Photodissociation of Manganese Oxide Cluster Cations - The Journal

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Photodissociation of Manganese Oxide Cluster Cations Joshua H. Marks, Timothy B. Ward, and Michael A. Duncan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01441 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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The Journal of Physical Chemistry

J. Phys. Chem. A

Photodissociation of Manganese Oxide Cluster Cations J. H. Marks, T. B. Ward, M. A. Duncan* Department of Chemistry, University of Georgia, Athens, Georgia 30602, U. S. A. *[email protected]

ABSTRACT Manganese oxide cluster cations are produced by laser vaporization in a pulsed nozzle source, and detected with time-of-flight mass spectrometry. The mass spectrum contains intense peaks for stoichiometries corresponding to (MnO)n+. Multiphoton photodissociation of these clusters yields smaller ions with the same stoichiometric ratio, either by sequential elimination of MnO units or by various fission processes with roughly equal efficiencies. Fragmentation of clusters containing excess oxygen also yields (MnO)n+ fragments. These apparently stable fragments are investigated further using density functional theory to determine their likely structures. The lowest energy structure for Mn2O2+ is found to be a planar ring, and those for Mn4O4+ and Mn6O6+ are cuboids. Mn3O3+ is predicted to have a six-membered ring structure and Mn5O5+ has a fused cube/ring configuration similar to the structure of the oxygen evolving center of Photosystem II. Open-shell, high spin configurations on individual manganese atoms couple anti-ferromagnetically and ferromagnetically to produce low-spin and high spin configurations on different sized clusters.

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INTRODUCTION Bulk manganese dioxide has applications as a component of dry cell batteries and as a pigment. However, the properties of manganese oxide nanoparticles and clusters differ dramatically from those of its solid. Manganese oxide clusters produced synthetically are well known as single molecule magnets,1-3 and small manganese oxide clusters form the active site in biological systems such as the photosynthetic protein, Photosystem II.4-6 Related nanostructured metal oxides including manganese have been investigated as water oxidation catalysts.7-10 To elucidate the structures and reactivities of such small transition metal oxides, many gas phase experiments have focused on these systems. Mass spectrometry,11-30 collision induced dissociation,11-14 reactivities,11-15,18 and photodissociation19-24 all reveal stable oxide stoichiometries, as only the most strongly bound clusters are believed to survive energetic production, reactions, and fragmentation processes. Photoelectron spectroscopy has been used to study both vibrational and electronic structure.31-41 Infrared spectroscopy has been measured in rare gas matrices42,43 and in the gas phase using mass spectrometry and tunable laser photodissociation.44-47 Ion mobility mass spectrometry has been used to identify the structural isomers present for specific stoichiometries.48-50 Computational studies, usually employing Density Functional Theory (DFT),51-59 have investigated structures and spin configurations for many small oxide systems. In the present study we investigate small manganese oxide cations with mass-selected ion photodissociation experiments. Much of the previous work on gas phase manganese oxide has employed mass spectrometry to determine the stoichiometries formed. Several groups have produced mass spectra with prominent peaks corresponding to (MnO)n+.25-27,29 The photodissociation studies of Kondow and coworkers focused on the elimination of Mn from MnnO+ (n = 2 – 5) and measured the dissociation threshold energy to determine binding energies.27 Zhou and coworkers 2 ACS Paragon Plus Environment

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examined the binding of atomic manganese to molecular oxygen in matrix isolation infrared studies.42,43 Bowen and coworkers reported the anion photoelectron spectra of small manganese oxide clusters and included theoretical predictions of spectra and spin states.36 The spin states of these clusters are particularly interesting because the half-filled d-shell of manganese forms the basis for their magnetism. In the only studies addressing the stabilities of multi-atom clusters, Mafuné and coworkers reported the use of temperature programmed desorption studies on cations of MnnOm+, finding an n:m ratio of 3:4 in the surviving clusters.29 Except for the work of Kondow and coworkers on small mono-oxide species,27 manganese oxide clusters have not been investigated with either collisional or photodissociation methods to probe their preferred stoichiometries. Photodissociation studies have been applied to a number of metal oxide systems, providing perhaps the most direct probe of relative cluster stability.19-24 These experiments reveal the production of specific fragment ions that may be produced repeatedly from different parents, the elimination of specific neutrals that can be inferred from fragment mass intervals, or the formation of specific stoichiometries that occur in many fragmentation processes. Although any one fragmentation event is difficult to interpret, the patterns of fragments for the dissociation of many different precursors can be understood in terms of the "survival of the fittest." Averaged over many dissociation events for different cluster sizes, stable clusters or stoichiometries will be produced more often. In some cases the detected stoichiometry may indicate an oxidation state in the clusters that is the same as that commonly seen in the bulk material.22,24 In other cases, less common or even new combining ratios may be revealed in the small clusters.19-21 In the case of vanadium and chromium oxides, the species found to be stable via gas phase photodissociation experiments had stoichiometries different from those of the known bulk oxides.19,20 However, these same clusters survived ligand-coating reactions and were able to be 3 ACS Paragon Plus Environment


isolated in solution, confirming their intrinsic stability.60 DFT calculations have been applied throughout the study of metal oxide clusters, predicting possible structures and their relative energies. Although DFT has potential problems with quantitative energetics or the relative energies of different spin states,56,61-64 it is the only viable way to compare multiple medium-tolarge clusters containing transition metals. In the present study, we employ mass-selected photodissociation of small gas phase clusters of manganese oxide to investigate their stability patterns, and complement this work with DFT computations.

EXPERIMENTAL SECTION Small manganese oxide cluster cations are produced by laser vaporization in a pulsed nozzle source.65 The third harmonic (355nm) of a Spectra-Physics INDI-HG Nd:YAG laser is used to vaporize metal from a rotating and translating metal rod. Helium seeded with approximately 3% oxygen by partial pressure is pulsed over the rod in the "turbo" source configuration with a 1 in. long x 5 millimeters dia. growth channel, as described previously.65 The cluster ions produced are skimmed into a differentially pumped chamber, where they are analyzed, mass selected and photodissociated with a reflectron time-of-flight mass spectrometer.66 Photodissociation employs the 355 nm output of a Spectra Physics GCR-150 Nd:YAG laser operating with a fluence of 15–60 mJ/cm2. This laser is timed to intersect the ion beam as it turns in the reflectron. The resulting fragmented and unfragmented ions are reaccelerated into a second flight tube for mass analysis and detected with a Hamamatsu R595 electron multiplier tube and a LeCroy digital oscilloscope. Computational studies are performed with the Gaussian 09 package67 using the M06L functional and the Def2-TZVP basis set. Geometry optimizations use "tight" convergence criteria for geometric parameters, and are confirmed with vibrational analysis in order to identify 4 ACS Paragon Plus Environment

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any imaginary frequencies indicative of transition states. Spin states were unrestricted, and all multiplicities possible from 1 or 2 through a maximum consisting of filled half shells on manganese atoms with a single unpaired s electron were explored. All optimized structures are tested with the "stable=opt" calculation in order to avoid spurious spin states. Isomer searches used multiple random starting geometries adjusted manually rather than any systematic structure generation. We also focus on cluster geometries identified in previous work on this same system.

RESULTS AND DISCUSSION The mass spectrum of manganese oxide cations produced with a gas mixture containing 3% oxygen is shown in Figure 1. As indicated, clusters with the (MnO)n+ stoichiometry are prominent throughout this spectrum. This mass spectrum is consistent with those reported by other labs in previous work.25-27 Reduced oxygen concentrations were found to produce substantially lower intensities for the larger masses. Greater concentrations of up to 10% oxygen were found to increase the intensities of clusters with oxygen exceeding the number of Mn. Regardless of oxygen concentration, ions with the number of oxygens less than that of Mn were not observed with appreciable intensity. This mass spectrum is somewhat different from that reported by Mafuné and coworkers,29 in which all masses were richer in oxygen than metal. This may be a result of the plasma in that experiment being confined into a longer 12 centimeter growth channel or to a higher concentration of oxygen. Mass-selected photodissociation was employed to investigate the fragmentation processes for these clusters. The results of these experiments are shown in Figures 2−4. In these fragmentation spectra, the intensity of the parent ion without laser excitation is subtracted from that with photo-excitation. The resulting data has the depletion of the selected parent ion plotted 5 ACS Paragon Plus Environment


in the negative direction and the photofragments plotted in the positive direction. In an ideal situation, charge conservation would cause the area of the depleted parent to equal the combined intensities of the fragments. However, it is not possible in our instrument to focus the ion optics equally on both regions of the spectrum. We therefore focus on the fragment ions, and the parent ion has a much smaller peak. Photodissociation was found to be much more efficient at 355 nm than at 532 nm, either because of greater absorption probability here or more efficient dissociation with the higher energy photons. However, the same fragment ions were observed at both wavelengths. Relatively high laser pulse energies (15–60 mJ/cm2) were required to detect photodissociation. This suggests that dissociation is a multiphoton process, which is not surprising for these strongly bound clusters (see computed dissociation energies in Table 1). At the 3.5 eV (80.7 kcal/mol) photon energy, the computed dissociation energies suggest that dissociation via the elimination of one MnO molecule should require at least two photons, but excess energy may be necessary to achieve dissociation in the 1−2 µsec time frame of the experiment.66 It is also conceivable that these clusters contain significant internal energy from their growth process, which would reduce the amount of energy required from photo-excitation. The condensation energy, i.e., the exothermicity from the formation of metal-oxygen bonds as these clusters grow, can lead to internal heating. As shown below, these spectra suggest that fragmentation may occur via the elimination of multiple MnO units, which is a higher energy process. In each of these fragmentation spectra we investigate different ion focusing conditions, adjusting the pulsed extraction timing from the molecular beam and the deflection plate voltages. The data collected employed conditions chosen to avoid as much as possible any bias for any specific clusters or mass range. Figure 2 shows the photodissociation mass spectra for the (MnO)n+ clusters for n = 3−6. As indicated, these clusters fragment to form various (MnO)n+ masses, apparently by the 6 ACS Paragon Plus Environment

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sequential loss of MnO. However, it is important to note that we cannot distinguish between the sequential loss of MnO units versus fission processes that eliminate neutral (MnO)n clusters. The relatively smooth distribution with multiple MnO decrements suggests that the former is more likely. However, it is likely a lower energy process to split a cluster into two pieces than to eliminate many small pieces. If fission is occurring, then the various channels necessary to account for the multiple fragments observed must have roughly equal efficiencies. In principle, laser power dependence studies of a multiphoton process can shift the ratio of product channels, allowing sequential fragmentation to be detected, but we do not see any change in the relative intensities of these fragment ions with different photodissociation laser powers. Regardless of the dissociation mechanism, it is still true that more stable clusters should survive and be produced more abundantly in these fragmentation processes. The Mn3O3+ fragment appears with greater intensity in each of these fragmentation processes, independent of mass spectrometer focusing. This suggests an increased stability for this ion compared to the other (MnO)n+. Manganese cation is also observed as a "terminal" fragment in these mass spectra. It has been shown previously by Kondow and coworkers27 that photodissociation of Mn2O+ results in MnO and Mn+. The Mn+ fragment is expected to carry the charge in this case because the ionization energy of Mn (7.4 eV) is lower than that of MnO (8.7 eV).11 By similar reasoning, we can conclude that the various (MnO)n clusters, which are produced as cation fragments, have ionization energies lower than that of the MnO neutral (MnO+ is not detected as a strong fragment ion). Figure 3 shows the photodissociation mass spectra of selected manganese oxide cluster cations which contain more oxygen than metal. The neutral fragments were not detected but are presumably atomic oxygen, diatomic oxygen, and MnO or (MnO)n. This figure also shows the fragmentation of Mn7O10+ to yield Mn7O8+ by the loss of O2. This is likely the result of the 7 ACS Paragon Plus Environment


energetically favorable loss of neutral O2, rather than the preferred formation of Mn7O8+, as this ion is not observed as a fragment from any other parent ions. A similar O2 elimination apparently occurs for Mn5O7+ and Mn6O8+, followed by the implied sequence of MnO mass losses seen before. This suggests that these ions have excess oxygen that is more weakly bound than that in the clusters with the 1:1 stoichiometry. The (MnO)3+ ion is again more intense than the other cation fragments. Figure 4 shows the fragmentation of larger clusters, which similarly demonstrate a preference for the formation of fragments containing equal amounts of manganese and oxygen. The fragmentation of Mn10O14+ to yield the Mn10O12+ indicates the loss of neutral O2, as does the fragmentation of Mn8O10+ to form (MnO)8+. Of particular interest is the relative intensity of the fragments formed here. The (MnO)n+ ions in the n = 3−6 range are more intense than others, and the (MnO)3+ and (MnO)6+ species are somewhat more intense than the others in this range. These particular cluster stoichiometries were the focus of previous computational work on iron oxides, where (M3O3)n clusters were identified as stable, and structures based on six-membered rings and stacked six-membered rings were proposed.68 1:1 stoichiometry clusters in this size range were also proposed to have ring structures in previous computational studies of manganese oxide.58,59 The (9,9) cluster is the next largest member of this (M3O3)n series. Unfortunately, we were unable to get stable enough signals for this cluster to measure its photodissociation. It is also apparent in Figure 4 that the mass peaks detected are broader than those in Figures 2 and 3, and this is particularly true for the largest clusters here. Some broadening is also noticeable in Figure 3 for the larger clusters (e.g., Mn7O10+). Such broad peaks indicate metastable fragmentation, i.e., that the clusters are dissociating on the same timescale of the acceleration out of the fragmentation zone in the mass spectrometer. According to ion simulations for our instrument, the relevant timescale is a few microseconds.66 Therefore, 8 ACS Paragon Plus Environment

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clusters with sharp fragment peaks are dissociating faster than this and those with broadened peaks are dissociating on about this same timescale. The experimental data presented here indicate that 1:1 stoichiometry clusters are produced as photofragments throughout the size range of clusters studied here. In certain cases, clusters with excess oxygen lose O2 or even an O atom, but all the other prominent fragments are (MnO)n+ cluster ions. Mn2O+ and Mn+ are also detected in nearly every fragmentation experiment, but their intensities are generally less than those of the nearby (MnO)n+ masses. A strong tendency for 1:1 stoichiometries for manganese oxide clusters was also reported by Kirilyuk and coworkers in mass spectrometry measurements of the clusters produced initially by laser vaporization.25 This differs significantly from the experiments of Mafuné and coworkers,29 where MnnOm+ stoichiometries of 3:4 were found in thermal evaporation/dissociation measurements. However, the data of Mafuné detected an increasing amount of the 1:1 masses at higher thermal desorption temperatures. This could indicate that the average amount of energy deposited thermally was less than that from our multiphoton excitation, so that the "core" stable clusters were not sampled effectively by evaporation. Unfortunately, the clusters in the Mafuné experiment were also not mass selected, making it difficult to identify specific fragmentation channels for specific clusters. The 1:1 stoichiometry seen here corresponds to the most common form of bulk manganese oxide, with the metal in essentially the +2 oxidation state (ignoring the charge). A similar stoichiometry was found for small iron oxide clusters,21 even though the +2 oxidation state is not the most common for iron. Our studies of vanadium, niobium and tantalum clusters found several different non-stoichiometric clusters with combining ratios unrelated to the bulk oxides.19 Similar stoichiometries were found for chromium oxide clusters.20 Our studies of yttrium and lanthanum found clusters with the expected +3 oxidation state,22 whereas the study 9 ACS Paragon Plus Environment


of cerium oxides found the expected +4 oxidation state.24 It is clear that some small oxide clusters can have unexpected combining ratios, but apparently this is not the case for manganese. To investigate the structures of these small manganese oxide clusters, we conducted computational studies using density functional theory at the M06L/Def2-TZVP level. We chose this level of theory because of its better performance for the spin configurations in these clusters.62,64 Figure 5 shows the lowest energy optimized structures found for some of the more prominent ion fragment clusters here. These studies identified and investigated numerous structural isomers and spin states for these systems, which are presented in the Supporting Information file for this article. As shown, both open ring and cuboid structures appear throughout these systems. Similar ring and cuboid structures have been proposed for other small oxide clusters having the 1:1 stoichiometry.48,49,57-59 In previous calculations on manganese oxide cations, Lang and coworkers used the spin density-functional theory molecular dynamics (BO-SDFT-MD) method and reported that the ring isomer for Mn4O4+ was slightly lower in energy than the cuboid found to be most stable here.58,59 Mafuné and coworkers focused their computations at the B3LYP/6-311+G* level on the oxygen-rich stoichiometries found in that study. They also reported a ring for the Mn4O4+ species,29 but did not report energetics for other isomers. For the abundant Mn3O3+ and the Mn6O6+ clusters, we find ring and stacked-ring structures analogous to those proposed by Khanna68 for corresponding iron oxide cluster cations and detected in ion mobility experiments on iron oxide by Misaizu.49 The ring is the lowest energy structure found here for Mn3O3+, but the stacked ring is not the lowest energy structure for Mn6O6+. Instead, Mn6O6+ is predicted to have a cubic structure. The predicted lowest energy structure for Mn5O5+ bears a striking resemblance to the structure of the oxygen evolving complex of Photosystem II, albeit with the substitution of a manganese atom for calcium. Our


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computational studies considered only clusters up to the (6,6) size, as the number of isomers and spin states becomes unmanageable for the larger species. The magnetism predicted for these clusters is particularly interesting. Because of the half filled d-shell of manganese, we find that high-spin electronic configurations are prominent for each manganese atom. The Mulliken spin densities of the calculated electronic wavefunctions were checked, and regardless of magnetic ordering, those structures composed of high-spin manganese atoms were generally most stable. The net spin for the overall cluster often involves the alignment or misalignment of these high spin configurations on individual atoms. The structures listed in Figure 5 along with the associated energies represent only those combinations of structure and multiplicity found to lie within 10 kcal/mol of the lowest energy structures. The low spin doublet was found to be the most stable for all structures containing even numbers of manganese and oxygen, e.g., Mn2O2+, Mn4O4+, and Mn6O6+ In these clusters the bonding to oxygen involves the s electrons and each manganese atom has a high spin d5 configuration; the low net spin results from anti-ferromagnetic alignment of these. However, in each of these cases, higher spin species are predicted to lie very close in energy (often within 1−2 kcal/mol). Because of the limitations of DFT theory for such effects, we are not particularly confident about these relative energies, but present the results as obtained. Accordingly, higher spin states were found to be stable for Mn3O3+ and the Mn5O5+. The next-to-lowest energy ring isomer of Mn4O4+ has an overall high spin state resulting from ferromagnetic alignment of the individual high-spin atoms. Lang and coworkers found the same spin configuration for this ion structure.58,59 For this particular cluster, there would be a sharp isomer dependence on the magnetic moment. If our computations can be trusted, there should be an alternation in magnetic moments for clusters with an even (low moment) versus odd (higher moment) number of metal atoms. However, the clusters would have to be cooled efficiently into their lowest electronic 11 ACS Paragon Plus Environment


states to see this effect. There are of course well known problems with DFT computations on spin states such as those seen here. Truhlar and coworkers have addressed this issue for the Mn3O3 cluster using a non-collinear spins DFT treatment.69 A similar treatment was employed on ligand-free and ligand-coated Co4O4 clusters.57 It is conceivable that these different spin configurations influence the fragmentation patterns for these clusters. However, nothing in the present fragment detection allows spin states to be identified, and because of the close energetics such states might be scrambled in the dissociation process anyway. The fragmentation experiments described here show in a convincing way that the 1:1 stoichiometry is preferred for small manganese oxide clusters, in contrast to suggestions from previous thermal desorption work.29 The computational studies on the small fragment ions with this stable stoichiometry suggest that they have a complex electronic structure resulting from open shell configurations on each manganese atom and their alignment or misalignment in different clusters. Similar electronic structure has recently been observed for the pure metal manganese dimer and trimer cations with x-ray magnetic circular dichroism (XMCD) measurements on cryogenically cooled and trapped ions.70 These small clusters have high-spin configurations on each metal atom with ferromagnetic coupling, producing high magnetic moments. In the case of the Mn4O4+ clusters, our computations at the DFT/M06L/Def2-TZVP level of theory produced slightly different relative energies for cuboid versus ring isomers compared to previous work.57,58 This sensitivity to computational method is not surprising considering the complexity of the electronic structure of manganese. Manganese oxide clusters seem to be an ideal prototype for examinations of the issues of electronic and geometric structure in a small, but difficult, system. Future work could address these issues with studies of the


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magnetism of these cluster ions using XMCD measurements,70 or studies of their geometric structures with ion mobility measurements48-50 or vibrational spectroscopy.44-47

CONCLUSIONS Manganese oxide cluster cations were produced from manganese metal and oxygen seeded helium gas in a laser vaporization pulsed nozzle source. Mass spectrometry revealed that clusters with the formula of (MnO)n+ were most abundant. Mass-selected photodissociation further showed that larger and more oxygenated cations fragment to yield products of the same (MnO)n+ stoichiometry. These results are consistent with manganese in the +2 oxidation state. M06L/Def2-TZVP calculations indicate that the most likely structures for the stable cations are cuboids or rings, with rectangular shapes predicted for larger species. Open-shell configurations on each manganese atom can couple anti-ferromagnetically or ferromagnetically, producing wide variations in the predicted magnetic moments for clusters with different structures and numbers of metal atoms.

AUTHOR INFORMATION Corresponding author: Email: [email protected]

ACKNOWLEDGEMENTS We gratefully acknowledge the generous support for this work from the Air Force Office of Scientific Research through grant no. FA9550-15-1-0088 (MAD).



SUPPORTING INFORMATION AVAILABLE: Full details of the computations, including the full citation for reference 67 and the structures, multiplicities, and energies for each of the clusters considered are available free of charge via at http://pubs.acs.org.


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Table 1. Relative energies calculated for different structural isomers and spin states for (MnO)n+ cluster cations. (MnO)n

+

structure

2S + 1

E (kcal/mol) BE[(MnO)n − MnO] (kcal/mol)

1

C∞v C∞v C∞v

5 7 3

0.0 +12.4 +29.9

-

2 2

D2h D2h

2 10

0.0 +0.1

132.7 132.6

3 3 3 3

D3h D3h D3h C2v

5 7 15 7

0.0 +7.1 +7.7 +8.3

110.0 103.0 102.4 101.7

4 4 4 4 4 4

Td Td Td D4h Td D4h

2 10 12 20 20 2

0.0 +1.2 +4.3 +7.9 +8.0 +8.1

100.6 99.4 96.2 92.6 92.6 92.4

5 5 5 5 5

Cs Cs Cs C1 C1

7 15 25 5 15

0.0 +1.1 +8.7 +8.8 +10.0

100.8 99.8 92.2 92.1 90.8

6 6 6 6 6 6

C2v C2v C2v C2v C2v C3

2 12 10 20 22 10

0.0 +1.0 +1.8 +5.6 +7.1 +7.6

133.7 132.7 131.9 128.0 126.6 126.0


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FIGURE CAPTIONS

Figure 1. Mass spectrum of manganese oxide cluster cations produced by laser vaporization of a manganese metal rod in a supersonic expansion containing 3% oxygen in helium.

Figure 2. Multiphoton photodissociation mass spectra of Mn3O3+, Mn4O4+, Mn5O5+, and Mn6O6+ clusters at 355 nm. The negative peaks indicate loss of the parent ion, and positive peaks indicate the cation products of fragmentation.

Figure 3. Multiphoton photodissociation mass spectra of Mn3O4+, Mn4O5+, Mn5O7+, Mn6O8+, and Mn7O10+ clusters at 355 nm. The negative peaks indicate loss of the parent ion, and positive peaks indicate the cation products of fragmentation.

Figure 4. Multiphoton photodissociation mass spectra of Mn8O10+, Mn9O11+, Mn10O14+, and Mn11O15+ clusters at 355 nm. The negative peaks indicate loss of the parent ion, and positive peaks indicate the cation products of fragmentation.

Figure 5. The lowest energy structures calculated at the M06L/Def2-TZVP level of theory for Mn2O2+, Mn3O3+, Mn4O4+, Mn5O5+, and Mn6O6+. Relative energies are shown in kcal/mol. The spin configurations for each of these clusters are given in Table 1.



Figure 1.


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Figure 2. 27 ACS Paragon Plus Environment


Figure 3.


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Figure 4. 29 ACS Paragon Plus Environment


Figure 5.


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Photodissociation of Manganese Oxide Cluster Cations - The Journal

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