (Ti,
Zr, Hf + O2 in argon).
(17) Citra, A.; Chertihin, G. V.; Andrews, L.; Neurock, M. J. Phys.
Chem. A 1997, 101, 3109 (Ni + O2 in argon).
(18) Chertihin, G. V.; Andrews, L.; Bauschlicher, C. W., Jr. J. Phys.
Chem. A 1997, 101, 4026 (Cu + O2 in argon).
(19) Zhou M. F.; Andrews, L. J. Phys. Chem. A 1998, 102, 8251 (Ta,
Nb + O2 in argon).
(20) Bare, W. D.; Souter, P. F.; Andrews, L. J. Phys. Chem. A 1998,
102, 8279. (Mo, W + O2 in argon).
(21) Zhou, M. F.; Andrews, L. J. Chem. Phys. 1999, 111, 4230 (Cr,
Mo, W + O2 in neon).
(22) Citra, A.; Andrews, L. J. Phys. Chem. A 1999, 103, 4182 (Ir + O2
in argon).
(23) Citra, A.; Andrews, L. J. Phys. Chem. A 1999, 103, 4845 (Rh +
O2 in argon).
(24) Citra, A.; Andrews, L. J. Mol. Struct. 1999, 189, 95 and unpublished
results (Ag, Au + O2).
(25) Zhou, M. F.; Citra, A.; Liang, B.; Andrews, L. J. Chem. Phys. A
2000, 104, 3457 (Re, Ru, Os + O2).
(26) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
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9450.
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V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-
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Ag-(η1-OO)- and Au-(η1-OO)2 were formed by electron-
-
capture processes. Doping with CCl4 to serve as an electron
trap gave the same neutral molecules but eliminated the anion
bands, which further supports the anion identifications.21,29-30
The good agreement with frequencies and isotopic frequency
ratios from BPW91 and B3LYP density functional calculations
further supports the vibrational assignments. The natural bond
orbital charge distributions in M(O2) molecules show increased
charge transfer in the order Au < Pd < Pt < Ag (2A′) < Li,
and there is an unusual matrix effect for the least ionic (AuOO)
of these.
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Acknowledgment. We acknowledge support for this re-
search from the National Science Foundation.
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