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Reactions of M+(H2O)n, n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D2O Reveal Water Activation in Mn+(H2O)n Christian van der Linde and Martin K. Beyer* Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstraße 40, 24098 Kiel, Germany S Supporting Information *

ABSTRACT: Reactions of M+(H2O)n, n < 40, M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, with D2O are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Isotopically highly enriched metals are used as applicable. Isotopic scrambling with formation of HDO is not observed for M = Cr, Fe, Co, Ni, Cu, and Zn, which indicates that these hydrated metal ions consist of a singly charged metal center and a hydration shell of intact, inactivated water molecules. In the vanadium case, HDO formation is observed in the size region where also hydroxide formation with evolution of molecular hydrogen occurs. For manganese, HDO formation occurs in the size regime n ≈ 8−20. Additional experiments show that, in this size regime, Mn+(H2O)n is slowly converted into HMnOH+(H2O)n−1 under the influence of room temperature blackbody radiation. The reaction is mildly exothermic; ΔH ≈ −21 ± 10 kJ mol−1.

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spectroscopy,37 or energy resolved collision induced dissociation (CID).38 Quantum chemical calculations by Ugalde and co-workers39−43 on the M+(H2O), M = V, Cr, Mn, Fe, Co, Ni, and Cu, potential energy surfaces show that the M+(H2O) complex is always the global minimum, and HMOH+ structures lie significantly higher in energy. This agrees with spectroscopic studies of V + (H 2 O), 23,30 Cr + (H 2 O), 24 Mn + (H 2 O), 25 Fe + (H 2 O), 4 4 , 4 5 Co + (H 2 O), 3 5 Ni + (H 2 O), 2 6 , 3 4 and Cu+(H2O),46 and the same holds true for Zn+(H2O).47 CID studies by Armentrout and co-workers show that the structure of M+(H2O)n, n = 1−4, M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, is a central metal ion with up to four solvating intact water molecules. These findings, however, cannot easily be extrapolated to larger clusters. V +(H2O) n undergoes intracluster redox reactions under the influence of room temperature blackbody radiation to eliminate atomic and molecular hydrogen, with a strong size dependence.48 H atom elimination is observed in a narrow size regime, n = 9−12, while H2 is formed over a broader range, n = 9−23.48 H2 formation is associated with a small activation energy since it is also observed at low temperatures, T ≈ 80 K.49 Even for Al+(H2O)n, the question is not definitively settled under which conditions and at which cluster size the hydride− hydroxide structure HAlOH+(H2O)n−1 is formed.50 For n = 1, Al+(H2O) is the global minimum on the potential energy

INTRODUCTION Transition metals are involved in numerous catalytic processes in nature and industry.1,2 They play an important role in biological systems like the active sites of enzymes, e.g., oxidases or nitrogenases,2,3 in industrial and technological applications,4−7 and in the atmosphere.8,9 Especially in photosynthesis, manganese complexes are involved.2 Since energy storage from alternative sources like wind power or solar cells is still a challenge, energy conversion by water photodissociation is also an important research topic. Metal ions, e.g., cobalt, are often present in the center of the catalyzing complexes and enzymes.5,10,11 Charge transfer processes from a hydrated metal ion in a low oxidation state to the surrounding solvent and activation of water by metal ions are of fundamental interest to get a deeper understanding of the process of hydrogen formation in aqueous environments.12−15 Hydrated singly charged metal ions M+(H2O)n, M = Cr, Mn, Fe, Co, Ni, Cu, in the gas phase undergo precipitation reactions with hydrogen chloride,16,17 similar to Ag+(H2O)n,18 and highly specific ion−molecule reactions with O2, CO2, and N2O.19 For a detailed understanding of these reactions, it is essential to know the key structural features of the reactant ions, like the oxidation state of the metal center. In particular, M+(H2O)n and HMOH+(H2O)n−1 have the same mass, but different chemical properties, with the metal center in oxidation states +I and +III, respectively. M+(H2O)n in the gas phase have been investigated with a wide range of experimental techniques,12−14,20−22 like infrared photodissociation in the O−H stretch region,23−29 photodissociation of hydrated monovalent transition metal ions with electronic excitation of the metal center,30−36 matrix isolation © 2012 American Chemical Society

Received: September 3, 2012 Revised: October 16, 2012 Published: October 17, 2012 10676

dx.doi.org/10.1021/jp308744p | J. Phys. Chem. A 2012, 116, 10676−10682

The Journal of Physical Chemistry A

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surface,51 while HAlOH+(H2O)n−1 becomes energetically preferred from n = 2,52−54 and evidence for this structure was found in infrared spectra.55 Calculations of larger clusters53 show that the barrier for the conversion of Al+(H2O)n to HAlOH+(H2O)n−1 becomes small at n = 8. Experimentally, H2 elimination is only observed for 11 ≤ n ≤ 24.56,57 Recent exchange experiments with D2O, however, suggest that clusters with n > 38 are present as Al+(H2O)n, i.e., a singly charged aluminum ion solvated by intact water molecules.50 Reactions of ionic water clusters with D2O reveal intracluster proton transfer.50,58−62 If proton transfer occurs, D2O and H2O are in equilibrium with HDO, reaction 1.

resided up to 2 s in the ICR cell at t = 0 s, reaction products can be observed as soon as the D2O reaction gas is present.

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RESULTS AND DISCUSSION

The results with V+(H2O)n are similar to our previous work with Al+(H2O)n.50 Characteristic mass spectra at different reaction delays are shown in Figure 1. At nominal 0 s, Figure 1a, uptake of up to 7 molecules of D2O is visible. This is reasonable since the ICR cell is filled with ions for 2 s, and the reactant gas is present at a constant pressure of 5.4 × 10−8 mbar. The V+(H2O)x(D2O)y peaks are found at odd numbers

proton transfer

D2 O + H 2O XooooooooooooooY 2HDO

(1)

Uptake of several D2O molecules by the cluster and concomitant evaporation of water molecules will statistically cause evaporation of HDO, which is easily detected in highresolution mass spectrometry.50,58 Additional water evaporation is caused by blackbody infrared radiative dissociation (BIRD),49,63−73 increasing the probability for HDO loss. With this technique, we have been able to show that no proton transfer occurs in (H2O)n− or O2−(H2O)n,58 while proton transfer is efficient in the presence of an excess proton58 in H+(H2O)n or a hydroxide ion in MgOH+(H2O)n.50 With a similar approach, Zatula et al. have recently shown that proton transfer is absent in hydrated alkali metal ions M+(H2O)n, M = Li, Na, K, Rb, and Cs, n ≤ 30.62 In the present work, this reaction is used to distinguish M + (H 2 O) n from HMOH+(H2O)n for first row transition metals M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn.

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EXPERIMENTAL SECTION All experiments were carried out on a modified Bruker/ Spectrospin CMS47X FT-ICR mass spectrometer.74−77 The instrument is equipped with an unshielded 4.7 T superconducting magnet, a Bruker infinity cell, an APEX III data station, a TOPPS ion optics power supply, and an ICC2 Infinity Cell Controller with BCH preamplifier. The [M,nH2O]+ ions were generated in an external laser vaporization source78−80 and were transferred by a system of electrostatic lenses to the ICR cell. To minimize the drift of the initial cluster size distribution, the vaporization laser (Continuum Surelite II, 532 nm) and frequency doubling crystal were heated by 20 laser shots, followed by another 20 laser shots at 10 Hz and typically 5 mJ pulse energy to fill the cell. Isotopically enriched targets were used where applicable: 52Cr (99.9%), 56Fe (99.7%), 58Ni (99.9%), 63Cu (99.3%), and 64Zn (99.4%) (STB Isotope Germany GmbH). Metal targets were prepared as described before.19 Deuterium oxide (Sigma Aldrich, 99.98 atom %) was present in the ultrahigh-vacuum region at a constant pressure of 1.1 × 10−8 to 5.4 × 10−8 mbar. Inevitable HDO impurities in the D2O reaction gas make the experiment difficult.58,59 The HDO content was minimized by using a new leak valve, and passivation of the system for 10 days, with repeated bake-out cycles.50 Electron ionization spectra of the reaction gas in the ICR cell were taken before and after the reaction. They allow to quantify the contribution of HDO impurities to less than 4% in the mass spectrum. To follow the reaction, mass spectra were taken after different reaction delays. The reaction delay is measured relative to the end of the laser vaporization cycle. Since clusters may have

Figure 1. Mass spectra of the reaction of V + (H 2 O) n and V(OH)2+(H2O)n−2 with D2O at a pressure of 5.4 × 10−8 mbar after (a) 0 s, (b) 2 s, and (c) 4 s. V+(H2O)x(D2O)y(HDO)z and V(OH)2+(H2O)x−2(D2O)y(HDO)z, with z = 0,2,4,..., connected by blue lines, and z = 1,3,5,..., connected by green lines. 10677



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of m/z. However, peaks with small intensities are present at even numbers, which are either due to elimination of HDO formed via proton transfer in reaction 1 or uptake of HDO. The HDO content in the reaction gas in the ultrahigh vacuum region is below 4%, high enough to account for the low intensities of even mass peaks at m/z > 400 amu, while their high contribution at m/z < 300 amu clearly indicates proton transfer. In this mass region, H2 elimination has taken place to a significant extent, and the resulting hydroxide ions enable proton transfer in the cluster. After 2 s, Figure 1b, this process has resulted in complete randomization of peaks at odd and even numbers for m/z < 300 amu or cluster sizes n < 14. For larger clusters, the contribution of clusters with even m/z values gradually levels off and has almost vanished at n > 24. This is in agreement with our earlier observation that formation of V(OH)2+(H2O)n−2 is observed only for n ≤ 24 in BIRD experiments of V+(H2O)n. V+ is oxidized to V3+, while hydrogen is reduced with the release of H2.48 After a delay of 4 s, Figure 1c, the mass spectrum contains clusters with n ≤ 22, due to BIRD. The distribution of odd and even numbered peaks is almost statistical, exhibiting almost perfect Gaussian shapes for n ≤ 15. The results show that V+(H2O)x(D2O)y occur only without HDO, while V(OH)2+(H2O)x−2(D2O)y(HDO)z are present with odd and even z in equal probability, with the exception of HDO uptake from the reaction gas. For M+(H2O)n, M = Cr, Fe, Co, Ni, Cu, and Zn, the situation is completely different. After 4 s, the mass spectra contain cluster with sizes n ≈ 9−23. Figure 2a shows the mass

9−11. Even more striking is the difference in the mass spectra after 10 s of reaction delay, Figure 3, showing cluster sizes n ≈

Figure 3. Mass spectra of (a) Zn+(H2O)n and (b) Mn+(H2O)n after 10 s at a D2O pressure of 1.9 × 10−8 mbar for zinc and 2.1 × 10−8 mbar for manganese. M+(H2O)x(D2O)y(HDO)z with z = 0,2,4,..., are connected by blue lines; M+(H2O)x(D2O)y(HDO)z with z = 1,3,5,..., are connected by green lines. While zinc shows only minor scrambling, probably from HDO content in the UHV, manganese shows a completely statistical distribution of species with odd and even z.

6−13. For zinc, Figure 3a, Zn+(H2O)x(D2O)y are still clearly dominant, while odd and even numbered mass peaks are completely statistical for manganese, Figure 3b. Since no atomic or molecular hydrogen loss is detected from Mn+(H2O)n, the results suggest that the oxidation of manganese stops after formation of a hydride−hydroxide, reaction 2. Mn+(H 2O)n → HMnOH+(H 2O)n − m − 1 + mH 2O

(2)

The mass spectra after 6 s, provided as Supporting Information (Figure S1), indicate that reaction 2 occurs for n ≤ 20 and proceeds more slowly than H2 formation in V+(H2O)n. Calculations for Al +(H2 O) n show that formation of HAlOH+(H2O)n−1 is exothermic by about 200 kJ mol−1.53,54 If formation of HMnOH+(H2O)n−1 from Mn+(H2O)n was comparatively exothermic, the intracluster reaction 2 should lead to evaporation of m ≈ 5H2O. BIRD experiments of size selected clusters, however, did not provide any evidence for such a heavily exothermic process. Evidence for a mild exothermicity of reaction 2 is provided by a different BIRD experiment. We followed the decay of Mn+(H2O)n and Fe+(H2O)n for 20 s without mass selection. The two experiment series were done with the same experimental parameters right after one another, with a small break for the target change. Plots of the average cluster size ⟨n⟩ and the difference of the two cluster species Δ⟨n⟩, Figure 4, show that, during the first 4 s, Fe+(H2O)n are on average Δ⟨n⟩ = 0.50 ± 0.10 water molecules smaller than Mn+(H2O)n. After 6 s, when the average cluster size has fallen below ⟨n⟩ = 20, the difference is reduced to Δ⟨n⟩ = 0.15 ± 0.10, suggesting that the decay of Mn+(H2O)n becomes slightly accelerated. Figures 2 and S1 showed that reaction 2 occurs mostly around 4−6 s, which coincides with the transition from Δ⟨n⟩ ≈ 0.50 to Δ⟨n⟩ ≈ 0.15. The BIRD results shown in Figure 4 are therefore consistent with the D2O exchange experiments. Reaction 2 proceeds slowly for n ≤ 20 and is mildly exothermic by ΔH(2) ≈ −21 ± 10 kJ mol−1.77,81,82 This value is estimated from the change in

Figure 2. Mass spectra of (a) Zn+(H2O)n and (b) Mn+(H2O)n after 4 s at a D2O pressure of 1.9 × 10−8 mbar for zinc and 2.1 × 10−8 mbar for manganese. M+(H2O)x(D2O)y(HDO)z with z = 0,2,4,..., are connected by blue lines; M+(H2O)x(D2O)y(HDO)z with z = 1,3,5,..., are connected by green lines. Both spectra are quite similar after 4 s for large clusters, while a significantly higher contribution of species with z = 1,3,5,..., is observed with M = Mn at small cluster sizes.

spectrum for M = 64Zn. Despite significant uptake of HDO from the impurity, peaks at even numbered m/z values dominate, especially in the low mass regime. The efficiency of the uptake seems to be independent of the metal ion. This clearly shows that H/D-exchange is absent in these systems, similar to the behavior of hydrated alkali metal ions, which also show no sign of isotopic scrambling as recently reported by Zatula et al.62 Manganese, however, exhibits again a different behavior, Figure 2b. While at higher masses, n ≥ 18, the zinc and manganese spectra look quite similar, the signature of HDO formation and elimination is evident for cluster sizes n = 10678



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although a significant number of D atoms are available in the clusters after 4 s. This indicates a strong kinetic isotope effect for reaction 2. Quite surprising is the behavior after an additional 300 s, Figure 5b, when BIRD leads to very slow loss of a water molecule from HMnOD+(D2O)3. HMnOD+(D2O)2 is present in minor amounts, while a completely exchanged DMnOD+(D2O)2 is the dominant product. On first sight, it looks like HDO was selectively lost from HMnOD+(D2O)3, which may even imply reduction of Mn(III) back to Mn(I) and formation of Mn+(D2O)3. More likely, however, is a very slow loss of D2O from HMnOD+(D2O)3, reaction 3, followed by efficient H/D-exchange in the collision with D2O, reaction 4. HMnOD+(D2 O)3 + hv → HMnOD+(D2 O)2 + D2 O (3) +

+

HMnOD (D2 O)2 + D2 O → DMnOD (D2 O)2 + HDO Figure 4. (a) Average cluster size ⟨n⟩ for manganese (orange triangle) and iron (blue open circle) as a function of time in a BIRD experiment without mass selection. (b) Difference in cluster size Δ⟨n⟩ between manganese and iron (black square). During the first 4 s, Fe+(H2O)n are on average about 0.50 water molecules smaller than Mn+(H2O)n. After 6 s, the difference goes down to Δ⟨n⟩ = 0.15, suggesting an accelerated decay of Mn+(H2O)n between 4 and 6 s. The dashed red lines indicate the average value for 0−4 s and 6−20 s.

(4)

We think that the reaction proceeds in these two steps because the collision complex in reaction 4 has a higher internal energy than HMnOD+(D2O)3 in reaction 3. Reaction 4 only occurs at these small cluster sizes because empty coordination sites at the metal center are probably required for H/Dexchange of the hydride H. Our interpretation of the intriguing behavior of HMnOD+(D2O)3 is further supported by D2O exchange experiments that start with a distribution of small clusters Mn+(H2O)n, n = 4−12, shown in Figure 6. After 4 s of reaction delay, Figure 6b, the HDO content is negligible in n = 4 and n = 5, and significantly higher and n = 7 and n = 8. After 10 s, complete exchange of H2O against D2O is achieved in n = 4, Figure 6c, featuring an intense peak corresponding to

Δ⟨n⟩ between 4−6 s, the period during which ∼70% of the Mn+(H2O)n clusters are converted to HMnOH+(H2O)n−1. The D2O exchange experiments after long reaction delays yield further evidence for the occurrence of reaction 2 with manganese. For Zn+(H2O)n after 10 s, Figure 3a, peaks are present, which show complete exchange of H2O against D2O, i.e., Zn+(D2O)n with n = 6−8. Following the reaction of Mn+(H2O)n with D2O for 60 s shows that one H atom is not exchanged during that period, Figure 5a. HMnOD+(D2O)3 is the dominant peak in the mass spectrum. This observation has interesting implications. First, the H atom that is not exchanged must be the hydride atom since the hydroxide ion is involved in the H/D-exchange mechanism, reaction 1. Second, reaction 2 does not seem to work with a D atom forming the hydride,

Figure 5. (a) Mass spectrum of the reaction of Mn+(H2O)n with D2O after 60 s at a pressure of 9.3 × 10−8 mbar. One H atom remains unexchanged. (b) After an additional 300 s, BIRD leads to slow water loss. Formation of the fully exchanged [Mn,(D2O)3]+ as the main product peak probably proceeds in two steps. Slow loss of D2O is followed by rapid exchange of HDO against D2O.

Figure 6. (a) Small initial Mn+(H2O)x(D2O)y(HDO)z, n = x + y + z, cluster distribution with n = 4−13 at a D2O pressure of 1.5 × 10−8 mbar. (b) The HDO content is negligible for n = 4,5. (c) After 10 s, complete exchange is observed for n = 4,5, while n = 6 is not fully exchanged. 10679



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Figure 7. (a) Decay of mass-selected Mn+(H2O)4 (green triangle) from the ion source to Mn+(H2O)3 (blue circle) and Mn+(H2O)2 (orange square). The curvature of the Mn+(H2O)4 intensity indicates the presence of a fast and slowly decaying isomer. Assuming that the slowly decaying ion is HMnOH+(H2O)3, we set its BIRD rate constant to a fixed value of kslow = 0.0023 s−1, the result obtained in the fit of panel b. The dashed lines show the two different isomers that were used for the fit. (b) BIRD kinetics of HMnOH+(H2O)3 (green triangle), which were mass selected in a BIRD experiment of larger clusters after 30 s, and their decay to HMnOH+(H2O)2 (blue circle). The shaded area denotes the noise level.

Mn+(D2O)4 and little HDO content. Complete exchange is also observed in n = 5, but the HDO content of this cluster size is appreciable. Already at n = 6, the H/D-exchange ends with HMnOH+(D2O)5 and a statistical HDO content. From the HDO content patterns in Figure 6c, one may estimate that about 50% of the clusters have undergone reaction 2. This places the lower size limit for reaction 2 to n ≈ 8, similar to the lower size limit for the intracluster reactions in Al+(H2O)n and V+(H2O)n.48,83 The D2O exchange experiments conclusively show that Mn+(H2O)4 is formed in the ion source, while BIRD of large clusters results in HMnOH+(H2O)3. Since Mn+(H2O)4 features a singly charged metal center, while Mn(III) is present in HMnOH+(H2O)3, the binding energy of the least strongly bound H2O should be much higher in HMnOH+(H2O)3 than in Mn + (H 2 O) 4 . Consequently, the BIRD rate of HMnOH+(H2O)3 should be significantly smaller. Experiments with mass selected ions corroborate this prediction. Figure 7a shows the decay of mass-selected Mn+(H2O)4 from the ion source, while Figure 7b displays the BIRD kinetics of HMnOH+(H2O)3, which were mass selected in a BIRD experiment of larger clusters after 30 s. While the intensity of Mn+(H2O)4 is reduced to 50% in less than 15 s, more than 60% of the initial HMnOH+(H2O)3 intensity remains after 180 s. In the Mn+(H2O)4 experiment, even the secondary product Mn+(H2O)2 is observed, while no further decay occurs from HMnOH+(H2O)2, Figure 7b. A quantitative fit of the BIRD kinetics of Mn+(H2O)4 in Figure 7a, however, requires two isomers of the reactant ion, as well as two isomers of the primary product. The contribution of the two isomers to the starting intensity shows ill convergence in the fit. Assuming that the slowly decaying ion in Figure 7a is HMnOH+(H2O)3, we set its BIRD rate constant to a fixed value of kslow = 0.0023 s−1, the result obtained in the fit of Figure 7b. With this boundary condition, we get improved convergence of the fit, and the decay of the initial intensity is reproduced very well. A fit in which all parameters are optimized results in a slightly smaller value of the total leastsquares error and is provided as Supporting Information. The BIRD experiments suggest that, under the conditions of this particular experiment, where the intensity of the m/z = 135 amu peak was optimized, it consists of 80% Mn+(H2O)4, decaying with a rate of k fast = 0.077 s −1 , and 20% HMnOH+(H2O)3. Both isomers seem to be formed in the laser vaporization source, while BIRD of large clusters leads to 100% HMnOH+(H2O)3. In the experiments displayed in Figure

6, the initial contribution of HMnOH+(H2O)3 must have been smaller. Most likely, the conditions for making larger clusters imply more efficient cooling and overall more rapid quenching of excited states of Mn+ in the plasma.

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CONCLUSIONS Exchange experiments with D2O together with BIRD experiments show that, among M+(H2O)n, M = Cr, Mn, Fe, Co, Ni, Cu, and Zn, only Mn+(H2O)n undergoes an intracluster redox reaction to form HMnOH+(H2O)n−1. The hydride H atom does not take part in the H/D-exchange for n ≥ 4, while all other H atoms are completely exchanged against deuterium after long reaction delays. The exothermicity of the water activation reaction is small, most likely in the range of ΔH(2) ≈ −21 ± 10 kJ mol−1. In comparison with Al+(H2O)n and V+(H2O)n, the low exothermicity is in line with the overall slow progress of the reaction. Comparison with Al+(H2O)n and V+(H2O)n also suggests that hydride−hydroxide formation at singly charged metal centers requires a hydration shell of at least 8 water molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Mass spectra of the reaction of Zn+(H2O)n and Mn+(H2O)n with D2O after 6 s reaction delay (Figure S1). Fit of BIRD kinetics of mass selected Mn+(H2O)4 and HMnOH+(H2O)3 without boundary conditions (Figure S2). Fit parameters with and without boundary conditions (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49-431-880-2830. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft, grant number BE2505/4-2, and the Fonds der Chemischen Industrie is gratefully acknowledged.

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