Solution structure of molybdic acid from Raman spectroscopy and DFT analysis

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Inorganica Chimica Acta 361 (2008) 1000–1007 www.elsevier.com/locate/ica

Solution structure of molybdic acid from Raman spectroscopy and DFT analysis Oyeyemi F. Oyerinde a, Colin L. Weeks a, Ariel D. Anbar b, Thomas G. Spiro a

a,*

Department of Chemistry, Princeton University, Princeton, NJ 08544, United States b Department of Geological Sciences, Arizona State University, United States Received 13 April 2007; accepted 13 June 2007 Available online 30 June 2007 Dedicated to Edward Solomon

Abstract Protonation of MoO4 2 produces the well-characterized polymolybdates, but at concentrations below 103 M the dominant species is monomeric molybdic acid, H2MoO4. It is likely to be the species adsorbed on manganese oxide, a process thought to control MoO4 2 levels in the ocean, because of the strong proton dependence of MoO4 2 adsorption. The molecular structure of H2MoO4 is elusive, since it occurs only in dilute solutions. Using 244 nm laser excitation, near resonance with O ! Mo charge-transfer electronic transitions of H2MoO4, we have detected a 919 cm1 Raman band assignable to msMo@O. Using DFT, we have computed geometries and vibrational modes for the various structures consistent with the H2MoO4 formula. We tested the computations on a series of Mo(VI) oxo complexes with known vibrational frequencies, at several levels of theory. Best agreement with experimental values, at reasonable computational cost, was obtained with the B3LYP functional, employing a LANL2DZ ECP basis set for Mo and the 6-311+G(2df,p) basis set for O and H. Among the possible H2MoO4 structures only those based on the MoO3 unit, with one, two or three coordinated water molecules, gave a scaled frequency for msMo@O that was within two standard deviations of 919 cm1. Best agreement was obtained for MoO3(H2O)3. The MoO2 and MoO structures gave frequencies that were too high. The Mo(OH)6 structure could be excluded, because its vibrational frequencies shift down strongly upon H/D exchange, whereas the 919 cm1H2MoO4 band shifts up 1 cm1 in D2O. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Molybdic acid; Density functional theory; Molybdenum; Molybdate

1. Introduction Molybdenum occurs widely in natural systems and its speciation is of interest. Although knowledge of the accessible forms of Mo is necessary to understand the distribution of Mo in natural waters, not all the species involved are well characterized. Under oxic conditions Mo occurs as MoO4 2 , with a uniform concentration of about 107 M in the oceans [1,2]. The maintenance of the Mo concentration is depen*

Corresponding author. Tel.: +1 609 258 3907; fax: +1 609 258 0348. E-mail address: [email protected] (T.G. Spiro).

0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.06.025

dent on the mechanisms of supply to and removal from the ocean. Supply is primarily via MoO4 2 dissolved in rivers. Various mechanisms of removal have been proposed including co-precipitation with iron sulfides under anoxic conditions [3], sorption of oxythiomolybdates on particulate organic matter [1,4] and adsorption and subsequent sedimentation with solid phase transition metal oxides [5]. Iron and manganese oxides play a key role in sequestering trace elements because they are common and because of their large surface areas and strong affinity for many elements [6]. Molybdenum has been shown to be concentrated approximately a hundred fold in marine ferromanganese crusts and nodules relative to seawater [7]. Under oxidizing

O.F. Oyerinde et al. / Inorganica Chimica Acta 361 (2008) 1000–1007

conditions, studies have shown that molybdenum is associated with solid phase Mn oxide [5] with a fractionation of Mo isotopes between the Mn oxides and seawater [8]. Intriguingly, laboratory experiments indicate that Mo isotopes are fractionated during adsorption to Mn oxide particles [8]. The magnitude of fractionation matches the isotopic offset between Mo in seawater and Mo in marine ferromanganese sediments, implying that Mo adsorption to Mn oxides exerts an important control on the marine Mo isotope budget [8]. These observations have been used to infer the redox conditions that existed when marine sediments were deposited [9]. It is possible to compute fractionation factors for adsorption equilibria from the vibrational frequencies of the adsorbed and desorbed species, and thus gain insight into both the fractionation mechanism and the mechanistic details of Mo adsorption to Mn oxides. However, the computations are hindered by the fact that the structures of both dissolved and adsorbed Mo are uncertain. The adsorption of oxyanions onto metal oxide surfaces often involves the protonation of the oxyanion and/or the protonation of the surface [6,10,11]. In hydrous manganese oxides, adsorption of molybdate increases markedly at pH values below 4 [7]. This is also the pH region where MoO4 2 takes up protons in solution. This process leads to condensation reactions, producing hepta- and octamolybdates. However, at concentrations below 104 M, the products remain mononuclear. The main product is the neutral molybdic acid, H2MoO4. Heptamolybdate has been suggested to be the species adsorbed on ferromanganese oxide crusts on the basis of a Mo extended X-ray absorption fine structure (EXAFS) analysis showing second shell scattering, which was fitted as Mo atoms [12]. However, it is unclear whether the second shell might have been due to Mn atoms at the oxide surface. Although concentrating molybdate at the surface might induce condensation reactions, it seems more reasonable to suppose that at the high dilution in the ocean molybdate is adsorbed to manganese oxides as H2MoO4. What is the structure of aqueous H2MoO4? There is no direct evidence, since most structural methods require high concentrations, where conversion to polymolybdates occurs. A theoretical study by Tossell suggested that MoO2(OH)2 is not the stable structure of molybdic acid, but rather that it dissociates into MoO3 and H2O. If so, it may be that the MoO3 species is adsorbed on oxide surfaces and is responsible for the isotope fractionations seen in nature and in experiments [13]. However, there is indirect experimental evidence for expansion of the MoO4 2 coordination sphere from the dilatometric study of Cruywagen et al. [14], who found a large volume decrease for the second protonation step, consistent with a decrease in free water via Mo hydration. Also consistent with this interpretation was the finding of large negative entropy and enthalpy changes. They considered alternative octahedral structures and predicted increased stability in the order Mo(OH)6 to MoO2(OH)2(H2O)2 to MoO3(H2O)3

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on the basis of a point charge model, with coordinated waters treated as dipoles. On the other hand Ozeki et al. [15] found best agreement between computed and experimental electronic absorption spectra for a distorted octahedral structure with the formula MoO2(OH)2(H2O)2, using molecular orbital calculations based on considerations of bond orders. In this study, we have observed a Mo@O stretching frequency of 919 cm1 in H2MoO4, using resonance Raman (RR) spectroscopy with UV excitation to provide the sensitivity needed to study solutions sufficiently dilute to prevent condensation. This datum provides an important constraint on the possible structures for this species. To evaluate this constraint, we have computed structures and vibrational spectra of the possible species via density functional theory, after determining appropriate scaling factors from vibrational spectra of various structurally characterized oxo-Mo(VI) complexes. The best agreement with the 919 cm1 Mo@O frequency is obtained for the 6-coordinate MoO3(H2O)3 complex, although several other species fall within the estimated uncertainty of the calculations. In a separate study, we evaluate H2 MoO4 =MoO4 2 isotope fractionation computationally, for comparison with the experimental data on molybdate absorption to manganese oxides [16]. 2. Methods 2.1. Experimental procedures Solutions were prepared from sodium molybdate, Na2MoO4 (Aldrich, >98% purity) at 0.10 and 10 mM, and adjusted to the desired pH with 0.1 M HCl (EMD) or DCl (Cambridge Isotopes). Sodium perchlorate, NaClO4, (100 mM, Aldrich, >98% purity) was used as an internal standard for the Raman intensities in the pH titration experiments. Resonance Raman spectra were obtained with excitation at 244 nm using an intracavity frequency-doubled Ar+ laser (Coherent Innova 300 FRED). The laser power measured at the sample was 2 mW. The scattered light was collected and focused onto the slit of a single spectrograph (Spex 1269, 3600 grooves/mm grating) equipped with a liquid nitrogen-cooled CCD camera (Roper Scientific). Nonresonant Raman spectra were obtained in spinning NMR tubes using the 488 nm excitation line from an Ar+ laser (Spectra Physics-2017). The laser power measured at the sample was 10 mW. The scattered light was collected and focused onto a single spectrograph (Chromex, 2400 grooves/mm grating) equipped with a notch filter (Kaiser Optical) and a liquid nitrogen-cooled CCD (Roper Scientific). Spectra were calibrated with cyclohexane and dimethyl formamide. Spectral acquisition times were typically 10 min. All the wavenumbers given were obtained by fitting the spectra with a sum of Lorentzian bands. The 1650 cm1 band of water was used as an internal standard for the Raman intensities for the cross-section calculations.

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UV–Vis absorption spectra were obtained on a Hewlett– Packard 8452A diode array spectrophotometer. A Corning 320 pH meter was used to measure the pH values of the solutions. The computer software VISUAL MINTEQ [17] was used to determine solution speciation at an ionic strength of 100 mM and 25 °C. Equilibrium concentrations of all MoVI species were calculated as functions of pH and total MoVI concentration. pH and anion concentrations were used as input data.

100

80

% Mo Species

1002

60

Mo8O26

40

4-

HMo7O24

H3Mo8O28

MoO4

H2Mo6O21

4-

Mo7O24

2.2. Theoretical calculations

3. Results and discussion

6-

-

HMoO4 0 2

3

4

5

6

pH Fig. 1. Mo speciation as a function of pH calculated from equilibrium constants [17] at [Mo] = 10 mM.

100

80

% Mo Species

Computations were performed with the GAUSSIAN 98 [18] and GAUSSIAN 03 [19] program packages. The calculations were carried out using DFT (BLYP, B3LYP, B3PW91 and VSXC functionals [20–26]), CCD or QCISD theory. The basis sets used were LANL2DZ ECP [27] for Mo and 6-31G*, 6-31+G*, 6-311+G*, 6-311+G(2d,p), 6311+G(2df,p), or 6-311+G(3df,p) [28–34] for all other atoms. Geometry optimizations were performed on all molecules and the optimized geometries from the same level of theory and basis set was used in the frequency calculations. To evaluate the effect of theory level and basis set choice on the calculated frequencies and determine appropriate scale factors, calculations were carried out for MoO4 2 , MoS4 2 , MoOS3 2 , MoO2 Cl4 2 , MoO2 F4 2 , [MoO2(NH2CH2CH2S)2], and [MoO3(dien)]. Subsequently the vibrational frequencies were calculated for possible structures of molybdic acid: MoO(OH)4, MoO(OH)4(H2O), MoO2(OH)2, MoO2(OH)2(H2O), MoO2(OH)2(H2O)2, MoO3, MoO3(H2O), MoO3(H2O)2, MoO3(H2O)3, and Mo(OH)6. In the frequency calculations the masses of the 95Mo, 35Cl, 32S, 19F, 16O, and 1H or D isotopes were used.

2-

5-

H2MoO4

20

5-

4-

H2MoO4

60

Mo8O26

40

20 -

HMoO4 0 5

4

3

2

1

|log[Mo](M)| Fig. 2. Mo speciation at pH 2.2 as a function of Mo concentration calculated from equilibrium constants [17].

3.1. Raman spectroscopy

40000

2 -1

-29

pH 6.37

Cross section x 10

60000

897

(cm sr )

208nm

6

217nm

80000

919

100000

Molar Absorptivity (M-1cm-1)

Molybdate solutions were studied at 10 and 0.1 mM concentrations, above and below the threshold for condensation reactions at low pH. At pH > 6, MoO4 2 dominated at all Mo concentrations, but at pH < 6 the condensation products had a steep concentration dependence, because of their high nuclearity. At high Mo concentrations hepta-, hexa- and octamolybdates became dominant successively as the pH was lowered (Fig. 1). However, mononuclear species remained dominant below 103 M. The speciation graph at pH 2.2 (Fig. 2) illustrates this point, showing a sharp rise in the octamolybdate concentration only above 103 M. Thus 0.1 mM is sufficiently dilute to ensure formation of mononuclear species. Fig. 3 shows absorption and UVRR spectra for neutral and acidic 0.1 mM solutions of Na2MoO4. The absorption spectra were in agreement with those reported for MoO4 2 and H2MoO4 [35]. The 244 nm Raman excitation was close

5 4 3 2 pH 6.37

pH 2.90

1 0 -1 880

900

920

940 -1

Raman Shift (cm )

20000

0 200

pH 2.90

300

400

500

Wavelength (nm) Fig. 3. UV–Vis absorption and 244 nm excited Raman spectra of 0.1 mM aqueous Na2MoO4 at the indicated pH values.

-1

Δ = 0 cm

pD 4.81

919

pH 4.83

-1

Δ = 1 cm

pD 1.75 pH 1.72

880

900

920

940 -1

Raman Shift (cm) Fig. 5. 244 nm excited Raman spectra of 0.1 mM aqueous Na2MoO4 in D2O (  ) and H2O (—) at the indicated pD/pH.

910 923

950 960 971

Actual Fit

2.0

920 960

897

951

2.0

pH 4.89 0.0

838

0.6 2.0

0.4 800

840

pH 6.68

0.0 840

880

920

Raman Shift 919 cm

100

-1

897 cm

pH 2.83

0.0

897

Crosssection x 10-29 (cm2sr-1)

to resonance with the UV absorption bands of both species. The UVRR spectra reveal that the well known 897 cm1 band [36], arising from the breathing vibration of MoO4 2 , was replaced by a new band at 919 cm1 when the pH was lowered from 6.37 to 2.9. When the intensities of these bands were tracked as a function of pH, they were seen to be in excellent agreement with the speciation curves calculated for MoO4 2 and H2MoO4 (Fig. 4). Thus the 919 cm1 band can confidently be attributed to H2MoO4. Because successive pKas for MoO4 2 protonation are similar, 3.89 and 3.61 [37], the monoprotonated HMoO4  remained a minority species, reaching a maximum of only 35% at pH 3.6. The UVRR spectra showed no indication of an extra peak for HMoO4  ; either the intensity was too low or there was overlap of the bands of the dominant species. To provide further insight into the nature of H2MoO4, we compared the 244 nm-excited UVRR spectra of 0.1 mM solutions in H2O and D2O (Fig. 5). The 897 cm1 MoO4 2 band did not shift, while the 919 cm1 H2MoO4 band shifted up 1 cm1. This slight upshift rules out coupling to an OH oscillator, and was consistent with H-bond effects on an oxo vibration (see below). When 10 mM Na2MoO4 was examined with 244 nm excitation, the 897 cm1 UVRR band was still present at neutral pH, but the 919 cm1 band could no longer be detected in acid solution (data not shown), reflecting condensation of H2MoO4 to polymolybdates. The absence of the UVRR signals from these species was puzzling; apparently they were insufficiently resonance-enhanced for detection. However, excitation at 488 nm, far from resonance, revealed that the 897 cm1 MoO4 2 band (along with a much weaker 838 cm1 band, which was due to the asymmetric stretch) was replaced at lower pH with a series of bands in the 900–980 cm1 region, arising from polymolybdate species (Fig. 6). The first species formed was heptamolybdate. A band at 939 cm1 which has been assigned [38]

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897

O.F. Oyerinde et al. / Inorganica Chimica Acta 361 (2008) 1000–1007

960

1000

(cm-1)

Fig. 6. 488 nm excited Raman spectra of 10 mM aqueous Na2MoO4, adjusted to the indicated pH values. The inset at the top is a deconvolution of the complex band envelope at pH 2.83.

-1

% Mo species

80

H2MoO4

2-

MoO4

60

40 -

HMoO4 20

0 2

3

4

5

6

pH Fig. 4. Mo speciation as a function of pH calculated from equilibrium constants [17] and experimental Raman intensities as a function of pH, at [Mo] = 0.1 mM.

to heptamolybdate can be seen at higher concentrations (>100 mM) (Supplementary Information, Fig. S1). At pH 4.9, a broad band was seen at 951 cm1, which must arise from a mixture of hexa- and hepta-molybdates (Fig. 2). As the pH was lowered further, the 897 cm1 MoO4 2 band continued to weaken, while the 951 cm1 band intensified and shifted to higher frequency. At pH 2.83 where octamolybdate dominates, the 897 cm1 MoO4 2 band has disappeared, replaced by a complex envelope of bands. The strongest of these at 960 and 971 have previously been assigned to octamolybdate [38]. However, a contribution at 950 cm1 is likely due to remnant hexamolybdate (Fig. 2). Assignment of weaker bands seen at 910 and 923 cm1 was uncertain.

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3.2. DFT analysis of structures and vibrations

1.72 Mo 1.72O

We carried out theoretical calculations to investigate which of the possible structures for molybdic acid are consistent with the results of the Raman experiments. The observed 919 cm1 band is at a frequency appropriate for Mo@O stretching. Table 1 lists reported Mo@O frequencies for a variety of Mo(VI) oxo complexes. The formula H2MoO4 is consistent with several species, related to one another by the addition or removal of water, and by the rearrangement of H atoms. These species include the trioxide, MoO3, the dioxo, dihydroxide, MoO2(OH)2, the monoxo, tetrahydroxide, MoO(OH)4, and the hexahydroxide, Mo(OH)6. All of these might be coordinated by additional water molecules. We considered all possibilities up to a coordination number of six. Trials with 7-coordinate complexes showed them to be unstable with respect to dissociation of water molecules. The computed structures are illustrated in Fig. 7 (additional details in Supplementary Information, Table S5). Energy minimization produces the geometries one would expect from a compromise between mutual repulsion of O atoms at short distances, and minimization of competition for p-donation into the mutually orthogonal empty dp orbitals on Mo(VI). Thus the strongly donating oxo ligands are arrange cis to one another, with O@Mo@O angles substantially less than 180°. The positions trans to the oxo ligands are either empty, or contain weakly donating water molecules, rather than hydroxide ions, which have intermediate donor strength. Table 2 lists the computed Mo@O stretching frequencies. Also listed are three vibrational frequencies for

Table 1 Comparison of MoO stretching frequencies Complex

mMoO (cm1)

MoO4 2 [36] MoOS3 2 [41] MoO2[NH2CH2CH2S]2 in Me2SO [42] MoO2[SC(CH3)2CH2NHCH2CH2NHCH2C(CH3)2S] in Me2SO [42] MoO2(dttd)a in CH2Cl2 [43] MoO2(cysOMe)2b (solid) [44] (NH4)3[MoO3F3]b (solid) [45] [MoO2Cl4]2 in H2O [45] [MoO2F4]2 in H2O [45] [MoO2(nap-(R)-btol)(py)]c (solid) [46] [MoO(l-O)(nap-(R)-btol)]n (solid) [46] [MoO3(dien)]d in H2O [47] Et4N[MoO3Tp*]e (solid) [48]

ms 897, mas 837 855 ms 912, mas 885 ms 915, mas 888

a

ms 922, mas ms 906, mas ms 900, mas ms 964, mas ms 951, mas 917, 900 930, 903 892, 845 924, 898

890 875 824 925 920

dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane. Raman spectra of MoVI complexes in solution are typically 5–15 cm1 higher than the corresponding values in solid state most likely due to some solid state effects [42]. c H2nap-(R)-btol = (R)-2-[(2-hydroxy-1-naphthyl)methylideneamino]butanol, py = pyridine. d dien = diethylenetriamine. e Tp* = hydridotris(3,5-dimethyl-1-pyrazolyl)borate. b

O

H2O

109

Mo

1.72 O

1.72

O 1.73 110

1.74

O 1.72 2.21

O 110

111

Mo

OH2

2.61

O

1.70

109

Mo

O

1.70

HO

1.73

112

112 1.73 O

O 1.73

OH2

2.43 2.43

Mo

OH2

2.44

OH2

1.90 1.90

109

109

O

O 1.73 OH 2 HO

109

O

O

1.73

2.20

109

HO

O

1.70

1.90 1.90

108

Mo

1.71

HO

O

2.67

H2O OH O

OH

1.70 1.92

106

2.51 2.67

Mo

O1.71

OH2 OH2

1.97

OH

HO 1.96 122 1.69 O 95

1.90

Mo

1.89

OH

1.96

OH

O 1.69 100 110

HO

1.99

OH

102

HO

OH

1.97

Mo

105

2.68

OH2

1.89

OH 1.91

HO

1.89

1.89 1.98

HO

Mo 1.98

OH 1.89 1.98

OH

OH

Fig. 7. DFT calculated geometries for the possible structures of molybdic acid.

Mo(OH)6, which fall in the region of Mo@O stretches, even though this species lacks M@O bonds. The high frequencies result from coupling of the multiple Mo–O–H bending coordinates. Because of these interactions, all three modes are predicted to shift strongly (213–254 cm1 – see supporting information) upon OH/D exchange. As noted above, the 919 cm1 band actually shifts up 1 cm1, consistent with solvent H-bonding to Mo@O bonds [39,40]. Consequently the Mo(OH)6structure can be ruled out. (Some of the tetrahydroxide species also showed dMoOH vibrations in this region, in addition to the Mo@O stretches.) For the oxo-containing species, Mo@O frequencies are computed between 940 and 1020 cm1, and decrease slightly when scaled (see below). The frequencies diminish from monoxo- to dioxo- to trioxo-species, consistent with diminished charge on the Mo(VI) atom as the number of strong donor oxo atoms increases. Within each of these classes, adding water molecules also diminish the frequencies slightly, water being a weak donor. (However adding water to MoO(OH)4 shifts the frequency up, perhaps reflecting the equatorial position of the intermediately donating hydroxide ligands in the resulting octahedral complex.)

O.F. Oyerinde et al. / Inorganica Chimica Acta 361 (2008) 1000–1007 Table 2 MoO stretching frequencies calculated (B3LYP theory, LANL2DZ ECP basis set for Mo and the 6-311+G(2df,p) basis set for all other elements) for candidate structures corresponding to the H2MoO4 empirical formula ± H2O Complex

Isotope Frequency Mode (cm1)

Scaled frequency (cm1)a

MoO3

95

Mo

963 964 995

mas mas ms

952 ± 70 953 ± 71 984 ± 73

MoO3(H2O)

95

Mo

964 969 985

mas mas ms

953 ± 71 958 ± 71 974 ± 72

952 962 979

mas mas ms

942 ± 70 951 ± 71 968 ± 72

Mo

948 949 964

mas mas ms

937 ± 69 938 ± 69 953 ± 71

Mo

1010 1019

mas ms

998 ± 74 1007 ± 75

MoO3(H2O)2

95

MoO3(H2O)3

95

MoO2(OH)2

95

MoO2(OH)2(H2O)

95

Mo

995 1018

mas ms

984 ± 73 1006 ± 74

MoO2(OH)2(H2O)2

95

Mo

990 1007

mas ms

979 ± 72 996 ± 74

MoO(OH)4

95

Mo

1020

m

1009 ± 75

MoO(OH)4(H2O)

95

Mo

1027

m

1016 ± 76

Mo(OH)6

95

Mo

894 899 968

a

Mo

mMoO + dMoOH mMoO + dMoOH mMoO + dMoOH

884 ± 65 889 ± 66 957 ± 71

Scale factor for m(MoO) = 0.989 ± 0.074 used (95% confidence level).

All of the computed Mo@O frequencies were distinctly higher than the experimentally observed 919 cm1 band of H2MoO4. In the case of MoO2 and MoO3 species, the frequency of interest was that of the symmetric stretching mode, ms, since the asymmetric stretches, mas, have lower Raman cross-sections (data not shown). Because the O@Mo@O angles in the energy-minimized structures were all <120° (cis structures), ms > mas in all cases. Thus even for the species with the lowest frequencies, MoO3(H2O)3, the scaled ms was 34 cm1 higher than the observed band. The scale factor was estimated by fitting reported vibrational frequencies of several reference compounds. These compounds included MoO4 2 , MoS4 2 and MoOS3 2 , for which all vibrational modes have been assigned, and several complexes of known structure (see Supplementary Information, Table S1 for comparison with computed structure parameters) that have Mo@O bonds (Table 3). There are extensive studies of scaling at various levels of theory for light atom molecules, and some studies for complexes of third row transition metals. However, we are unaware of theoretical vibrational calculations for fourth row transition metals. Consequently we tested various levels of theory for the reference compounds, using the computed

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Table 3 Scale factors calculated using the B3LYP functional for MoO4 2 , MoOS3 2 , MoO2 Cl4 2 , MoO2 F4 2 , [MoO2(NH2CH2CH2S)2], and [MoO3(dien)] using the LANL2DZ ECP basis set for Mo and 6311+G(2df,p) for the light elements Compound

Vibrational modes

Experimental frequency (cm1)a

Scale factor

MoO4 2

m2(E) m4(T2) m3(T2) m1(A1) m6(E) dMoS3) m3(A1) dMoS3) m5(E) q(MoS3) m2(E) ms(MoS) m4(E) mas(MoS) m1(A1) m(MoO) d(MoF) d(MoO) mas(MoO) ms(MoO) d(MoO) mas(MoO) ms(MoO) mas(MoO) ms(MoO) ms(MoO) Average SD Average SD

317(2) 325(10) 837(4) 897(1) 181(3)

1.064 1.035 1.013 1.033 1.146

192(3)

1.091

263(5)

1.019

464(4)

1.067

480(10)

1.071

855(4)

0.965

293 385 920(7) 951(10) 392 925(5) 964(10) 885 912 892d

1.12261 1.05769 1.01996 1.02922 1.062 0.989 1.007 0.941 0.939 0.949 1.031 0.046 0.989 0.037

MoOS2 3

MoO2 F4 2 b

MoO2 Cl4 2 b [MoO2(NH2CH2CH2S)2]b [MoO3(dien)]b,c All modes m(MoO)

a Experimental data from Refs. [36] ðMoO4 2 Þ, [41] ðMoOS3 2 Þ, [45] ðMoO2 Cl4 2 , and MoO2 F4 2 ), [42] [MoO2(NH2CH2CH2S)2], and [47] [MoO3(dien)]. b Experimental data are not available for all of the vibrational modes of these compounds. The listed scale factors represent the computed modes that match the assignment of the experimental spectra. c dien = diethylenetriamine. d Only the msMo@O mode was used to determine the scale factors because only one masMo@O mode was observed experimentally. In the calculations there were two different frequencies for the masMo@O modes so the experimental data could not be unambiguously assigned.

scale factors (SF = observed/computed frequency) as a guide. The computations were carried out using density functional theory with the widely used B3LYP functional and wavefunctions constructed from the standard LANL2DZ ECP basis set for Mo and the 6-311+G(2df,p) basis set for the other elements. We also tested other theoretical methods and other basis sets (Supplementary Information, Tables S2 and S3). Using the minimal 6-31G* light-atom basis set for comparison, we tried the functionals BLYP, B3PW91 and VXSC, in addition to B3LYP, but found a similar spread of SF’s in each case. Some narrowing of this spread was obtained with the fully ab initio QCISD and CCD methods, but at considerably higher computational

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cost. Increasing the light atom basis set from 6-31G*, 6-31+G*, 6-311+G*, and 6-311+G(2d,p) to 6-311+ G(2df,p) likewise narrowed the spread of scaling factors, but a further increase to 6-311+G(3df,p) brought no improvement. Most of the SF’s are >1, i.e. the experimental frequencies were higher than computed values. This was particularly noticeable for bending modes, for which the SFs were systematically higher than for the stretching modes. Probably this trend was due to additional restoring forces for bond angles from ionic and H-bonding interactions in the solid and polar liquid media in which the experimental frequencies were determined. However, several of the Mo@O stretches have SFs <1, i.e. the experimental frequencies were lower than computed. The largest of these discrepancies were observed for MoOS3 2 , [MoO2(NH2CH2CH2S)2], and [MoO3(dien)]. We attribute these discrepancies to solvent H-bonding to the oxo atoms, an effect that should be pronounced when the auxiliary ligands have S and N atoms, which are strong donors and enhance the negative charge on the oxo groups. This effect was not captured by the in vacuo computations. Because of this effect, it was not obvious which scaling factor to apply to the candidate molybdic acid structures. For the purpose of Table 1, we averaged the SFs for all the Mo@O stretches in Table 2, and used an uncertainty estimate twice the standard deviation (95% confidence interval). The inclusion of the lower frequency Mo@O stretches raises the uncertainty estimate to 70 cm1. When this uncertainty was added to the 919 cm1 frequency, all of the trioxo species were acceptable candidates, but none of the dioxo or monoxo species were; their ms values were too high. Based on these frequency comparisons, we concluded that MoO3(H2O)3 was the likeliest structure for molydic acid. We also compared the DFT-computed energies for 4-, 5-, and 6-coordinate structures (Supplementary Information, Table S7). In all cases the MoO2 species were computed to be the most stable tautomer, followed by MoO and MoO3 species; among the 6-coordinate complexes Mo(OH)6 was least stable. However, the MoO3 species have the highest dipole moments, and should be stabilized most by solvation. A trial calculation with the polarized continuum model (PCM) reduced the energy difference between MoO2(OH)2(H2O)2 and MoO3(H2O)3 from 14.3 to 6.2 kcal/mol. PCM does not include explicit H-bonds from the solvent, which are likely to further stabilize MoO3(H2O)3. Comparisons between complexes of different coordination numbers can not be made because the dissociated water molecules are themselves stabilized by solvation. However, since 6-coordination is readily accessible to Mo(VI), it seems likely that molybdic acid is 6coordinate. As noted in the introduction, the dilatometric studies of Cruywagen et al. support this view. Their conclusion, based on a simple point charge model, that its structure is MoO3(H2O)3, is in accord with our vibrational analysis.

4. Conclusions UVRR spectroscopy identifies a vibrational band of molybdic acid at 919 cm1, indentifiable with Mo@O stretching via its slight upshift in D2O. DFT computation of vibrational frequencies for all plausible structures showed that only trioxo-species gave Mo@O symmetric stretches that agreed with the 919 cm1 frequency within the estimated uncertainty of the scale factor determined from an extensive data base of oxo-Mo(VI) complexes. Among the trioxo-species, MoO3(H2O)3 gave the best agreement with experiment. This is also the structure deduced by Cruywagen et al. from a point charge model and the dilatometric evidence that H2MoO4 is 6-coordinate [14]. It is likely that MoO3(H2O)3 is the species that adsorbs on manganese oxides in the ocean environment. Computation of the isotope fractionation for this adsorption process will be reported elsewhere. Acknowledgements This work was supported by NSF Grant OCE-0526495. Our thanks to Dr. Andrzej Jarzecki for fruitful discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2007. 06.025. References [1] G.R. Helz, C.V. Miller, J.M. Charnock, J.F.W. Mosselmans, R.A.D. Pattrick, C.D. Garner, D.J. Vaughan, Geochim. Cosmochim. Acta 60 (1996) 3631. [2] K.K. Bertine, K.K. Turekian, Geochim. Cosmochim. Acta 37 (1973) 1415. [3] J.L. Morford, S. Emerson, Geochim. Cosmochim. Acta 63 (1999) 1735. [4] K.K. Bertine, Marine Chem. 1 (1972) 43. [5] G.B. Shimmield, N.B. Price, Marine Chem. 19 (1986) 261. [6] L.S. Balistrieri, T.T. Chao, Geochim. Cosmochim. Acta 54 (1990) 739. [7] N. Takematsu, Y. Sato, S. Okabe, E. Nakayama, Geochim. Cosmochim. Acta 49 (1985) 2395. [8] J. Barling, A.D. Anbar, Earth Planet. Sci. Lett. 217 (2004) 315. [9] G.L. Arnold, A.D. Anbar, J. Barling, T.W. Lyons, Science 304 (2004) 87. [10] W. Yao, F.J. Millero, Environ. Sci. Technol. 30 (1996) 536. [11] M.J. Vissenberg, L.J.M. Joosten, M.M.E.H. Heffels, A.J. van Welsenes, V.H.J.S. de Beer, R.A. van Santen, J.A.R. van Veen, J. Phys. Chem. B 104 (2000) 8456. [12] T. Kuhn, B.C. Bostick, A. Koschinsky, P. Halbach, S. Fendorf, Chem. Geol. 199 (2003) 29. [13] J.A. Tossell, Geochim. Cosmochim. Acta 69 (2005) 2981. [14] J.J. Cruywagen, E.F.C.H. Rohwer, Inorg. Chem. 14 (1975) 3136. [15] T. Ozeki, H. Adachi, S. Ikeda, Bull. Chem. Soc. Jpn. 69 (1996) 619. [16] C.L. Weeks, A.D. Anbar, L.E. Wasylenki, T.G. Spiro, J. Phys. Chem., in press. [17] J.P.A. Gustafsson, VISUAL MINTEQ, Royal Institute of Technology in Stockholm, Stockholm, 2006.

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