Coordination and Solvation of the Au+ Cation: Infrared

Article pubs.acs.org/JPCA

Coordination and Solvation of the Au+ Cation: Infrared Photodissociation Spectroscopy of Mass-Selected Au(H2O)n+ (n = 1− 8) Complexes Yuzhen Li, Guanjun Wang, Caixia Wang, and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Gold cation−water complexes with attached argon atoms are produced via a laser vaporization supersonic cluster source. The [Au(H2O)nArx]+ (n = 1−8; x = 1 or 2) complexes are each mass selected and studied by infrared photodissociation spectroscopy in the OH stretching frequency region to explore the coordination and solvation structures of the Au+ cation. Density functional calculations have been performed, and the calculated vibrational spectra are compared to the experimental spectra to identify the gas-phase structures of the Au(H2O)n+ complexes. Confirming previous theoretical predications, the first coordination shell of the Au+ cation contains two water molecules forming a linear O−Au+−O arrangement; subsequent water molecules bind to the two H2O ligands of the Au(H2O)2+ core ion via hydrogen bond forming of the second hydration shell, which is complete at n = 6. For the complexes with n ≤ 7, the experimental spectrum can in general be assigned to the predicted global minimum structure. However, the spectrum suggests that two or more conformers coexist for the n = 8 complex, indicating that the identification of a single global minimum becomes less important upon increasing the number of solvating water molecules.

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that the first two water molecules bind strongly to Au+ (40.1 and 44.9 kcal/mol, respectively). The binding energy decreases from 23.0 to 16.1 kcal/mol from 3 to 6 H2O molecules and remains at a constant value around 10.4 kcal/mol for large Au(H2O)n+ clusters with n = 7−10.24 In contrast to other cationic alkali-metal and transition-metal hydrates,6−8 the gold ion tends to have a low coordination number of two due to the relativistic effect. This is contrasted to that of Ag(H2O)n+, which has the coordination number of three.41 Theoretical investigations of the Au(H2O)n+ complexes containing up to 10 water molecules were carried out by several groups, which predicted that the first two water molecules bind directly to the Au+.67−76 The addition of subsequent H2O molecules results in the formation of two rings of water molecules, while each of them contains 4 units leading to a dumbbell structure for Au(H2O)8+. The ninth H2O molecule occupies a position above the Au+ cation, and one H2O molecule from each (H2O)4 binds to the ninth molecule, resulting in distorted (H2O)4 cages and the beginning of a semidroplet formation around the cation. The charge on the cation is transported to the outer H2O molecules, leading to a positively charged outer shell screening the central Au atom.76 Although the calculated binding energies are in good agreement with the collision induced dissociation experiments, the optimized structures are

INTRODUCTION Hydrated metal ions play an important role in many chemical and biological systems, which have been the subject of intensive experimental and theoretical studies.1−5 Gas phase clusters have been commonly used to probe the fundamental interactions involved in solvation. 6−76 Mass spectrometry measurements9−23 and collision induced dissociation16−24 provided cation-solvent binding energies, whereas structural information was obtained from electronic25−32 and infrared (IR) spectroscopy.33−53 In particular, infrared photodissociation spectroscopy offers one of the most direct and generally applicable approaches to investigate the mass-selected cation−water complexes for exploring the solvation structures in the gas phase because certain vibrations of solvent molecules (particularly in the OH stretching region) are extremely sensitive to changes in their binding environment.33−53 On the theoretical front, computational chemistry has probed the structures, binding energies, and vibrational spectra of these hydrated metal ions with ever-increasing proficiency.54−76 Gold is special because it shows a local maximum for the relativistic effect in the periodic table, so it has been regarded as a relativistic element.77 The Au(H2O)n+ ions have attracted attention in a number of experimental20,23,24 and theoretical studies.67−76 Marinelli et al. first discovered that the second water molecule binds more strongly to Au+ than the first one.20 This phenomenon was confirmed by several subsequent experimental23,24 and computational studies.67−69,76 The collision induced dissociation (CID) investigation indicates © 2012 American Chemical Society

Received: September 24, 2012 Revised: October 21, 2012 Published: October 22, 2012 10793

dx.doi.org/10.1021/jp3094963 | J. Phys. Chem. A 2012, 116, 10793−10801

The Journal of Physical Chemistry A

Article

Table 1. Incremental Binding Energies (BE, in kcal/mol) of the Studied Au(H2O)n+ Clusters and the Ar-Atom Binding Energies (Ar-BE, in kcal/mol) with These Clusters (Including Zero-Point Energy and Basis Set Superposition Error Corrections)a complex

BE

literature

complex

Ar-BE

literature

Au(H2O)+

37.2

40.1 ± 2.3b, 39.2 ± 2.3c, 37.2d, 34.0e, 40.2f, 36.0g, 35.1h, 35.9i

Au(H2O)2+ Au(H2O)3+ Au(H2O)4+ Au(H2O)5+ Au(H2O)6+ Au(H2O)7+ Au(H2O)8+

41.6 16.4 15.3 11.8 10.9 7.7 8.8

44.9 23.0 20.7 18.4 16.1 10.4 10.4

[Au(H2O)Ar]+ [Au(H2O)Ar2]+ [Au(H2O)2Ar]+ [Au(H2O)3Ar]+ [Au(H2O)4Ar]+ [Au(H2O)5Ar]+ [Au(H2O)6Ar]+ [Au(H2O)7Ar]+ [Au(H2O)8Ar]+

14.3 1.5 1.3 1.0 0.7 0.6 0.2 0.3 0.2

14.4h 1.6j, 2.2j 1.5k

± ± ± ± ± ± ±

3.4b, 4.6b, 4.6b, 3.4b, 3.4b, 2.3b, 2.3b,

46.1 ± 4.6c, 39.8d, 40.1e, 48.1f 16.9d, 17.4e, 16.8f 15.4d, 16.1e, 20.3f 12.3d, 14.1e 11.1d, 12.8e 7.5d 8.1d

a

The BE and Ar-BE values from previous available experimental (in italics) and theoretical studies are included for comparison. The Ar-BE value of [Au(H2O)Ar2]+ is the binding energy of the second Ar atom in this complex. bRef 24. cRef 23. dRef 76. eRef 68. fRef 67. gRef 70. hRef 72. iRef 71. j Ref 73. kRef 74.

very floppy systems and thus require spectroscopic confirmation. In this article, mass selected Au(H2O)n+ (n = 1−8) cluster cations are systematically studied by infrared photodissociation spectroscopy in the gas phase in the OH stretching frequency region. Since the binding energies of Au+−water are greater than the infrared photon energies in the symmetric and asymmetric OH stretching frequency region of the complexes, the method of attaching one or more argon atoms (Ar-tagging) is employed.33−44 In addition, density functional theory calculations are carried out to analyze the experimental IR spectra. The coordination and hydration structures of the ions are identified from a comparison of the experimental and theoretical spectra.

fragment ion of interest as a function of the dissociation IR laser wavelength and normalizing to parent ion signal. Typical spectra were recorded by scanning the dissociation laser in steps of 2 cm−1 and averaging over 250 laser shots at each wavelength. Quantum chemical calculations were performed to determine the molecular structures and to help assignment of the vibrational frequencies of the observed species. Geometry optimization and harmonic vibrational frequency analysis were performed with the most popular hybrid B3LYP density functional theory (DFT) method with the augmented Dunning correlation-consistent basis set aug-cc-PVTZ for H, O, and Ar and the same basis set with pseudopotential aug-cc-PVTZ-PP for Au.79 This whole basis set will be simply denoted as aVTZ for brevity. The ultrafine (99590) integral grid and the tight convergence criterion (the maximum force on the atom allowed is 6.0 × 10−5 Eh a0−1 in Gaussian09) were used for all the DFT calculations. The basis set superposition error (BSSE) correction was made using the counterpoise method.80 Our main purpose is to predict the IR spectra of Ar-tagged Au(H2O)n+ rather than locate all the possible structural conformers at high levels of theory. For [Au(H2O)nAr]+, [Au(H2O)Ar2]+, and Au(H2O)n+ clusters with n = 1−8, we find that our optimized ground state structures are in good agreement with those previous DFT and high level ab initio calculations.67−76 All these calculations were performed using the Gaussian09 program.81 Theoretical predicted IR spectra were obtained by applying Lorentzian functions with the theoretical harmonic vibrational frequencies scaled by a factor of 0.9782 and a 5 cm−1 full width at half-maximum (fwhm).

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EXPERIMENTAL AND COMPUTATIONAL METHODS The experimental apparatus equipped with a collinear tandem time-of-flight mass spectrometer and a laser vaporization supersonic cluster source has been described in detail previously.78 Briefly, the 1064 nm fundamental of a Nd:YAG laser (Continuum, Minilite II, 10 Hz repetition rate, and 6 ns pulse width) was used to vaporize a rotating gold metal target. The gold cation−water complexes with attached Ar atoms were produced from the laser vaporization process in expansions of mixed helium/argon gas seeded with water using a pulsed valve (General Valve, Series 9) at 1.0−1.5 MPa backing pressure. After free expansion, the cation complexes were skimmed and analyzed using a Wiley−McLaren time-of-flight mass spectrometer. The cations of interest were each mass selected and decelerated into the extraction region of a second collinear time-of-flight mass spectrometer, where they were dissociated by a tunable IR laser. The fragment and parent cations were reaccelerated and mass analyzed by the second time-of-flight mass spectrometer. The infrared source used in this study is generated by a KTP/KTA optical parametric oscillator/amplifier system (OPO/OPA, Laser Vision) pumped by a Continuum Powerlite 8000 Nd:YAG laser, which is tunable from 2000 to 5000 cm−1. The wavenumber of the OPO laser is calibrated using NH3 absorptions. The IR beam path is purged with nitrogen to minimize absorptions by air. Fragment ions and undissociated parent ions are detected by a dual microchannel plate detector. The ion signal is amplified, collected on a gated integrator, and averaged with a LabView based program. The photodissociation spectrum is obtained by monitoring the yield of the

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RESULTS AND DISCUSSION The Au+−water binding energies (see Table 1) are greater than the infrared photon energies in the O−H stretching frequency region of the complexes; therefore, it is impossible to obtain the infrared spectra of the Au(H2O)n+ complexes with single photon dissociation. The method of rare gas tagging with Ar is employed. The Ar binding energy of the [Au(H2O)Ar]+ complex was predicted to be 14.4 kcal/mol,72 which still exceeds the IR photon energy. The binding energy of the second Ar atom in the [Au(H2O)Ar2]+ complex predicted to be 2.2 kcal/mol is lower than the IR photon energy.73 Therefore, the doubly tagged [Au(H2O)Ar2]+ complex is selected to record the infrared photodissociation spectrum for the n = 1 complex. As can be seen in Table 1, the Ar binding energies of 10794



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Figure 1. Mass spectrum of the [Au(H2O)nArm]+ complexes (n = 1−8; m = 0, 1, 2) formed by pulsed laser vaporization of a gold metal target in an expansion of helium/argon mixture doped with water.

the n ≥ 2, complexes are less than 1.5 kcal/mol and decrease with the increasing of the coordination number n. A typical mass spectrum of the ion complexes produced by the laser vaporization supersonic cluster source is shown in Figure 1. The spectrum is composed of progressions of mass peaks due to mononuclear gold cation−water complexes with the stoichiometry of [Au(H2O)nArm]+. The cation complexes of interest are each mass-selected and subjected to infrared photodissociation. When the IR laser is on a resonance with the O−H stretching of the complexes, photofragmention of the cations involving the loss of tagged argon atom is observed. The observed band positions are listed in Table 2. [Au(H2O)Ar2]+. The IR photodissociation spectrum of the [Au(H2O)Ar2]+ complex obtained by monitoring the loss of one Ar atom is shown in Figure 2(i). The spectrum contains five bands centered at 3528, 3557, 3643, 3657, and 3683 cm−1. Since the [Au(H2O)Ar2]+ complex is expected to have just two vibrational fundamentals in the OH stretching frequency region corresponding to the symmetric and antisymmetric stretching of the water ligand, the observed additional bands can be assigned to vibrations of other structural isomers or combination bands. Previous calculations predicted that the [Au(H2O)Ar2]+ complex has two conceivable isomeric structures.73 The structure with one Ar atom attached to an H atom of the H2O ligand and the other Ar atom bound to Au+ is a little more stable than another isomer with both Ar atoms coordinated to the Au+ center. These two structures were also found from the present calculations at the same level of theory. However, the less stable structure with both argon atoms coordinated to the Au+ center converged to the more stable isomer when a large basis set (aug-cc-PVTZ for the H, O, and Ar atoms and aug-cc-PVTZ-PP for the Au atom) was used for geometry optimization, which suggests that the isomer with both argon atoms coordinated to Au+ is not a stable structure. We therefore assign the two main features at 3528 and 3643 cm−1 to the symmetric and antisymmetric OH stretching modes of the [Au(H2O)Ar2]+ complex, which are red-shifted by 129 and 113 cm−1 from the corresponding bands in free

H2O.82 The red-shifts for Au+ are larger than those measured for other late transition metal cation−water complexes, which are usually determined to have structures with the tagged Ar atoms attached directly to the metal cations.33,39 The tag atom binds on the OH group of water in [Au(H2O)Ar2]+ inducing larger red-shifts. The weak bands at 3557, 3657, and 3683 cm−1 should be attributed to combination bands involving a rare gas vibration and the O−H stretching vibrations. Similar weaker vibrational bands have been seen in the infrared spectra for many cation−water complexes when these systems were studied with rare gas tagging.7,33,39−52 [Au(H2O)2Ar]+. The infrared photodissociation spectrum of the [Au(H2O)2Ar]+ complex is shown in Figure 3(i). The spectrum exhibits four main features with comparable intensities at 3548, 3602, 3658, and 3682 cm−1. These bands can be assigned to the symmetric and antisymmetric OH stretching vibrations of the complex. In addition, very weak vibrational bands at 3716 and 3750 cm−1 are observed, which are assumed to be combination bands. To make a more quantitative investigation of this spectrum, DFT calculations were performed on the Au(H2O)2+ and [Au(H2O)2Ar]+ complexes, and the calculated spectra are compared to the experimental spectrum in Figure 3. In agreement with previous theoretical calculations, the ground state Au(H2O)2+ complex has a C2 structure with both the H2O molecules binding directly to the Au+ cation (Figure S2, 2a, Supporting Information) . The Ar atom prefers to bind to the H atom rather than bind to the metal center. Upon Ar tagging, the C2 symmetry of Au(H2O)2+ is reduced to C1. Both the Au(H2O)2+ and [Au(H2O)2Ar]+ complexes have four OH stretching modes, which are all IR-active. On the basis of the calculations, the spectrum of Au(H2O)2+ (Figure 3(iii)) exhibits only two bands; the two symmetric stretching modes are predicted to be almost degenerate at 3615 and 3619 cm−1, and the two antisymmetric stretching modes are degenerate at 3699 cm−1. The spectrum of the Ar tagged complex (Figure 3(ii)) was predicted to have four well-separated bands at 3542, 3619, 3676, and 3700 cm−1, which matches almost perfectly 10795



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Table 2. Comparison between the Experimentally Observed and Calculated (Scaled, in cm−1) Band Positions of the ArTagged Au(H2O)n+ (n = 1−8) Complexes (the Calculated IR Intensities Are Listed in Parentheses in km/mol) complex

exptl

calcd

n=1

3528 3643 3548 3602 3658 3682 2812 3560 3644 3660 3730 2899 3623 3645 3665 3735 2977 3099 3147 3633 3665 3739 3125 3185 3647 3735 3071 3205 3547 3557 3651 3709 3717 3739 3017 3167 3213 3391 3455 3493 3545 3651 3685 3715 3739

3521(634) 3665(271) 3542(570) 3619(151) 3676(260) 3700(226) 2917(2102) 3563(473) 3668(56) 3686(252), 3692(161) 3756(152) 2989(3011), 3016(835) 3648(396) 3674(75), 3674(25) 3699(151) 3764(148), 3764(144) 3063(2057) 3173(1171) 3191(1615) 3654(370) 3675(42), 3676(42), 3677(26) 3765(110), 3767(133), 3767(154) 3183(2453) 3208(775), 3221(1701), 3225(973) 3664(89), 3676(40), 3676(40), 3677(23) 3757(172), 3766(115), 3767(67), 3767(194) 3131(1132), 3155(2252) 3225(708), 3239(1618) 3525(292) 3545(542) 3643(53), 3676(34), 3678(31) 3727(183) 3740(201), 3743(67) 3767(107), 3769(139)

n=2

n=3

n=4

n=5

n=6

n=7

n=8

Figure 2. Experimental IR spectrum of [Au(H2O)Ar2]+ (i) and the simulated vibrational spectra of [Au(H2O)Ar2]+ (ii), [Au(H2O)Ar]+ (iii), and Au(H2O) + (iv) in the OH-stretching region.

Figure 3. Experimental IR spectrum of [Au(H2O)2Ar]+ (i) and the simulated vibrational spectra of 2a_Ar (ii), 2a (iii), and 2b_Ar (iv) in the OH-stretching region.

[Au(H2O)3Ar] +. The infrared photodissociation spectrum of the [Au(H2O)3Ar]+ complex is illustrated in Figure 4(i). The spectrum contains four sharp bands at 3560, 3644, 3660, and 3730 cm−1 in the free OH stretching frequency region. In addition, a broad band centered at 2812 cm−1 was observed. The observation of such large red-shifted OH stretching vibration indicates that the complex involves hydrogen bonding between the H2O molecules. The observation of only one hydrogen bonded OH stretching band suggests that only one hydrogen bond is involved in the complex. Previous theoretical calculations predicted that the most stable structure of Au(H2O)3+ involves an Au(H2O)2+ core, while the third H2O molecule binds to one of the first shell H2O molecules leading to the beginning of the second hydration shell.76 The next high energy structure with one H2O in the first hydration shell and two H2O in the second hydration shell was predicted to lie 23.8 kcal/mol above the ground state structure.76 Our DFT calculations produce very similar results (Figure S3, Supporting Information, and Table 3). The hydrogen bonded H···O distance was predicted to be 1.574 Å, about 0.6 Å shorter than that in the neutral water

with the experimental spectrum. The experimentally observed 3548 and 3658 cm−1 bands are assigned to the symmetric and antisymmetric stretching modes of the Ar-tagged H2O ligand, while the 3602 and 3682 cm−1 bands are attributed to the symmetric and antisymmetric stretching mode of the H2O ligand without Ar-tagging. Another less stable structure with one-fold coordination (Figure S2, 2b, Supporting Information) was predicted to have a strong characteristic hydrogen bonded OH stretching vibration around 2800 cm−1 (Figure 3(iv)). The experimental spectrum is very clean in this frequency region. Therefore, the 2b structure can be ruled out. 10796



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cm−1 band belongs to the hydrogen bonded OH stretching vibration; the 3560 and 3644 cm−1 bands are attributed to the symmetric and asymmetric stretching modes of the Ar-tagged H2O subunit; the 3660 and 3730 cm−1 bands can be assigned to the stretching vibrations of the remaining free OH groups. [Au(H2O)4Ar]+. The infrared photodissociation spectrum of the [Au(H2O)4Ar]+ complex shown in Figure 5(i) is quite


Table 3. Relative Energies (kcal/mol) of the Au(H2O)n+ and [Au(H2O)nAr]+ Isomers Calculated at the B3LYP/aVTZ Level (the Structures Are Shown in Figures S1−S8, Supporting Information)

n=2 n=3 n=4 n=5 n=6 n=7 n=8

isomers

Au(H2O)n+

[Au(H2O)nAr]+

a b a b a b a b a b a b a b c

0.0 22.8 0.0 24.5 0.0 2.4 0.0 5.8 0.0 2.9 0.6 0.0 0.0 1.4 1.4

0.0 10.9 0.0 25.0 0.0 2.1 0.0 5.5 0.0 2.6 0.5 0.0 0.0 1.2 1.4


similar to that of [Au(H2O)3Ar]+. The spectrum contains sharp bands at 3623, 3645, 3665, and 3735 cm−1, which are due to stretching vibrations of the free OH groups. In addition, a strong band centered at 2899 cm−1 was observed in the hydrogen bonded OH stretching frequency region. The band is broad and asymmetric, suggesting the involvement of more than one mode in this band. Previous DFT calculations predicted that the ground state Au(H2O)4+ has a structure with the third and fourth H2O bound to different H2O in the first shell (see Supporting Information, Figure S4, 4a). The structure with both the third and fourth H2O bound to the same H2O (Figure S4, 4b) is slightly higher in energy than the ground state structure. Previous MP2/6-31G(d) calculations predicted that the 4a structure is more stable than the 4b isomer by 1.6 kcal/mol;68 PBE99 calculations gave an energy difference of 2.3 kcal/mol.76 Present calculations at the B3LYP level predicted an energy gap of 2.4 kcal/mol. Similar to Au(H2O)3+, the argon atom prefers to attach to the first-shell H2O in [Au(H2O)4Ar]+. The simulated spectra of [Au(H2O)4Ar]+ are compared to the experimental spectrum in Figure 5. Apparently, the calculated spectrum for the argon-tagged ground state structure 4a_Ar agrees well with the experiment. Thus, the ground state structure of Au(H2O)4+ can be confirmed to be 4a. The broad band observed at 2899 cm−1 is assumed to be an unresolved doublet involving both the antisymmetric and symmetric stretching modes of the two hydrogen bonded OH groups, which were calculated at 2989 and 3016 cm−1 with only 27 cm−1 splitting. The bands observed at high frequency region are associated with the stretching vibrations of the free OH groups. Note that there are several weak bands at 3059, 3089, and 3573 cm−1 in the spectrum of [Au(H2O)4Ar]+. These bands fit well

dimer,82 which implies that the charge on the Au+ ion polarizes the H-donating H2O in the first hydration shell forming the stronger charge-enhanced hydrogen bond.83 Our calculations indicate that the Ar atom prefers to attach to the first shell H2O ligand rather than attach to the second shell H2O. The calculated IR spectra of Au(H2O)3+ and argon-tagged [Au(H2O)3Ar]+ are compared to the experimental spectrum in Figure 4. The simulated IR spectra of the complexes with and without Ar-tagging are essentially the same except that the band positions of the argon-tagged complex are slightly shifted from those of untagged complex. The hydrogen bonded OH stretching is blue-shifted, whereas the other free OH stretching modes are red-shifted. The shifts of most vibrational modes are less than 20 cm−1, but the vibration involving the tagged OH exhibits a much larger shift of about 61 cm−1. The theoretical spectrum of argon-tagged [Au(OH2) 2−H2O]+ structure (Figure 4(ii)) matches the experimental spectrum very well, thus confirming that the most stable structure of Au(H2O)3+ is the hydrogen bonded [Au(OH2)2−H2O]+ form as predicted by density functional calculations. As listed in Table 2, the 2812 10797



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with the predicted spectrum of 4b_Ar (Figure 5(iv)) and are tentatively assigned to the less populated 4b isomer. [Au(H2O)5Ar]+. The photodissociation spectrum for [Au(H2O)5Ar]+ shown in Figure 6(i) has three broad bands in the


at the present B3LYP level of theory. The structure 6b can be regarded as adding the sixth H2O molecule to the first hydration shell H2O of structure 5b. On the basis of the calculations, argon tagging does not change obviously the structure and relative stability of the complexes. As can be seen in Figure 7, the calculated spectrum for the more stable isomer 6a agrees well with the experimental spectrum. Therefore, we can determine that the observed complex corresponds to the more stable structure 6a. [Au(H2O)7Ar]+. The experimental infrared spectrum of [Au(H2O)7Ar]+ is illustrated in Figure 8(i). The experimental


hydrogen-bonded OH stretching frequency region, which indicates that there are three hydrogen bonds in [Au(H2O)5Ar]+. Consistent with the experimental expectation, the most stable structure of Au(H2O)5+ was predicted to include three hydrogen bonds (Figure S5, 5a, Supporting Information). In this structure, two H2O molecules occupy the first hydration shell, and the other three H2O molecules occupy the second hydration shell. The next higher energy structure 5b (Figure S5, 5b, Supporting Information) with the completion of a ring of H2O molecules on one side was predicted to be 5.8 kcal/mol higher in energy than the global minimum structure (Table 3). The ring with Cs symmetry is composed of four H2O molecules where the fourth H2O molecule lies on the Cs axis. Previous calculations at the PBE99 level of theory predicted that the ring structure lies 6.1 kcal/mol higher in energy than the 5a structure.76 In 5b, the third hydration shell is formed. The relative stability between 5a and 5b does not change much upon Ar-tagging (Table 3). The simulated spectra of the Au(H2O)5+ complex in both structural forms are compared to the experimental spectrum in Figure 6. Apparently, only the calculated spectrum of the more stable 5a_Ar structure matches the experiment. Therefore, the observed Au(H2O)5+ complex can be clearly attributed to the 5a structure. [Au(H2O)6Ar]+. Figure 7 displays the experimental and simulated infrared spectra of [Au(H2O)6Ar]+. The experimental spectrum is quite simple and involves only four bands: two in the free OH stretching region at 3647 and 3735 cm−1 and two in the hydrogen bonded frequency region at 3125 and 3185 cm−1. Such simple spectrum implies that the Au(H2O)6+ complex has high symmetry. Previous theoretical calculations predicted that the most stable structure of Au(H2O)6+ has C2 symmetry, in which the four second hydration shell H2O molecules bind directly to the two first hydration shell H2O molecules via four hydrogen bonds (Figure S6, 6a, Supporting Information). This structure is more stable than the next stable structure (Figure S6, 6b, Supporting Information) by 3.3 kcal/ mol.76 This energy difference was predicted to be 2.9 kcal/mol


spectrum is more complicated than the small complexes reported above. At least six bands in the upper frequency region and two broad bands in the low frequency region can clearly be resolved. Previous DFT calculations found two energy degenerate structures for Au(H2O)7+,76 as presented in Figure S7, Supporting Information. The 7a structure has a four10798



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CONCLUSIONS To explore the coordination and solvation structures of the Au+ ion, IR spectra of mass-selected Ar-tagged Au(H2O)n+ (n = 1− 8) complexes are measured via IR photodissociation spectroscopy in the OH stretching frequency region. The cluster cations are produced via a laser vaporization supersonic cluster source. Density functional calculations at the B3LYP/aVTZ level have been performed, and the calculated vibrational spectra are compared to the experimental data to identify the gas-phase structures of the complexes. Although argon tagging introduces small shifts in the spectra, it does not change obviously the structure and relative stability of the complexes, particularly for the larger complexes. Thus, the argon-tagged complexes still can provide reliable spectra to interpret the structure and bonding of the Au(H2O)n+ complexes. The infrared spectra of the n = 1 and 2 complexes involve only stretching vibrations of free OH groups, while the hydrogen bonded stretching vibrations appear at n = 3, which confirmed previous theoretical predictions that the first coordination shell of the Au+ cation contains two water molecules. Subsequent water molecules bind to the two H2O of the Au(H2O)2+ core ion via a hydrogen bond forming on the second hydration shell, which is complete at n = 6. The results are similar to the Cu+−water complexes but differ from the Ag+−water complexes, which have the first shell coordination number of 3. For the complexes with n ≤ 7, the experimental spectrum can in general be assigned to the predicted global minimum structure. However, more than one conformer coexists for the n = 8 complex, indicating that the identification of a single global minimum becomes less important upon increasing the number of solvating water molecules.

membered ring structure, while the ring is opened in 7b. Both structures can be regarded as adding the seventh H2O molecule to the Au(H2O)6+ core ion in forming the third hydration shell. Present calculations at the B3LYP level found that the 7a structure is less stable than 7b by 0.6 kcal/mol. The simulated infrared spectra of the complexes are also listed in Figure 8 for comparison. It is clear that only the calculated spectrum for the argon-tagged complex 7a matches the experimental spectrum, which indicates that the experimentally observed complex has a 7a structure. The 7b structure was predicted to have a characteristic OH stretching vibration at 3405 cm−1. The experimental spectrum in the 3300−3500 cm−1 region is very clean, and no band was observed in this region. Thus, the 7b structure can be ruled out. The observation of 7a instead of 7b also implies that 7a should be more stable than 7b, although 7b was predicted to be more stable at B3LYP or equally stable at PBE99. [Au(H2O)8Ar] +. The experimental spectrum of [Au(H2O)8Ar]+ shown in Figure 9(i) is very complicated,

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ASSOCIATED CONTENT

S Supporting Information *

Calculated geometries, vibrational frequencies, and intensities; complete ref 81. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Experimental IR spectrum of [Au(H2O)8Ar]+ (i) and the simulated vibrational spectra of 8a_Ar (ii), 8b_Ar (iii), and 8c_Ar (iv) in the OH-stretching region.

AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 21-6564-3532. E-mail: [email protected]. Notes

suggesting the involvement of more than one conformer. Previous calculations suggest that the most stable isomer of Au(H2O)8+ has a dumbbell structure with two rings of water molecules each containing 4 units (Figure S8, 8b, Supporting Information). This ring structure is more stable than the open ring isomer with a small energy difference of less than 1 kcal/ mol.76 Three minimum-energy structures are identified for Au(H2O)8+ at the B3LYP level, as depicted in Figure S8, Supporting Information. Besides the dumbbell structure 8b, 8a and 8c represent the structures with opening of the ring on one side and a four-membered H2O ring on another side. The only difference between 8a and 8c is the attachment of the last H2O molecule. Our calculation results indicate that 8b and 8c are isoenergetic and are less stable than 8a by only 1.4 kcal/mol (Table 3). The calculated infrared spectra of argon-tagged complexes of 8a, 8b, and 8c are compared with the experiment in Figure 9. It is clear that none of the simulated spectra match the experiment, suggesting that different isomers are involved, which makes the identification of a single global minimum structure impossible.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation (21173053) and Ministry of Science and Technology of China (2010CB732306 and 2012YQ220113-3).

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