Compositions and Structures of Vanadium Oxide ... - ACS Publications

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Compositions and Structures of Vanadium Oxide Cluster Ions VmOn± (m = 2−20) Investigated by Ion Mobility Mass Spectrometry Jenna W. J. Wu,† Ryoichi Moriyama,† Hiroshi Tahara,† Keijiro Ohshimo,†,‡ and Fuminori Misaizu*,† †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai 980-8576, Japan

‡

S Supporting Information *

ABSTRACT: Stable compositions and geometrical structures of vanadium oxide cluster ions, VmOn±, were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V2O4)(V2O5)(m−2)/2+ and (VO2)(V2O5)(m−1)/2+, depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V2O5)m/2− and (VO3)(V2O5)(m−1)/2−, were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V2O5)m/2± where m = 6−12 had right polygonal prism structures except for the anionic V12O30−, and for the clusters of odd numbers of vanadium m, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V12O30−, which had a CCS almost equal to that of the neighboring smaller V11O28−, should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V11O28−. anionic,9,14−16 and neutral11,12,17 species. The structures of cationic clusters were suggested by Duncan and co-workers for group 5 metal oxides (MnOm; n = 1−5; M = V, Nb, and Ta).5 Bernstein and co-workers worked on small, neutral vanadium oxide clusters with density functional theory.11,12 For neutral vanadium oxide clusters having an even number of vanadium atoms, which are expressed by (V2O5)x up to x = 12, right “polygonal” prism and polyhedral structures were suggested by quantum chemical calculations by Vyboishchikov and Sauer.17 The structures of the anionic vanadium oxide clusters with an odd number of vanadium atoms, expressed as (VO3)(V2O5)1−3−, were suggested by infrared multiphoton dissociation spectroscopy and calculations.14 In this work, Asmis and co-workers obtained the vibrational spectra of structural isomers such as pyramid structure and bridged structures of V5O13−, which had an extra top of vanadium atom on the even sized polygonal prism structures. Structures of neutral, cationic, anionic (V2O5)1−3, and anionic (VO3)(V2O5)1−2− are shown in Figure 1 as examples of stable structures from past studies. The study of small gas-phase vanadium oxide cluster anions by Wang and co-workers based on anion photoelectron spectroscopy also gave insights into the electronic structures of the cluster anions and into V−O chemical bonding properties.18,19

1. INTRODUCTION A vanadium atom can take various oxidation states within the range of +2 to +5. Bulk phase vanadium oxides exist in various chemical compositions with different oxidation states, such as VO with V (+II), and V2O5 with V (+V). Among these compositions, bulk V2O5 is found to be the most stable because the vanadium atoms are fully oxidized in the bulk solid crystals. Bulk V2O5 is also well-known as an oxidation catalyst, which is used widely in industries. Some examples of its industrial usage are catalytic oxidation of SO2 to SO3 in the synthesis of sulfuric acid1 and selective oxidation of hydrocarbon molecules.2 In other applications, bulk vanadium oxide is studied as cathode materials of lithium batteries or as thin-film materials in optical switching devices.3 Gas-phase vanadium oxide clusters (VmOn) were studied experimentally and were often complemented by theoretical studies to achieve a better understanding of their reactivity and their roles in catalysis. Experimentally, mass spectrometry was applied to studies not only on cationic4−8 and anionic9 clusters, but also on neutral10−12 vanadium oxide clusters. The collisioninduced dissociation studies by Castleman and co-workers on the small vanadium oxide clusters ions of m = 2−7 indicated that VO2, VO3, and V2O5 were the main building blocks for both cationic6 and anionic9 species. To understand the formation process and structures of vanadium oxide clusters, theoretical studies on structures based on V2O5 units were also examined for small cationic,5,6,13 © XXXX American Chemical Society

Received: April 4, 2016 Revised: May 9, 2016

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DOI: 10.1021/acs.jpca.6b03403 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

(injection energy, Einj) of 50−250 eV by a pulsed electric field. The 100 mm long ion drift cell with an applied 10 V/cm electric field was filled with 0.80 Torr of helium buffer gas, and it was further cooled to 190 K with liquid nitrogen. When cluster ions were injected into the cell especially with high Einj, ions had collision-induced dissociations by the collisions with the He atoms at the inlet of the cell as noted below. Finally, product ions from these reactions passed through the cell. In our previous study for sodium fluoride cluster cations, stable magic number cluster cations were clearly observed as collisioninduced dissociation products under high Einj conditions.32 The velocity of the cluster ions injected into the cell was decreased by collisions with He buffer gas atoms at the inlet of the cell. However, because of a balance between deceleration by buffer gas collisions and acceleration by the applied electric field, E, inside the cell, drift velocity of the ions, vd, becomes a constant value that is proportional to E, that is

Figure 1. Structures of neutral, cationic, and anionic even sized clusters V2O5, V4O10, V6O15 and anionic odd sized clusters V3O8− and V5O13− calculated in previous studies.13,14,17 Red spheres are vanadium atoms, and blue spheres are oxygen atoms. The basic framework structures are shown in the figure, and vanadium oxide clusters are formed by placing vanadium atoms on the vertices of these polyhedrons.

vd = KE

(1)

in which the coefficient K is known as ion mobility.23 An equation of the ion mobility K of thermalized ions drifting through the buffer gas with an applied electric field was given from the kinetic theory33 as

In addition, experiments based on reactivity of vanadium oxide cluster ions were also studied as models of catalytic oxidation reaction on the bulk crystals, such as the reaction with SO2,20 H2O,21 and some organic molecules.22−24 Ion mobility mass spectrometry (IM-MS), which is a combination of both ion mobility spectrometry (IMS) and mass spectrometry (MS), is a powerful method to elucidate the geometrical structures of gas-phase cluster ions.25−27 Structural information on cluster ions is obtained as collision cross sections (CCSs), which can be evaluated by measuring the mobility of ions inside a buffer-gas-filled ion drift cell of IMS. Although it is difficult to determine detailed structural parameters of each cluster ion directly from CCSs, IM-MS is easily applicable for larger sized cluster ions by a combination with quantum chemical calculations. In this study, the most abundant compositions of vanadium oxide cluster cations and anions (VmOn±, m ≤ 20) were first determined by mass spectra from IM-MS, in which collisioninduced dissociation of the cluster ions occurred. Specific geometrical structures of these compositions with growing patterns partially discussed by previous studies5,11,12,14,17 were calculated by quantum chemical calculations. Finally, the structures were assigned by comparison of theoretical CCSs derived from calculated structures with experimental CCSs measured by IM-MS.

3e ⎛ 2π ⎞ ⎜ ⎟ 16N ⎝ kBμTeff ⎠

1/2

K=

1 (1,1)

Ω

(2)

where e is the elementary charge, N the number density of the buffer gas, kB the Boltzmann constant, μ the reduced mass of the ion and the buffer gas atom, and Ω(1,1) a collision integral representing an average of momentum transfer cross section over collision energy and orientations. When we treat the ion and the neutral as hard sphere without internal states, the collision integral is reduced to the hard-sphere CCS, Ω. The term Teff, effective temperature of the ions, is given by TBG + mBvd2/3kB, where TBG is the buffer gas temperature and mB is the mass of a buffer gas atom. After exiting the ion drift cell, the ions were reaccelerated by another pulsed electric field in an acceleration region of the TOF mass spectrometer. We denote the time difference, Δt, between ion injection pulse and reacceleration pulse as “arrival time”. An effective time, teff, that ions spent in the cell was derived from subtracting the time that ions flew outside of the cell, tex, from the arrival time, Δt, i.e., teff = Δt − tex. The drift velocity, vd, is easily calculated by dividing the length of the ion drift cell (100 mm) by teff. Therefore, we can estimate the ion mobilities and CCSs of the cluster ions by measuring the arrival time for each cluster ion with eqs 1 and 2.28−32 In the IM-MS measurement, we obtained a series of TOF mass spectra sequentially by scanning the arrival time automatically with LabVIEW program. Then, these data were summarized in a two-dimensional (2D) plot of mass number versus arrival time. In IMS, the ratio of the drift electric field, E, to the number density of buffer gas, N, is an important parameter. Typically, in other IMS experiments, the E/N values were 1.5−10 Td (1 Td = 10−17 V cm2).33,34 It is desirable to keep E/N low for measuring the precise CCSs of ions in the low field limit (E/N → 0 Td) because the mobility of ions in high E/N conditions deviates from the data in the low field. On the other hand, the amount of the cluster ions after the separation decreases at low E/N conditions because of scattering by many collisions with buffer gas. Therefore, we searched for the highest possible E/N conditions for determining the structures of cluster ions. In the

2. EXPERIMENTAL AND COMPUTATIONAL METHODS IM-MS experiments were performed using a home-built vacuum apparatus composed of a cluster ion source, an ion drift cell, and a reflectron time-of-flight (TOF) mass spectrometer. Details of this experimental setup and procedures for IM-MS were reported elsewhere.28−31 Vanadium oxide cluster ions, VmOn±, were generated by a combination of laser vaporization of a vanadium rod and supersonic expansion of a 5% O2/He gas mixture. Stagnation pressure of the O2/He mixture gas was 3 atm. Vaporized vanadium atoms/ions and O2 molecules collide with one another and grow to large vanadium oxide cluster ions in the region immediately after the vaporization laser irradiation point. The generated cluster ions were injected into the ion drift cell with a kinetic energy B


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Figure 2. Mass spectra of vanadium oxide cluster cations and anions with injection energies of (a) cations in 50 eV, (b) anions in 50 eV, (c) cations in 250 eV, and (d) anions in 250 eV condition. Compositions of some corresponding stable species are as labeled.

3. RESULTS AND DISCUSSION 3.1. Compositions and Arrival Time Distributions of Vanadium Oxide Cluster Ions. 3.1.1. Composition Assignment. Mass spectra of vanadium oxide cluster cations and anions (VmOn±), obtained by IM-MS measurement, both in injection energy of 50 and 250 eV conditions, are shown in Figure 2. These mass spectra of the gas-phase cluster ions were obtained by summing all the ion signals exiting the ion drift cell with spatial distributions depending on their mobilities. Panels a and b of Figure 2 show the mass spectra for both vanadium oxide cluster cations and anions, respectively, observed in low injection energy condition (Einj = 50 eV) with the range from 0 to 2000 mass number. From these mass spectra, multiple species of different number of oxygen atom n can be observed for each vanadium atom number m. For example, when m = 5, V5O11−14+ were observed in Figure 2a for cluster cations and V5O12−14− were observed in Figure 2b for cluster anions. On the other hand, Figure 2c,d shows the mass spectra of both cations and anions observed under high Einj condition (250 eV). Compared with the lower injection energy condition, the number of species with different oxygen number n in each m series decreases in Figure 2c,d. For the same example as above, when m = 5, V5O11−13+ and V5O12−13− were observed. Also, the ion signals in Figure 2c,d of cluster sizes m ≤ 5 are relatively higher than cluster sizes m > 5. In this injection energy condition, collision-induced dissociation was more likely to occur at the inlet of the ion drift cell and larger clusters were dissociated into more stable smaller clusters or into stable compositions. Cluster ions that cannot be further dissociated are considered stable and abundantly observed in this condition. The most abundant species found in the 250 eV condition can also be observed in the 50 eV condition; however, some of them were not the major species at the 50 eV condition. For m = 5, V5O12+ was mainly observed at 250 eV, while ion signal of V5O13+ was more intense than V5O12+ at the 50 eV condition. From the mass spectra in Figure 2c,d, the stable compositions can be assigned as the most abundantly observed

present experiments, conditions were optimized with E/N = 24 Td when buffer gas pressure and temperature in the ion drift cell were 0.80 Torr and 190 K, respectively. The structures of vanadium oxide cluster ions, VmOn±, were calculated for 2 ≤ m ≤ 12 by B3LYP/6-311+G(d) level of theory with Gaussian 09.35 Theoretical CCSs of the cluster ions were calculated from these ionic structures by using the projection approximation method,36 which is included in MOBCAL program.37 In this approximation, we assumed that the orientationally averaged projected area is exactly equal to the CCS. The projection approximation method requires the hard sphere atomic radii of each atom in a molecule and does not require the charge distributions of a molecule. The hard sphere radii of vanadium and oxygen atoms in the cluster cations and anions were determined so that theoretical CCSs of V8O20± reproduce experimental CCSs of V8O20+ cation and V8O20− anion, where the experimental CCSs were 122 ± 5 and 123 ± 5 Å2, respectively. The radius ratio of vanadium and oxygen atoms was fixed to account for the radius relation of crystal vanadium oxide (V5+: O2− = 0.495:1.21).38 The resulting hard sphere radii for cations and anions coincided to be V (0.62 Å) and O (1.51 Å). In addition, a radius of 1.15 Å for neutral He atom, the buffer gas atom, was used in the calculation.36 The fitted theoretical CCSs of V8O20± were 122 and 123 Å2, respectively, under projection approximation (PA). The theoretical CCSs of tetrahedral structured V4O10± were therefore estimated to be 78 and 79 Å2. As a reference, the CCSs of V8O20± were both 133 Å2, and those of V4O10± were both 82 Å2 under the exact hard sphere approximation (EHSS), which is another evaluation method of CCSs in MOBCAL. When the hard-sphere contact distance is the same as that of PA, it was reported that the CCSs by EHSS are in general overestimated compared to those by PA.39 Because we found that these differences between PA and EHSS do not affect the results and discussion in the present study, simple PA was adopted for calculating CCSs throughout this study. C


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occur in this low Einj condition, but the possibility of the dissociation should be much less than in the high Einj condition. Additionally, by observing the peak intensity of each vanadium atom number series m from the mass spectra in Figure 2, it was clear that cluster ions having an odd number of vanadium atoms were generally more stable than those with an even number of vanadium atoms. The stability of odd sized clusters, (VO2)(V2O5)(m−1)/2+ and (VO3)(V2O5)(m−1)/2−, could be explained by the electron shell closing of the clusters. The ionized form of these clusters are of closed-shell stoichiometry, in which all of the vanadium atoms take the most stable and common +V oxidation state, and oxygen atoms of −II oxidation state. The stability of some small (m = 1−7) cationic6 and anionic14 compositions were also discussed in past studies. In contrast to the closed-shell odd sized cluster ions, cluster ions with an even number of vanadium atoms such as (V2O4)(V2O5)(m−2)/2± and (V2O5)m/2± contained vanadium atoms of two different oxidation states, +IV and +V. Uneven number of electrons in the even sized cluster ions form open-shell species. The fact that all odd sized ions were closed shell, which is also electronically stable, and even sized ions are open-shell species should be one of the reasons why the intensities of all odd sized ions were almost always higher than those of neighboring even sized ions in the mass spectra. The odd size series stability is clearly visible for the first time in the present mass spectra by the use of IM-MS, in which cluster ions can be successfully separated and had collision-induced dissociation before mass analysis. As noted above, the even sized (V2O5)m/2± clusters were the second or third most prominent for cations and the most prominent for anions in the mass spectra. This composition corresponds to the polyhedron and polygonal prism structures in Figure 1, as calculated in previous studies.14,17 However, oneoxygen deficient compositions, (V2O4)(V2O5)(m−2)/2+, were the predominant composition for even sized cations. This difference of the primary compositions in cations and anions could be explained by the difference on the distributions of a lone pair of electrons. It was reported by Schwarz and co-workers that for the small polyhedral (V2O5)m/2+ cations such as V4O10+, spin density distribution was localized at one of the bonds between a terminal oxygen and a vanadium atom.42 Figure 3a shows the spin density distribution around the terminal oxygen of the tetrahedral structured V4O10+, which was also calculated in this study. Spin density distributions of V6O15+ and V8O20+ are also

compositions. We realized that all vanadium oxide cluster ions have compositions containing the V2O5 unit depending on whether the number of the vanadium atom is even or odd. For even sized cations, (V2O4)(V2O5)(m−2)/2+ was the most prominent composition, whereas (V2O5)m/2+ was the second or third most prominent composition in this condition. For even sized anions, the composition of (V2O5)m/2− was the most abundant and (V2O4)(V2O5)(m−2)/2− was the second most abundant. For example, V10O24+ was the most prominent and V10O25+ was the third most prominent for cations. On the other hand, V10O25− was the most prominent and V10O24− was the second most prominent for anions. With odd sized clusters, compositions of (VO2)(V2O5)(m−1)/2+ for cations and (VO3)(V2O5)(m−1)/2− for anions were the most prominent. As examples of odd sized ions, V5O12+ and V5O13− were predominant for cations and anions, respectively. The composition found here for vanadium oxide cluster cations was consistent with the abundant species found by Kurokawa and Mafuné for cluster sizes m = 2−10.8 Also, in previous photodissociation experiments4,5,14 and a collision-induced dissociation experiment,6 the compositions observed in this study were commonly produced as fragments from various parent ions (m < 10). The commonly produced fragments are VO2+, V2O4+, V3O7+, V4O9+, V5O12+, and V6O15+ or VO3−, V2O5−, V3O8−, V4O10−, V5O13−, and V6O15−, along with other ions with different number of oxygen atoms. The present results of the most abundantly observed compositions were also consistent with the observations made by Castleman et al. The observation was that anions tend to have a one-oxygen richer composition than the cations,6,9 and in addition, we are able to extend this observation to cluster sizes up to m = 20. Using the impulsive collision theory40 under multiple collisions (see Supporting Information), the total averaged internal energy transferred to the cluster ions from 250 eV relative kinetic energy collisions with He buffer gas was estimated. The estimated energy was 33 eV on average for cluster sizes larger than V4O10±. This internal excitation energy can be partitioned to internal degrees of freedom, and finally it can exceed bond dissociation energy of the cluster ion. After the cluster ions lose their kinetic energies by the collisions and reach constant drift velocities, they rarely dissociate by the collisions with light He buffer gas inside the cell. In the mass spectra, signals of the resultant ions were observed after such dissociations. Therefore, they were assigned as stable compositions with respect to collision-induced dissociations for vanadium oxide cluster cations and anions. On the other hand, total averaged energy transferred to the cluster ions from the initial relative kinetic energy at Einj = 50 eV was estimated to be 6.5 eV for ions larger than V4O10± by the impulsive collision theory noted above. Moreover, centerof-mass collision energy of one collision, which is defined by the product of the reduced mass of the collision pairs (a vanadium oxide cluster ion and a He atom) and the relative velocity,41 was estimated to be smaller than 1 eV for each collision under 50 eV injection energy condition with ions larger than V4O10±. By comparison, Castleman and co-workers reported collision-induced dissociation of vanadium oxide cluster ions with xenon gas with the center-of-mass collision energy of 3−5 eV.6 Though energy of a single collision is smaller in the present experiment, dissociation is possible because the clusters undergo multiple events of collisions at the inlet of the ion drift cell. Therefore, dissociation could still

Figure 3. (a) Calculated spin density distribution of V4O10+, V6O15+, and V8O20+. The distribution was localized around one terminal oxygen atom. (b) Calculated spin density distribution of V4O10−, V6O15−, and V8O20−, which was delocalized around vanadium atoms. D


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The Journal of Physical Chemistry A shown in Figure 3a assuming their structure to be triangular prism and square prism, which will be discussed in a later section. The spin density distributions are probably localized on the antibonding orbitals between the terminal oxygen atom and the vanadium atom; therefore, they cause extension of the corresponding bonds. When the spin density localization property is extrapolated, this extended and destabilized V−O bond in (V2O5)m/2+ cations could dissociate easily by collisions with He buffer gas in high Einj conditions at the inlet of the cell in our experiment. Therefore, (V2O4)(V2O5)(m−2)/2+, the oneoxygen deficient species, became the most prominent for cations as a result of the dissociation of terminal oxygen. On the other hand, for the anions, spin density distribution was delocalized around d orbitals of all vanadium atoms in V4O10−, and a vanadium atom in V6O15− or V8O20−, which was discussed by Asmis and co-workers,15 as shown in Figure 3b. Thus, the extra electron in the anion is not localized on the antibonding orbital, which causes extension of the corresponding bond length. As a result, it is suggested that (V2O5)m/2− was less likely to dissociate into (V2O4)(V2O5)(m−2)/2− rather than the cationic species (V2O5)m/2+. 3.1.2. Collision Cross Section Analysis with ATD. The analysis and discussion were focused on the compositions (V2O5)m/2± for cluster ions having an even number of vanadium atoms and (VO2)(V2O5)(m−1)/2+ and (VO3)(V2O5)(m−1)/2− for those having odd number of vanadium atoms, because these were predominant species found in mass spectra measured in the high injection energy condition. However, to estimate accurate experimental CCSs, arrival time distributions (ATDs) of each ion should be measured under the low injection energy condition (Einj = 50 eV). The high Einj condition not only causes collision-induced dissociation resulting in stable species but also causes a longer deceleration time for the cluster ion until the ion reaches a constant drift velocity (vd). As a result, the injected ions tend to have shorter arrival times; thus, the high Einj condition finally results in an underestimation of CCSs. At the low Einj condition, cluster ions with various oxygen atoms were observed rather than the high Einj condition (such as V5O13+ is observed in 50 eV spectrum in Figure 2), yet the desired (in this case, V5O12+) for analysis were still observable. Figure 4 shows typical ATDs obtained under the Einj = 50 eV condition for vanadium oxide cluster cations and anions with the range between m = 2−20. The designated cluster compositions are (V 2 O 5 ) m/2 + (m; even) and (VO 2 )(V2O5)(m−1)/2+ (m; odd) for cations, along with (V2O5)m/2− (m; even) and (VO3)(V2O5)(m−1)/2− (m; odd) for anions. Red solid curves in the ATD plots are Gaussian functions used to fit the experimental plots and to obtain the representative arrival times as the center positions of the distributions. These ATDs were constructed from the 2D plots of arrival time versus mass of these ions shown in Figure S2 by scanning the mass of cluster ions at specific arrival times. The ATDs of vanadium oxide cluster cations shifted to a longer time monotonically with increasing m from 2 to 20, along with a gradual increase of bandwidth of ATDs, as shown in Figure 4. Similarly, the ATDs of the anions shifted to a longer arrival time from m = 2−11. However, different from the cations, a stepwise growth of ATD was observed for the cluster anions with sizes around m = 12−18. For example, arrival time of the even sized (V2O5)m/2− ion at m = 12, V12O30−, almost coincided with that of the neighboring odd sized (VO3)(V2O5)(m−1)/2−, m = 11, V11O28−. Furthermore, the broad

Figure 4. Arrival time distributions of vanadium oxide cluster cations (left panel) and anions (right panel) in the range m = 2−20, obtained with injection energy of 50 eV. The red curves are Gaussian functions that fit the experimental values shown as white circles.

ATDs of odd sized anion m = 13, 15, 17 and 19, can be fitted with two Gaussian functions rather than those of neighboring even sized anions. This result indicates the possibility that a signal originating from multiple structural isomers was included in these ATDs. 3.2. Prediction of the Cluster Structures by Quantum Chemical Calculation. To reveal the structures of the observed cluster ions in IM-MS, optimized structures of the cluster ions, VmOn±, for the compositions noted in section 3.1.2 were calculated by quantum chemical calculations with B3LYP/ 6-311+G(d) level up to m = 12. As for the structures obtained with the present calculation level, the geometries and the bond lengths of the ions agreed well with other calculations reported previously.42,43 For example, bond lengths between the vanadium atom and terminal oxygen in the tetrahedral V4O10± are 1.55 Å for cations and 1.59 Å for anions, whereas 1.72 Å was found for cations42 and 1.81 Å for anions.43 Because the stable composition of vanadium oxide cluster cations and anions had an even and odd size dependency, the structures should also be considered based on such composition dependency. As briefly discussed in the Introduction, neutral, cationic, and anionic (V2O5)x 0/± (x = 2, 3, 4) species were proposed to be tetrahedral and right polygonal prism structures,15 and odd sized (VO3)(V2O5)1−3− anionic species were pyramid and bridged structures.14,15 In addition to these structures discussed in past studies, we will propose in this study some new cluster ion structures based on their parity dependency and also suggest a patterned structural growth with increasing cluster size. First, the even sized (V2O5)m/2±, structures for m = 6−12 as patterned right polygonal prism structures and polyhedron structures for m = 4 and 12 are shown in Figure 5, in which both structures for m = 12 are newly proposed in this study. All of the vanadium atoms configure the framework structures as vertices of the polyhedrons (V4O10± and V12O30±) and the right polygonal prism structures [(V2O5)m/2±, m = 6, 8, 10, 12]. It is found that 60% of the oxygen atoms locate on every edge of the framework structures forming V−O−V bonds, and the remaining 40% oxygen atoms bond to vanadium atoms on the vertices as terminal oxygen. This oxygen percentage applies for all of the right polygonal prism structures and the E


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Figure 5. Optimized structural candidates of the clusters with even number of V atoms (left panel) and those with odd number of V atoms (middle and right panels) calculated with B3LYP/6-311+G(d). Relative energies are indicated for the two isomeric structures.

polyhedron structures in Figure 5. The even sized clusters grow with an increment of V2O5 in the polygonal prism structure with an expansion of the polygon, from triangle, to tetragon (square), to pentagon, and then to hexagon. Next, two structural candidate patterns of the odd sized cluster ions were predicted for both (VO2)(V2O5)(m−1)/2+ and (VO3)(V2O5)(m−1)/2− as shown in Figure 5. These structural candidates of each odd sized composition were derived from the neighboring even sized ions by an addition of VO2 or VO3. For the first structural candidate, an extra VO2 or VO3 was added to one edge of the triangle in the triangular prism of V6O15± to form the bridged structure of V7O17+ or V7O18−. For the second type of structural candidate, pyramid structure of V7O17+ or V7O18−, an extra VO2 or VO3 was added to one face of the triangular prism V6O15±, and the extra oxygen in the anions became terminal oxygen on the top of the five coordinated vanadium atom. Following these patterns, new structures for bridged and pyramid structures were proposed for odd sized ions of m = 9 and 11. 3.3. Structure Assignments by Comparisons between Experimental and Theoretical Results. Experimental and theoretical CCSs of cationic and anionic clusters are shown in Figure 6 and Tables 1 and 2. All experimental data were measured under the low injection energy condition (50 eV). Experimental CCSs of even sized (V2O5)m/2+ and odd sized (VO2)(V2O5)(m−1)/2+ were plotted in Figure 6a for m = 2−20. Similarly, experimental CCSs of even sized (V2O5)m/2− and odd sized (VO3)(V2O5)(m−1)/2− were plotted in Figure 6b. The errors of experimental CCSs were estimated from standard deviations of the data obtained by three (cations) and six (anions) independent measurements. All of the theoretical CCSs of the proposed structures in Figure 5 were also plotted in Figure 6. As noted in section 2, in the calculations of CCSs by the projection approximation method in MOBCAL, the hard sphere radii of V and O atom were determined so that theoretical CCSs of right tetragonal prism (cubic) of V8O20±

Figure 6. Experimental collision cross sections (blue marks, Ωexp.) and theoretical collision cross sections (colored marks, Ωcalc.) for both cluster (a) cations and (b) anions. The experimental collision cross sections of m = 2−20 were measured in 50 eV injection energy condition. The calculated cross sections are indicated for m = 2−12.

can reproduce the experimental CCSs of cation (122 ± 5 Å2) and anion (123 ± 4 Å2). As for cations, the theoretical CCSs of the right polygonal prism structure pattern had good agreement with experimental CCSs of even sized clusters, (V2O5)m/2+, for the sizes m = 6−12 as shown in Figure 6a. Trigonal prism structure of V6O15+ had an experimental CCS of 104 ± 5 Å2 and a theoretical CCS of 102 Å2 right within the standard deviation. Likewise, the prism structures of V10O25+ and V12O30+ had good matches in CCSs with experimental values: 142 Å2 to 142 ± 7 Å2 and 162 Å2 to 161 ± 8 Å2, respectively. The polyhedron structure of V12O30+, which had the same CCS as the right hexagonal prism structure (162 Å2), was more F


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V7O17+ are clearly broader than those of V2O5+, V4O10+, and V6O15+. This feature is most likely due to the coexistence of isomers. However, it is not possible to explain the origin of the broad ATDs with only the two isomers shown in Figure 5 because experimental CCSs are always slightly larger than both of them, as shown in Table 1. Not only these isomers but also other structures which are not discussed here may coexist in V3O7+, V5O12+, and V7O17+. For the anions, the right polygonal prism structure pattern agreed well with experimental values up to m = 10 for even sized vanadium oxide cluster anions, (V2O5)m/2− as shown in Figure 6b. The theoretical CCSs of V6O15− was 102 Å2 in comparison with experimental CCS 102 ± 3 Å2, and similarly, V10O25− was 142 Å2 in comparison to experimental CCS 140 ± 4 Å2. As described in section 3.1.2, V12O30− had a smaller arrival time close to that of V11O28−, 151 ± 5 Å2. Therefore, the structure of V12O30− should be more compact than the right hexagonal prism structure or the polyhedron structure both with theoretical CCSs of 162 Å2, which will be considered in the next paragraph. On the other hand, for the odd sized anionic clusters, bridged structures are slightly more stable in energy of approximately less than 0.2 eV than pyramid structures for m = 5, 9, and 11. However, the pyramid structure of m = 7 is 0.22 eV more stable than the bridged structure. When the experimental and theoretical CCSs are compared, the more compact pyramid structures had better agreement with experimental values. Because the two types of structures had relatively close energy, the pyramid structure pattern can be assigned to the odd sized (VO3)(V2O5)(m−1)/2− (m = 5, 7, 9, and 11) for their better match in experimental CCSs. The experimental CCS of V12O30−, 151 ± 5 Å2, was relatively close to the experimental CCS of the neighboring smaller odd size species V11O28− (150 ± 5 Å2); therefore, they should have structures of similar theoretical CCSs. A structure derived from V11O28− pyramid structure was proposed as shown in Figure 7. An additional VO2 was added to the cavity within the pyramidal

Table 1. Experimental CCSs (Ωexp) and Theoretical CCSs (Ωcalc) Calculated by MOBCAL of Vanadium Oxide Cluster Cations Ωcalc (Å2) cation +

V2O5 V3O7+ V4O10+ V5O12+ V6O15+ V7O17+ V8O20+ V9O22+ V10O25+ V11O27+ V12O30+ V13O32+ V14O35+ V15O37+ V16O40+ V17O42+ V18O45+ V19O47+ V20O50+

Ωexp (Å2) 63 73 81 93 104 113 122 131 141 151 161 166 176 183 193 198 208 213 221

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3 3 5 5 8 5 11 7 8 8 7 7 8 8 9 9 9 9

linear

prism

bridged

pyramid

88

85

110

105

129

127

150

146

polyhedron

54 68 78 102 122 142 162

162

energetically stable and an equally possible structure (not plotted in Figure 6a). For the two structural candidates of odd sized vanadium oxide cluster ions, (VO2)(V2O5)(m−1)/2+, bridged structures were 2−4 Å2 larger in CCSs in comparison with the pyramid structures. The bridged structures were also more stable energetically by approximately 1.6−0.5 eV than the pyramid, as shown in Figure 5. Thus, even though both of the structural candidates had theoretical values well within the standard deviation of the odd sized cluster ions, the bridged structure pattern can be assigned to V5O12+, V7O17+, V9O22+, and V11O27+. In addition, the ATDs of V3O7+, V5O12+, and

Table 2. Experimental CCSs (Ωexp) and Theoretical CCSs (Ωcalc), Calculated by MOBCAL, of Vanadium Oxide Cluster Anionsa Ωcalc (Å2) Ωexp (Å ) 2

anion V2O5− V3O8− V4O10− V5O13− V6O15− V7O18− V8O20− V9O23− V10O25− V11O28− V12O30− V13O33− V14O35− V15O38− V16O40− V17O43− V18O45− V19O48− V20O50− a

61 71 ± 4 79 ± 3 93 ± 3 102 ± 3 113 ± 3 123 ± 4 131 ± 3 140 ± 4 150 ± 5 151 ± 5 149 ± 6 162 ± 5 170 ± 4 175 ± 6 182 ± 6 189 ± 5 195 ± 6 202 ± 8

linear

prism

bridged

pyramid

94

90

117

112

136

132

155

152

polyhedron

56 70 79 102 123 142 162

162

165 ± 6 183 ± 5 196 ± 7 208 ± 6

The experimental CCSs of structural isomers are also included. G


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proposed for odd sized cluster ions. Experimental collision cross sections of the vanadium oxide cluster cations and anions have been measured using ion mobility mass spectrometry. To assign the structures of the cluster ions of the size m = 2−12, comparisons between experimental CCSs and theoretical CCSs from proposed structures were made. As a result, CCSs of even sized cluster cations (m = 6−12) were assigned to be the right polygonal prism pattern, except that m = 12 was also polyhedron; odd sized ones (m = 5−11) were assigned to be bridged structures. The even sized cluster anions (m = 6−10) were assigned to be right polygonal prism structures, and odd sized ones (m = 5−11) were assigned to be pyramid structures. Here, we experimentally confirmed a structural growth pattern for the cluster ions with even number and odd number of vanadium atoms, whether growing by increasing the size of polygonal prism or by expanding the sizes of bridged and pyramid structures. On the other hand, due to a structural transition found for anions at around m = 12, which can be clearly observed in the ATD plot and mass versus arrival time plot, a captured typed structure was proposed and fitted to the experimental CCS. The proposed captured typed structure suggested the cluster anion of m = 12 has a higher density than the cluster cation, and such a structure should be considered for cluster anions of m ≥ 12.

Figure 7. Additional structural candidate of V12O30− derived from the pyramid structure of V11O28− by an addition of VO2 into the cavity inside the pyramid, optimized by B3LYP/6-311+G(d). The corresponding theoretical collision cross sections (Ωcalc.) calculated by MOBCAL are also shown as labeled.

structure of V11O28− to form V12O30−, and it was optimized with B3LYP/6-311+G(d). The structural optimization suggested that this “pyramid capture” structure was relatively more compact (153 Å2) than the right hexagonal prism and the polyhedron structures (162 Å2) and was similar to the pyramid structure of V11O28− (152 Å2). Even though this pyramid capture structure provided a better theoretical-to-experimental value comparison, we cannot ignore the fact that this structure was about 1.28 eV less stable than the hexagonal prism and 1.43 eV less stable than the high-symmetry polyhedron structure. Species like V12O30− were necessary to have structures with higher density; thus, it can possibly include a VO2 inside the structural cavity of the odd sized species to decrease the overall CCS, as described in Figure 7. Meanwhile, by observing the experimental CCS growth of the cluster anions carefully in Figure 6b and Table 2, we can see that the bulkier isomer for sizes m = 13, 15, 17, and 19 may be an extension of odd sized cluster ion growth similar to those of cluster cations. In fact, the larger isomer for sizes m = 13, 15, 17, and 19 had experimental CCSs similar to those of cluster cations with the same number of vanadium atoms. For example, the cationic V13O32+ had an experimental CCS of 166 ± 7 Å2, and the anionic V13O33− was 165 ± 6 Å2; moreover, V15O37+ was 183 ± 8 Å2, whereas V15O38− was 183 ± 5 Å2. This observation suggests that the larger structural isomers of anions for sizes m = 13−19 may have structures similar to those of cations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03403. Estimation of the total internal excitation energy, arrival time versus mass 2D plot of vanadium oxide cluster cations and anions, and the complete author list of ref 35 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 22 795 6577. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS), the Research Seeds Quest Program (JST), and the Murata Science Foundation. Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan.

4. CONCLUSION In this study, we have observed abundant compositions of vanadium oxide cluster cations and anions as a result of collision-induced dissociation products formed at the inlet of the ion drift cell. The compositions had a dependency on the even and odd number of vanadium atoms. The compositions predominantly found for cations were (V2O4)(V2O5)(m−2)/2+ for even size and (VO2)(V2O5)(m−1)/2+ for odd size; similarly, the compositions found for anions were (V2O5)m/2− and (VO3)(V2O5)(m−1)/2−. These representative compositions have been proposed for the first time and were found to be applicable at least to mass number up to 2000 (approximately m = 20). The structures of the even and odd sized cluster ions were also predicted by quantum chemical calculations for the geometries based on previous studies for m = 2−12. The right polygonal prism structures were calculated for even sized cluster cations and anions; pyramid and bridged structures were

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REFERENCES

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