Infrared spectra and structures of the coinage metal dihydroxide molecules

Article abstract of DOI:10.1021/ic051201c

Laser-ablated Cu, Ag, and Au atoms react with H₂O₂ and with H₂ + O₂ molecules during condensation in excess argon to give four new IR absorptions in each system (O-H stretch, M-O-H bend, O-M-O stretch, and M-O-H

Full text of DOI:10.1021/ic051201c

Inorg. Chem. 2005, 44, 9076−9083
Infrared Spectra and Structures of the Coinage Metal Dihydroxide
Molecules
Xuefeng Wang and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, P.O. Box 400319, CharlottesVille, Virginia 22904-4319
Received July 18, 2005
Laser-ablated Cu, Ag, and Au atoms react with H2O2 and with H2
+
O2 molecules during condensation in excess
H bend, O O stretch, and M−O−H
argon to give four new IR absorptions in each system (O H stretch, M
−
−O
−
−M
−
deformation modes) that are due to the coinage metal M(OH)2 dihydroxide molecules. Isotopic substitution (D2O2,
18O2, 16O18O, D2, and HD) and comparison with frequencies computed by DFT verify these assignments. The
2
calculations converge to Bg ground electronic state structures with C2h symmetry, 111
−117
°
M
−O
−H bond angles,
and substantial covalent character for these new metal dihydroxide molecules, particularly for Au(OH)2. This is
probably due to the high electron affinity of gold owing to the effect of relativity.
Introduction
molecule has been observed in solid argon and in the gas
phase, and AuH2 has been prepared in solid hydrogen.7-10
The neutralization of aqueous gold chloride solutions (HAuCl4)
with Na2CO3 leads to an amorphous brown precipitate of
gold(III) aquoxide, or Au2O3‚nH2O, which is sometimes
called Au(OH)3.11 In addition, the anion Au(OH)4- is thought
to form in strongly basic solutions,1,5 and the crystal
structures of two alkaline earth metal tetrahydroxoaurate-
(III) salts have been determined.12 Finally, dimethyl gold-
(III) hydroxide is known as a tetrameric ring compound.13
Silver and copper, on the other hand, form monobasic
hydroxides of moderate strength.1 The CuOH and AgOH
molecules have been characterized in the gas phase14-16 and
by calculations at several levels of theory including relativ-
istic effects.4,17 In addition, CuOH has been prepared in solid
argon from the photochemical reaction of Cu and H2O.18
Copper dihydroxide is obtained as a blue precipitate from
The coinage metals show decreasing chemical reactivity
going down the family from copper to silver to gold. Gold
is unreactive, but gold combines with strong chemical
oxidizing agents such as chlorine or aqua regia.1 Although
binary gold compounds are limited to pairing with the most
electronegative elements, stable gold hydrides have been
prepared in spectroscopic quantities.2,3 Gold hydroxide
molecules have not been observed experimentally, but
relativistic calculations have recently been performed for
AuOH.4 Gold aquoxide or hydroxide solid materials prepared
by the electrochemical oxidation of gold anode surfaces in
-
solutions are not well-defined, but Au(OH)2 is a possible
hydrolysis product.5 A solid produced by cooling annealed
gold films in H2 and then O2 was examined by early electron
diffraction methods and determined to have the Au(OH)2
stoichiometry,6 but this work has not been substantiated.
Monomeric gold(II) complexes are scarce, although the AuO
(7) Citra, A.; Andrews, L. THEOCHEM 1999, 489, 95.
(8) Okabayashi, T.; Koto, F.; Tsukamoto, K.; Yamazaki, E.; Tanimoto,
M. Chem. Phys. Lett. 2005, 403, 223.
* Author to whom correspondence should be addressed. E-mail: lsa@
virginia.edu.
(9) (a) Wang, X.; Andrews, L.; Manceron, L.; Marsden, C. J. Phys. Chem.
A 2003, 107, 8492. (b) Andrews, L.; Wang, X.; Manceron, L.;
Balasubramanian, K. J. Phys. Chem. A 2004, 108, 2936.
(10) Pyykko, P. Angew. Chem., Intl. Ed. 2004, 43, 4412.
(11) Schwarzmann, E.; Fellwock, E. Z. Naturforsch. 1971, 26b, 1369.
(12) (a) Jones, P. G.; Sheldrick, G. M. Acta Crystallogr., Sect. C 1984,
40, 1776. (b) Jones, P. G.; Schelbach, R.; Schwarzmann, E. Z.
Naturforsch. 1987, 42b, 522.
(13) Glass, G. E.; Konnert, J. H.; Miles, M. G.; Britton, D.; Tobias, R. S.
J. Am. Chem. Soc. 1968, 90, 1131.
(14) Trkula, M.; Harris, D. O. J. Chem. Phys. 1983, 79, 1138.
(15) Jarman, C. N.; Fernando, W. T. M. L.; Bernath, P. F. J. Mol. Spectrosc.
1990, 144, 286; 1991, 145, 151.
(1) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdV.
Inorg. Chem., 6th ed.; Wiley: New York, 1999.
(2) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure,
Vol. IV, Constants of Diatomic Molecules; Van Nostrand Reinhold:
New York, 1979.
(3) (a) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2001, 123, 12899. (b)
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9076 Inorganic Chemistry, Vol. 44, No. 24, 2005
10.1021/ic051201c CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/18/2005

Coinage Metal Dihydroxide Molecules
alkali hydroxide and cupric ion solution, but warming
dehydrates this compound to the oxide. However, Cu(OH)2
is soluble in concentrated strong base to give deep blue
anions [such as Cu(OH)42-], but there is no evidence for
pure solid Ag(OH)2.1 In this paper, we report the preparation
of copper, silver, and gold dihydroxide molecules through
the reaction of excited metal atoms with hydrogen peroxide
molecules and with mixtures of O2 and H2. Similar reactions
with H2 have formed coinage metal hydrides, where mono-
hydride species dominate the product yield,3,9 but in contrast,
the metal dihydroxide is the major hydroxide molecule
formed here. This is particularly noteworthy as the +2
oxidation state is less common for silver and gold. A
preliminary communication on the Au(OH)2 molecule has
been published.19
Figure 1. Infrared spectra for the copper and hydrogen peroxide reaction
products in solid argon at 10 K. (a) Cu + H2O2 deposited for 60 min, (b)
after 240-380 nm irradiation, and (c) after λ > 220 nm irradiation; (d) Cu
+ D2O2 deposited for 60 min, (e) after 240-380 nm irradiation, and (f)
after λ > 220 nm irradiation.
Experimental and Theoretical Methods
Experimentally, laser-ablated Cu, Ag, and Au atoms were reacted
with H2O2 molecules in an argon stream during condensation onto
a 10 K infrared transparent window, as described previously.3,20
Urea-hydrogen peroxide (Aldrich) at room temperature in the
sidearm of a Chemglass Pyrex-Teflon valve provided H2O2
molecules to the flowing argon reaction medium (4 mmol in 60
min). A deuterium-enriched urea-D2O2 complex was prepared by
adapting earlier procedures to exchange urea and H2O2 with D2O.21
Argon matrix infrared spectra were recorded on a Nicolet 750
spectrometer after sample deposition, after annealing, and after
irradiation by a mercury arc lamp (Philips, 175 W, globe removed).
Experiments were also performed with H2 and O2 mixtures in order
to introduce 18O2 into the reaction products, as done for the alkaline
earth metal dihydroxide molecules.22
electrons).25 All geometrical parameters were fully optimized, and
harmonic frequencies were calculated analytically at the optimized
structures.
Results and Discussion
Reactions of coinage metal atoms with H2O2 and with H2
+ O2 mixtures are investigated, and the major metal
dihydroxide products are identified by matrix infrared
spectroscopy through isotopic substitution and comparison
to frequencies calculated by density functional theory.
Copper. Laser-ablated copper atoms were deposited with
H2O2 in an argon stream during condensation at 10 K, and
infrared spectra of this sample are illustrated in Figure 1.
The strong 3586.2 and weaker 3576.3 and 3573.4 cm-1 bands
are common to H2O2 experiments,21,23 but the sharp 3634.7
cm-1 band is unique to copper, as are the sharp bands at
773.5, 769.4, 719.4, and 468.8 cm-1 also labeled Cu(OH)2
in the lower wavenumber region. Annealing to 22 K
sharpened but did not increase these new bands, whereas
the H2O2 dimer absorptions increased about 20%, and the
OH radical absorption at 3553.0 and 3548.0 cm-1 decreased
as well.26 Visible irradiation had no effect on the spectrum.
However, ultraviolet (240-380 nm) irradiation exciting the
Theoretically, the structures and vibrational frequencies of the
Cu(OH)2, Ag(OH)2, Au(OH)2, CuOH, AgOH, and AuOH molecules
were calculated using methods employed for gold hydrides3 and
mercury dihydroxide.23,24 The 6-311++G(3df,3pd) basis was used
for H, O, and Cu. Relativistic effects were accounted for in the
SDD pseudopotential and basis for silver and gold (19 valence
(17) (a) Illas, F.; Rubio, J.; Centellas, F.; Virgili, J. J. Phys. Chem. 1984,
88, 5225. (b) Bauschlicher, C. W. Int. J. Quantum Chem. Symp. 1986,
20, 563. (c) Mochizuki, Y.; Takada, T.; Murakami, A. Chem. Phys.
Lett. 1991, 185, 535.
(18) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. J Phys. Chem. 1985,
89, 3541.
(19) Wang, X.; Andrews, L. Chem. Commun. 2005, 4001.
(20) Andrews, L. Chem. Soc. ReV. 2004, 33, 123 and references therein.
(21) (a) Pettersson, M.; Tuominen, S.; Rasanen, M. J. Phys. Chem. A 1997,
101, 1166. (b) Pehkonen, S.; Pettersson, M.; Lundell, J.; Khriachtchev,
L.; Rasanen, M. J. Phys. Chem. A 1998, 102, 7643.
(22) (a) Andrews, L.; Wang, X. Inorg. Chem. 2005, 44, 11. (b) Wang, X.;
Andrews, L. J. Phys. Chem. A 2005, 109, 2782.
2
2
copper S f P transition27 increased the above bands by
25%, and the more intense ultraviolet light in the full arc (λ
> 220 nm) increased the bands another 20% (Figure 1b,c).
The OCuO doublet at 823.1 and 818.7 cm-1 and weak CuOH
bands at 727.5 and 632.5 cm-1 also increased on UV
irradiation.18,28
(23) (a) Wang, X.; Andrews, L. Inorg. Chem. 2005, 44, 108. (b) Wang,
X.; Andrews, L. J. Phys. Chem. A 2005, 109, 3849.
The analogous experiment with D2O2 gave 2651.3 and
2645.7 cm-1 bands common to D2O2 investigations21 and a
sharp 2681.4 cm-1 band unique to copper along with sharp
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian, Inc.:
Pittsburgh, PA, 1998
(25) Schwerdtfeger, P.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P.
D. W. J. Chem. Phys. 1989, 91, 1762.
(26) Cheng, B.-M.; Lee, Y.-P.; Ogilvie, J. F. Chem. Phys. Lett. 1988, 109,
151.
(27) (a) Vala, M.; Zeringue, K.; Shakhs Emampour, J.; Rivoal, J.-C.;
Pyzalski, R. J. Chem. Phys. 1984, 80, 2401. (b) Ozin, G. A.; Gracie,
C. J. Phys. Chem. 1984, 88, 643.
(28) Chertihin, G. V.; Andrews, L.; Bauschlicher, C. W., Jr. J. Phys. Chem.
A 1997, 101, 4026.
Inorganic Chemistry, Vol. 44, No. 24, 2005 9077

Wang and Andrews
D2 + O2 reactions with Cu are compared in Figure 3. The
new product absorptions with D2 + O2 at 2681.5, 768.04,
764.02, and 549.8 cm-1 are within 0.1 cm-1 of the absorp-
tions formed with D2O2 as the reagent. The HD + O2 mixture
gave the following diagnostic information: (a) an intermedi-
ate copper isotope doublet at 770.42 and 766.33 cm-1, (2)
two bending modes at 704.4 and 526.0 cm-1, (3) a new O-H
stretching mode at 3636.3 cm-1, which is 1.5 cm-1 higher
than the frequency with H2, and (4) a new O-D stretching
mode at 2684.1 cm-1, which is 2.6 cm-1 higher than the
frequency with D2.
Hence, we have identified the new Cu(OH)2 molecule from
four vibrational modes (O-H stretch, O-Cu-O stretch,
Cu-O-H bend, and Cu-O-H out-of-plane deformation).
Mixed isotopic precursors have verified the participation of
two O-H(O-D) oscillators, two equivalent O atoms, and a
single Cu atom in this new dihydroxide molecule.
Figure 2. Infrared spectra for the copper, oxygen, and hydrogen reaction
products in solid argon at 10 K. (a) Cu + 0.4% O2 + 6% H2 deposited for
60 min, (b) after λ > 220 nm irradiation, and (c) after annealing to 20 K;
(d) Cu + 0.1% 16O2 + 0.2% 16O18O + 0.1% 18O2 + 6% H2 deposited and
irradiated with λ > 220 nm irradiation and (e) after annealing to 22 K; (f)
Cu + 0.2% 18O2 + 6% H2 deposited and irradiated with λ > 220 nm and
(g) after annealing to 24 K.
The assignments to Cu(OH)2 are confirmed by the match
of frequencies and isotopic shifts computed by density
functional theory. Table 1 compares the observed and
calculated frequencies. First, the ground-state molecule is
associated bands at 767.9, 763.9, and 549.8 cm-1. These
bands exhibited the same annealing and photolysis behavior
as their H2O2 counterparts. A sharp, weak 3636.2 cm-1 band
was observed from HDO2 in the D2O2 sample. Weak CuOD
bands were also observed at 635.1 and 533.5 cm-1.18
The sharp doublet at 773.5 and 769.4 cm-1 exhibits the
2.3-1.0 relative intensity for 63Cu and 65Cu in natural
abundance and demonstrates the participation of a single Cu
atom in the new product molecule. Furthermore, the mag-
nitude of the 63Cu and 65Cu isotopic shift indicates that copper
is vibrating between two oxygen masses (63/65 frequency
ratio 1.0053) and not against a single oxygen mass as in the
diatomic CuO molecule (63/65 ratio 1.0031).28
Experiments were performed with Cu and H2 + O2
mixtures in order to introduce 18O2 into the new product
molecule. Infrared spectra are compared in Figure 2; the
deposited samples contain common HO2 and ArnH+ bands,29-32
and weak product absorptions, but full arc irradiation
increases the latter bands, and annealing sharpens new
absorptions at 3634.9, 773.67, 769.59, and 719.6 cm-1, which
are 0.2 cm-1 higher than the Cu and H2O2 reaction products.
The reaction with 18O2 gives substantial shifts with new
absorptions at 3623.7, 750.0, 746.1, and 711.6 cm-1, as listed
in Table 1. The H2 and 16O2 + 16O18O + 18O2 reagent
produced diagnostic isotopic multiplet absorptions. The triplet
of copper isotopic doublets with new 16O18O components at
762.6 and 758.5 cm-1 verifies the participation of two
equiValent oxygen atoms, as does the triplet at 719.6, 716.6,
and 711.6 cm-1. The new absorptions at 3637.3 and 3625.4
cm-1, in addition to the pure isotopic bands at 3634.9 and
3623.7 cm-1, arise because both (16O-H) and (18O-H)
stretching modes are active in the new mixed isotopic
product, which must, therefore, contain two OH subunits.
The participation of two OH subunits is confirmed by the
reaction of Cu and HD + O2, and spectra for H2, HD, and
2
converged to be a planar Bg state with C2h symmetry, as
shown in Figure 4, and only ungerade modes are allowed
in the IR spectrum. Second, the four strongest infrared-active
fundamentals calculated are observed in the spectrum: the
calculated O-H stretching and Cu-O-H bending and
deformation modes are 5.1, 0.5, and 10.3% higher than
observed, but the calculated O-Cu-O stretching mode is
0.8% lower than observed. Third, the calculated and observed
isotopic shifts agree very well: the calculated and observed
63Cu-65Cu shift for the bu O-Cu-O mode are both 4.1
cm-1, but the calculated 16O-18O shift (26.7 cm-1) slightly
exceeds the observed value (23.7 cm-1), whereas the
calculated H-D shift (3.5 cm-1) is just below the observed
value (5.7 cm-1). This almost pure antisymmetric O-Cu-O
stretching mode is described accurately but not perfectly by
the B3LYP calculation. The 16O-Cu-18O counterpart is
predicted to be 14.5/26.7 ) 0.543 ) 54.3% of the way up
from 18O-Cu-18O to 16O-Cu-16O and observed to be 12.6/
23.7 ) 0.532 ) 53.2% of the way up. The calculated bu
16O-H/18O-H frequency ratio (1.003 27) slightly exceeds
the observed ratio (1.003 09). In the Cu(16OH)(18OH) mol-
ecule of lower symmetry, both O-H stretching modes are
observed, and they couple slightly sharing the intensity for
their double statistical population in the 16O2 + 16O18O +
18O2 isotopic sample. The weaker 16O-H (more symmetric)
mode is predicted to be 1.6 cm-1 higher than the bu
Cu(16OH)2 value, and it is observed to be 2.4 cm-1 higher,
whereas the stronger 18O-H (more antisymmetric) mode is
predicted to be 2.3 cm-1 higher than the bu mode for
Cu(18OH)2 and is observed to be 1.7 cm-1 higher. Likewise,
in the Cu(OH)(OD) molecule, both O-H and O-D stretch-
ing modes are observed as predicted. In addition, both Cu-
O-H and Cu-O-D bending modes are allowed in this
molecule of lower symmetry, and both are observed.
(29) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1963, 38, 2627.
(30) Smith, D. W.; Andrews, L. J. Chem. Phys. 1974, 60, 81.
(31) Milligan, D. E.; Jacox, M. E. J. Mol. Spectrosc. 1973, 46, 460.
(32) Wight, C. A.; Ault, B. S.; Andrews, L. J. Chem. Phys. 1976, 65, 1244.
9078 Inorganic Chemistry, Vol. 44, No. 24, 2005

Coinage Metal Dihydroxide Molecules
Table 1. Observed and Calculated Frequencies (cm-1) for Cu(OH)2 in the C2h Structure
Cu(OH)2
calcdb
Cu(OH)(OD)
Cu(OD)2
calcd
Cu(18OH)2
calcd
mode
obsda
obsd
calcd
obsd
obsd
O-H stretch
3836.0 (ag, 0)c,d
3832.0 (bu, 256)
767.5 (bu, 98)
722.9 (bu, 197)
695.3 (ag, 0)
3636.3
2684.1
770.4
3834.0 (127)
2791.4 (76)
765.7 (110)
710.1 (105)
612.8 (5)
517.8 (32)
460.2 (141)
181.6 (11)
106.5 (1)
2792.9 (0)
2789.8 (152)
764.0 (118)
613.2 (0)
541.5 (92)
498.4 (0)
394.5 (107)
176.2 (11)
103.0 (1)
3823.4 (0)
3819.5 (250)
740.8 (44)
717.7 (240)
691.7 (0)
O-H stretch
3634.9
773.7
719.6
2681.5
768.0
3623.7
750.0
711.6
O-Cu-O stretch
Cu-O-H bend
Cu-O-H bend
O-Cu-O stretch
Cu-O-H deform
O-Cu-O bend, in
O-Cu-O bend, out
704.4
549.8
609.6 (ag 0)
526.0
576.3 (0)
468.8
517.5 (au, 176)
187.2 (bu, 11)
109.8 (au, 1)
513.4 (173)
181.4 (11)
106.5 (1)
a Observed in solid argon. b Calculated at B3LYP/6-311++G(3df,3pd) level of theory. c Mode symmetry in C2h, infrared intensity, km/mol. d Strong
absorptions calculated for Cu(16OH)(18OH) are 3834.3 (90), 3821.1 (163), 755.3 (79), 720.9 (208), and 515.4 cm-1 (175 km/mol).
Figure 5. Infrared spectra for the silver and hydrogen peroxide reaction
Figure 3. Infrared spectra for the copper, oxygen, and hydrogen reaction
products in solid argon at 10 K. (a) Ag + H2O2 deposited for 60 min, (b)
products in solid argon at 10 K. (a) Cu + 0.4% O2 + 6% H2 deposited for
after 240-380 nm irradiation, and (c) after λ > 220 nm irradiation; (d) Ag
60 min, (b) after λ > 220 nm irradiation, and (c) after annealing to 24 K;
+ D2O2 deposited for 60 min, (e) after 240-380 nm irradiation, and (f)
(d) Cu + 0.4% O2 + 6% HD deposited and irradiated with λ > 220 nm
after λ > 220 nm irradiation.
irradiation and (e) after annealing to 30 K; (f) Cu + 0.4% O2 + 6% D2
deposited and irradiated with λ > 220 nm irradiation and (g) after annealing
to 24 K.
very weak band was observed at 744.3 cm-1 (labeled m for
monohydroxide AgOH). The Ag reaction with D2O2 shifted
these absorptions to 2644.7, 522.4, and 619.5 cm-1 and to
538.5 cm-1, and a weak HDO2 reaction product was observed
at 3589.7 cm-1. The Ag and H2 + O2 reaction was performed
for 18O2 substitution, and the stronger product bands shifted
to 3571.3, 699.2, and 589.9 cm-1, as listed in Table 2.
Reaction with H2 and 16O2 + 16O18O + 18O2 gave a quartet
at 3594.4, 3582.3, 3575.1, and 3571.3 cm-1 and a triplet at
702.7, 700.9, and 699.2 cm-1. Reaction with D2 + O2 gave
the same stronger bands as D2O2, but the HD + O2 reaction
produced new absorptions at 3589.3, 2651.0, 689.8, and
Figure 4. C2h structures computed for the Cu(OH)2, Ag(OH)2, and Au-
(OH)2 molecules at the B3LYP/6-311++G(3df,3dp) level of theory using
the SDD pseudopotential for Ag and Au.
523.4 cm-1. The mixed isotopic experiments identify a new
Ag-containing molecule with two OH(OD) groups and two
equivalent oxygen atoms, and Ag(OH)2 is, thus, identified.
Like Cu(OH)2, the ground state of Ag(OH)2 is calculated
The CuOH and CuOD monohydroxide molecules are
observed, here, as weak bands for bending and Cu-O
stretching modes, in agreement with previous matrix isolation
work.18 The stronger bending mode measured in the gas
phase for CuOH and CuOD is 15 and 3 cm-1 higher,14 and
the argon matrix values sustain a reasonable matrix shift.
Silver. Laser-ablated silver atoms were reacted with H2O2
during condensation in excess argon, and infrared spectra
are illustrated in Figure 5. The metal-dependent bands labeled
Ag(OH)2 now appear at 3582.3 and 702.7 cm-1 and very
weakly at 614.6 and 454.0 cm-1. As with copper, the silver
product bands increase on UV irradiation. In addition, a new,
2
to be Bg in C2h symmetry, and the agreement between
observed and calculated frequencies displayed in Table 2
attests to this assignment. The calculation using the SDD
pseudopotential for silver predicts the O-H stretching 6.1%
too high, the Ag-O-H bending mode 2.6% too high, the
O-Ag-O stretching mode 6.6% lower and weaker than
observed, and the Ag-O-H deformation mode 9.1% too
high, which is not as good as the agreement found for Cu-
(OH)2 using the all-electron basis for copper. The calculation
does correctly predict the blue deuterium shift for the
O-Ag-O stretching mode. The bu Ag-O-H bending and
Inorganic Chemistry, Vol. 44, No. 24, 2005 9079

Wang and Andrews
Table 2. Observed and Calculated Frequencies (cm-1) for Ag(OH)2 in the C2h Structure
Ag(OH)2 Ag(OH)(OD)
calcdb
calcd
Ag(OD)2
calcd
Ag(18OH)2
mode
obsda
obsd
obsd
obsd
calcd
O-H stretch
3808.7 (ag, 0)c,d
3801.7 (bu, 452)
731.1 (bu, 245)
705.0 (ag, 0)
576.4 (bu, 2)
495.5 (au 160)
459.5 (ag, 0)
142.6 (bu, 6)
68.1 (au, 1)
3589.8
2651.0
689.8
616.2
523.4
3805.2 (225)
2770.4 (122)
719.2 (137)
585.5 (25)
524.9 (29)
448.5 (3)
439.2 (127)
139.0 (6)
67.0 (0)
2772.8 (0)
2768.0 (243)
596.7 (81)
530.2 (0)
518.2 (58)
374.3 (94)
438.9 (0)
135.5 (6)
64.2 (0)
3796.1
3789.1
726.4
701.9
553.0
491.9
434.3
137.2
65.6
O-H stretch
3582.3
702.7
2644.7
619.5
3571.3
699.2
Ag-O-H bend
Ag-O-H bend
O-Ag-O stretch
Ag-O-H deform
O-Ag-O stretch
O-Ag-O bend, in
O-Ag-O bend, out
614.6
454.0
522.4
589.9
a Observed in solid argon. b Calculated at B3LYP/6-311++G(3df,3pd)/SDD level of theory. c Mode symmetry in C2h, infrared intensity, km/mol. d Strong
absorptions calculated for Ag(16OH)(18OH) are 3806.1 (117), 3791.7 (334), 728.9 (241), 565.9 (3), and 493.6 cm-1 (158 km/mol).
O-Ag-O stretching modes are near enough to mix, and
upon deuteration, the Ag-O-D bending mode shifts below
the O-Ag-O stretch and forces it to higher wavenumbers,
although the observed interaction is not as much as predicted.
As for copper, the quartet of O-H stretching modes is
predicted for the silver and 16O2 + 16O18O + 18O2 reaction
product, although the observed splittings are slightly more
than calculated. Likewise, the O-H and O-D stretching
modes for Ag(OH)(OD) are predicted above the pure isotopic
bands as observed, and the Ag-O-H and Ag-O-D
bending modes are both observed.
The weak, new bands at 744.3 and 538.5 cm-1, just above
the strong bending modes for Ag(OH)2 and Ag(OD)2, have
the ratio 1.3822, which is appropriate for the bending modes
Figure 6. Infrared spectra for the gold and hydrogen peroxide reaction
of AgOH and AgOD. The AgOH bands have been calculated
products in solid argon at 10 K. (a) Au + H2O2 deposited for 60 min, (b)
at 766 cm-1 at the DK3-CCSD(T) level4 and at 787 cm-1 as
after 240-380 nm irradiation, and (c) after λ > 220 nm irradiation; (d) Au
the strongest IR absorption at the B3LYP level,33 which
+ D2O2 deposited for 60 min, (e) after 240-380 nm irradiation, and (f)
after λ > 220 nm irradiation.
supports the assignment of the above weak bands to AgOH
and AgOD. Our calculation for CuOH also gives param-
7. Deposition with laser-ablated Au atoms gave weak product
eters33 in good agreement with the previous higher-level
bands including OAuO,7,34 but UV irradiation increased these
calculation.4
absorptions at essentially the same wavenumbers observed
Gold. Infrared spectra of the Au + H2O2 reaction products
are illustrated in Figure 6. The major, new absorptions at
3565.9, 884.9, 676.6, and 532.5 cm-1 [labeled Au(OH)2]
with H2O2. The major product absorptions were about 4 times
stronger with H2O2. Spectra with the H2 + 16O2 + 16O18O +
18O2 reagent mixture produced multiplet absorptions near
increased substantially upon UV irradiation, but weaker
3560, 880, and 660 cm-1. Investigations were also done with
bands at 3571.3 and 840.8 cm-1 increased only slightly.
Annealing the sample to 20 K decreases the 3571.3 and 840.8
cm-1 absorptions with no effect on the former bands. Near-
ultraviolet (240-380 nm) and full-arc (λ > 220 nm)
irradiation markedly increased the former absorptions (Figure
6b,c) and slightly increased a weak, new 931.0 cm-1 band
D2 + O2 and D2 + 18O2 mixtures, and shifted product
absorptions were observed in each case. Experiments with
HD and O2 gave the unique mixed H/D product with new
absorptions (Table 3). Figure S1 (Supporting Information)
compares spectra of deuterated reaction products.
The sharp major reaction product absorptions at 3565.9,
(labeled m for monohydroxide AuOH), a weak OAuO
884.9, 676.6, and 532.5 cm-1 are grouped by common
band7,34 at 817.9 cm-1, the 3571.3 and 840.8 cm-1 pair, and
behavior upon photolysis and annealing and formation from
a weak AuH absorption3 at 2226.6 cm-1, and subsequent
both H2O2 and H2 + O2 reagents at several sample
annealing to 30 K decreased these bands by about 30%. The
gold atom reaction was also done with D2O2, and Figure 6
also presents representative spectra. The major, new absorp-
tions [labeled Au(OD)2] shifted as listed in Table 3.
concentrations. From the higher IR frequencies, O-H
stretching and Au-O-H bending modes are identified.19
Next, the 676.6 cm-1 absorption shifts to 645.7 cm-1 with
18O2 (16/18 ratio 1.0478), and the band becomes an asym-
Complementary experiments were performed with H2 +
metric triplet at 676.6, 666.2, and 645.7 cm-1 with statistical
O2 as the reagent, and infrared spectra are shown in Figure
16,18O2, which is precisely what is observed for the antisym-
(33) (a) B3LYP calculation for AgOH. Structure: Ag-O, 2.042 Å; O-H,
metric stretching mode of the linear O-Au-O molecule (16/
18 ratio 1.0506).7,34 Hence, the new molecule contains an
O-Au-O subunit with equivalent oxygen atoms. Accord-
ingly, the isotopic vibrational spectra definitively identify
the new molecule Au(OH)2.19
0.963 Å, angle 107.4°. Frequencies: 3826 cm-1 (36 km/mol), 787
cm-1 (63 km/mol), 483 cm-1 (31 km/mol). (b) B3LYP calculation
for CuOH using SDD. Structure: Cu-O, 1.780 Å; O-H, 0.963 Å,
angle 109.0°. Frequencies: 3820 cm-1 (42 km/mol), 810 cm-1 (78
km/mol), 609 cm-1 (37 km/mol).
(34) Wang, X.; Andrews, L. J. Phys. Chem. A 2001, 105, 5812.
9080 Inorganic Chemistry, Vol. 44, No. 24, 2005

Coinage Metal Dihydroxide Molecules
Table 3. Observed and Calculated Frequencies (cm-1) for Au(OH)2 in the C2h Structure
Au(OH)2
calcdb
Au(OH)(OD)
Au(OD)2
calcd
Au(18OH)2
Au(18OD)2
mode
obsda
obsd
calcd
obsd
obsd
calcd
obsd
calcd
O-H stretch
3783.0 (ag, 0)c,d
3776.4 (bu, 335)
906.5 (bu, 218)
865.9 (ag, 0)
641.2 (bu, 39)
590.4 (ag, 0)
555.5 (au, 158)
194.7 (bu, 6)
121.0 (au, 0)
3569.1
2635.5
862.1
3779.7 (167)
2751.4 (94)
888.3 (127)
682.8 (82)
619.7 (5)
573.9 (1)
491.5 (123)
189.3 (6)
2753.9 (0)
2748.9 (187)
708.6 (170)
651.1 (0)
601.7 (1)
563.8 (0)
417.6 (87)
184.0 (6)
114.4 (0)
3770.5
3764.0
900.3
862.6
611.7
557.7
551.7
186.3
115.9
2736.7
2731.8
668.8
637.6
584.8
541.0
412.2
177.1
110.2
O-H stretch
3565.9
884.9
2632.3
711.3
3555.1
879.0
2616.5
686.5
Au-O-H bend
Au-O-H bend
O-Au-O stretch
O-Au-O stretch
Au-O-H deform
O-Au-O bend, in
O-Au-O bend, out
692.6
676.6
532.5
648.1
645.7
530.7
612.9
474.4
119.0 (0)
a Observed in solid argon. b Calculated at B3LYP/6-311++G(3df,3pd)/SDD level of theory. c Mode symmetry in C2h, infrared intensity, km/mol. d Strong
absorptions calculated for Au(16OH)(18OH) are 3780.5 (89), 3766.5 (244), 903.5 (213), 630.7 (36), and 553.6 cm-1 (158 km/mol).
stretching modes in Au(16OH)(18OH) and the asymmetric
triplet mixed isotopic O-Au-O stretching mode, in agree-
ment with the observed band separations. In summary, the
B3LYP calculation accurately predicts subtle details in the
spectra of mixed isotopic molecules, which provides further
substantiation for the observation and characterization of Au-
(OH)2.
The weaker bands at 3571.3 and 840.8 cm-1 (Figure 6)
shift to 3561.0 and 837.1 cm-1 with 18O2 and to 2636.3 and
693.9 cm-1 with D2O2 (H/D ratios 1.3547 and 1.2117) and
are also due to O-H stretching and Au-O-H bending
modes. These bands decrease upon first annealing and
increase only 30% upon UV irradiation. Our B3LYP
Figure 7. Infrared spectra for the gold, oxygen, and hydrogen reaction
calculation predicts almost the same energy C2V Au(OH)2
products in solid argon at 10 K. (a) Au + 0.4% O2 + 6% H2 deposited for
structure with a strong O-H stretching mode 12 cm-1 higher
60 min, (b) after 240-380 nm irradiation, and (c) after λ > 220 nm
irradiation; (d) Au + 0.1% 16O2 + 0.2% 16O18O + 0.1% 18O2 + 6% H2
and a Au-O-H bending mode 36 cm-1 lower than those
deposited and irradiated with λ > 220 nm; (e) Au + 0.4% 18O2 + 6% H2
for the C2h structure. The weaker 3571.3 and 840.8 cm-1
deposited and irradiated with λ > 220 nm. W2 denotes water dimer
bands are in good agreement with this relationship, and these
bands are assigned to the minor population of Au(OH)2
trapped in the C2V structure.
absorptions.
The vibrational assignments to Au(OH)2 are substantiated
by excellent agreement between calculated and observed
isotopic frequencies, and this agreement likewise confirms
that the computed C2h structure is the ground-state config-
uration of Au(OH)2. Calculations for the Au(OH)2 molecule
converged to the C2h structure illustrated in Figure 4 and
provided the frequencies listed in Table 3. Notice that the
three modes involving O-H stretching and bending are
predicted to be 5.9, 2.4, and 4.3% too high by the B3LYP
calculation, which is the expected relationship.35 However,
the O-Au-O mode prediction is low (5.2%), just as was
found for Hg(OH)2 (4.8% low), Cu(OH)2, and Ag(OH)2. The
Au(OH)2 molecule was also calculated with the BPW91
density functional, and the structure was essentially the same
(1.947 Å, 0.975 Å, and 108.8°), but the frequencies were
slightly lower, as expected.35
Relativistic calculations predict the AuOH triatomic
molecule to have the 3727, 955, and 569 cm-1 frequencies,4
and our B3LYP/SDD results of 3793 cm-1 (65 km/mol
infrared intensity), 957 cm-1 (60 km/mol), and 513 cm-1
(26 km/mol) and the structure are in excellent agreement.36
The strong bending mode of AuOH is, thus, predicted to be
51 cm-1 higher than this bending mode for Au(OH)2 in the
C2h structure, and the new 931.0 cm-1 band is 46 cm-1
higher. The weak 931.0 cm-1 band and the deuterium
counterpart at 743.2 cm-1 (H/D ratio 1.2527) are produced
upon co-deposition with H2O2 and D2O2, and they increase
slightly upon UV irradiation. The 931.0 cm-1 band is favored
relative to Au(OH)2 in experiments with less total H2O2.
These bands are appropriate for assignment to AuOH and
AuOD, which provides the first experimental evidence for
the gold monohydroxide molecule.
The B3LYP calculation predicts the unobserved ag O-H(D)
stretching modes for Au(OH)2 and Au(OD)2 to be higher
than the strong observed bu modes. This means that the
uncoupled O-H and O-D stretching modes in Au(OH)-
(OD) will be higher than the observed bu bands by half of
the ag-bu mode separation, and that is precisely what is
observed and calculated (Table 3). Furthermore, the calcula-
tion indicates coupling between the 16O-H and 18O-H
The Au(OH)2- molecular anion is isoelectronic with Hg-
(OH)2, and our calculated dihedral C2 structures are almost
-
the same.23,37 In contrast, Au(OH)4 is computed to have a
planar C4h structure with slightly shorter Au-O bond lengths
(36) B3LYP calculation for AuOH. Structure: Au-O, 1.990 Å; O-H,
0.966 Å, angle 104.6°. Frequencies: 3793 cm-1 (65 km/mol), 957
cm-1 (60 km/mol), 513 cm-1 (26 km/mol).
-
(37) At the B3LYP/6-311++G(3df, 3pd)/SDD level of theory, Au(OH)2
has the C2 structure with Au-O, 2.045 Å; O-H, 0.961 Å; Au-O-H
angle, 105.0°; O-Au-O angle, 176.9°; and dihedral angle, 90°.
(35) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
Inorganic Chemistry, Vol. 44, No. 24, 2005 9081

Wang and Andrews
(2.022 Å), and these gold hydroxide anions are discussed in
the Supporting Information.
computed for gold in the Au(OH)2 molecule, and the Ba-
(OH)2 molecule is ionic, and the solid is a strong base.22,42
Family Trends. Several comparisons within the coinage
metal family are of interest. All form the M(OH)2 dihydrox-
ide molecules with C2h symmetry structures as major
products, and all give a small yield of the monohydroxide.
Our observation of the same CuOH absorptions from Cu
photochemical reactions with H2O2 and D2O2, as found with
H2O and D2O in solid argon,18 support these assignments
and the slightly higher bending mode (743, 537 cm-1)
measurements in the gas phase14 for CuOH and CuOD. This
agreement lends credence to our similar identifications of
AgOH and AgOD, and of AuOH and AuOD, from reactions
with H2O2 and D2O2. Gold alone forms a minor C2V isomer
Au(OH)2 molecule. Absorptions are observed for OCuO and
OAuO, as found before, but not for OAgO.7,28,34 Weak MH
absorptions are observed for all metals in the experiments
with H2 + O2; however, we have no evidence for CuOO,
but the AgO2 complex was strong, and AuOO was detected
as reported previously.7 This indicates that Cu is the most
reactive and Ag is the least reactive under the conditions of
these experiments.
Natural charges, bond orbital analysis,38,39 and structures
for Cu(OH)2, Ag(OH)2, and Au(OH)2 are presented in Table
S1 (Supporting Information) and Figure 4. Although the
O-H stretching frequency is lowest for Au(OH)2, the
computed charge on gold and the Au-O-H angle are
smallest, which suggests increasing covalent character from
Cu(OH)2 to Ag(OH)2 to Au(OH)2. Note that the s electron
population is substantially higher on gold, and the O-H bond
length is longest for gold as well. A straightforward rationale
for the increase in covalent character in the coinage metal
dihydroxide series from Cu to Ag to Au is the increase in
electron affinities of the metal atoms (28, 30, and 53 kcal/
mol, respectively).40 Thus, the high degree of covalent
character in the Au(OH)2 molecule is probably due, in large
part, to the high electron affinity of gold, which comes from
relativity.10 One must remember that the OH (3568 cm-1)2
and OH- (3556 cm-1)41 fundamentals are so close that the
O-H stretching frequency itself is not a measure of ionic
character. A similar relationship has been found for the Zn-
(OH)2, Cd(OH)2, and Hg(OH)2 molecules.23 The bent
M-O-H bond has been offered as a sign of covalent
character,4,17 and in contrast, the more ionic group 2 metal
dihydroxides have essentially linear M-O-H bonds.22,42 The
experimental bond angles of CuOH (110.1°) and AgOH
(107.8°) follow the same trend as that which we compute
for Cu(OH)2 and Ag(OH)2, and AuOH has not yet been
observed in the gas phase.
B3LYP calculations on the series of OMO dioxide
molecules are only an approximation, but quartet states are
lower in energy than doublet states.34 However, the anti-
symmetric O-M-O stretching frequencies for OCuO and
OAuO in the doublet states are in accord with our observa-
tions, but the doublet OAgO state with the highest energy
has an imaginary frequency and is not observed here. These
calculations support the suggestion of stable high-energy
quartet state intermediates for the excited metal atom reaction
with O2 and H2.
Reaction Mechanisms. The ground-state metal atoms
appear to be unreactive toward insertion, as observed
previously,3,7 but the excited metal atoms are reactive.
Therefore, the insertion with hydrogen peroxide is straight-
forward for excited metal atoms, which are formed in the
ablation process or by subsequent mercury arc irradiation.27,43
The [M(OH)2]* intermediate is relaxed by the matrix, and
the dihydroxide molecule is stabilized. We observe a small
yield of CuOH, AgOH, and AuOH upon sample deposition.
The H2O2 reaction has been used as a source for CuOH
molecules in a hollow cathode sputtering source with argon
carrier gas;14,15 however, the dihydroxide molecule might be
stabilized in a nozzle beam expansion experiment.
The same dihydroxide products are formed with the H2
+ O2 reagent, and this reaction appears to proceed through
the [OMO]* intermediate. We believe the M(2P) + O2(3Σ)
reaction forms the [OMO]* intermediate in a quartet state
for immediate reaction with adjacent H2 molecules. Unre-
acted [OAgO]* decomposes upon relaxation to give AgO,
2
but [OCuO]* and [OAuO]* relax to give the stable Πg
states, which are observed here.7,28,34 The M(OH)2 molecules
are very stable: reaction 2 is exothermic for ground-state
metal atoms (∆E ) -111, -66, and -80 kcal/mol,
respectively, for Cu, Ag, and Au). Furthermore, decomposi-
tion of the M(OH)2 molecules to the metal monoxide and
water in gas-phase reaction 3 are endothermic processes (∆E
) 56, 30, and 43 kcal/mol, respectively, for Cu, Ag, and
Au). Note that Ag(OH)2 is the least stable and AgO is the
only monoxide observed here (Figure 5). Finally, the M(OH)2
radicals have large electron affinities (EA ) 59, 77, 73 kcal/
mol, respectively) and form stable anions, reaction 4, and
-
these anions can react favorably to make the M(OH)4
anions. Reaction 5 energies are exothermic (-45, -29, -40
kcal/mol, respectively). However, charged species are not
major products in these systems.
M*(2P) + H2O2 f [M(OH)2]* f M(OH)2
(1a)
We conclude that solid Au(OH)2 will be a weak base. In
contrast, the positive charge calculated on the barium center
in the Ba(OH)2 molecule22 is approximately double that
M*(2P) + H2O2 f [M(OH)2]* f MOH + OH (1b)
M*(2P) + H2 + O2 f [OMO]* + H2 f M(OH)2 (2)
M(OH)2 (2Bg) f MO (2Π) + H2O (1A1)
M(OH)2 (2Bg) + e- f M(OH)2- (1A1)
(3)
(4)
(5)
(38) Reed, A. J.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(39) Frenking, G.; Frohlich, N. Chem. ReV. 2000, 100, 717.
(40) Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1985, 14, 73.
(41) Rosenbaum, N. H.; Owrutsky, J. C.; Tack, L. M.; Saykally, R. J. J.
Chem. Phys. 1986, 84, 5308.
M(OH)2- + H2O2f M(OH)4
-
(42) Kaupp, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 491.
9082 Inorganic Chemistry, Vol. 44, No. 24, 2005

Coinage Metal Dihydroxide Molecules
Conclusions
substantial covalent character for these new metal dihydrox-
ide molecules. The Au(OH)2 molecule is the most covalent
in the series, owing to the effect of relativity and the high
electron affinity of gold. We expect solid Au(OH)2 to be
weak as a base. This matrix-isolation work provides the first
experimental evidence for gold hydroxide molecules.
Reactions of laser-ablated Cu, Ag, and Au atoms with
H2O2 and with O2 + H2 during condensation in excess argon
give four new IR absorptions in each system (O-H stretch,
M-O-H bend, O-M-O stretch, and M-O-H deforma-
tion) that are due to the M(OH)2 molecules, with weaker
bands for the MOH molecules. Reagent isotopic substitution
(D2O2, 18O2, 16O18O, D2, and HD) and comparison with
frequencies computed by the B3LYP density functional
substantiate these vibrational assignments. The calculations
Acknowledgment. We appreciate support from NSF
Grant CHE 03-52487 to L.A.
Supporting Information Available: Gold hydroxide anions,
Figure S1 and Table S1. This material is available free of charge
via the Internet at http://pubs.acs.org.
2
converge to C2h symmetry structures in the Bg ground
electronic state with 111-117° M-O-H angles and reveal
(43) Gruen, D. M.; Bates, J. K. Inorg. Chem. 1977, 16, 2450.
IC051201C
Inorganic Chemistry, Vol. 44, No. 24, 2005 9083