Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions

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Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry Ryoichi Moriyama, Ryuki Sato, Motoyoshi Nakano, Keijiro Ohshimo, and Fuminori Misaizu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b02431 • Publication Date (Web): 13 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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The Journal of Physical Chemistry

Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry

Ryoichi Moriyama,1 Ryuki Sato,1 Motoyoshi Nakano,1 Keijiro Ohshimo,1,2 and 5

Fuminori Misaizu1*

1. Department of Chemistry, Graduate School of Science, Tohoku University, 6–3 Aoba, Aramaki, Aoba-ku, Sendai 980–8578, Japan 2. 10

Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi,

Aoba-ku, Sendai 980-8576, Japan

Received:

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* Corresponding author. Tel. +81 22 795 6577, Fax: +81 22 795 6580 E-mail address: [email protected] (F. Misaizu)

1 ACS Paragon Plus Environment


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Abstract Structural assignments of gas phase chromium oxide cluster anions, CrmOn− (m = 1-7), have been achieved by comparison between experimental collision cross sections measured by ion mobility mass spectrometry and theoretical collision cross sections of optimized structures by 5

quantum chemical calculations. In the mass spectrum, significant magic behavior between the numbers m and n was not observed for CrmOn−, while wide ranges of compositions were observed around CrmO2m+2− to (CrO3)m− as reported previously. The (CrO3)m− (m = 3-7) ions were assigned to have monocyclic-ring structures for m = 3-5 and bicyclic rings for m = 6 and 7. In addition, gradual structural change from these cyclic structures of (CrO3)m− to three

10

dimensional structures of CrmO2m+2− was found for m = 4-7. The energy levels of molecular orbitals of a calculated monocyclic structure of Cr5O15− were also found to be consistent with previous results of photoelectron spectroscopy, although those of the bicyclic isomer exhibited a different behavior. Moreover, the observation of abundant ions generated by collision induced dissociations at the inlet of the ion drift cell indicates that the larger sized (CrO3)m− (m

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> 5) series were unstable and easily dissociated to smaller ions.


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1. Introduction Transition metal oxides have been widely investigated in various research fields such as catalytic chemistry, photochemistry, and material science for the purpose of seeking practical use. In the first row transition metal oxides, their properties sensitively depend on the metal 5

element; for example, titanium oxide as a photocatalyst, vanadium oxide as a strong oxidant, iron oxide and cobalt oxide as coloring materials, and zinc oxide as a semiconductor. Among these metal oxides, chromium oxide also has a unique property depending on its oxidation state. For instance, CrO3 (+VI) is a strong oxidant, whereas CrO2 (+IV) shows a ferromagnetic property. The former chromium oxide is used as a catalyst,1 and the latter is used as a material

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of magnetic tape.2 Chromium oxide clusters have been expected either to be partial structures of the above bulk material, or to exhibit novel properties which are characteristic to the cluster structures. Both of experimental and theoretical studies of chromium oxide clusters were actively reported so far.3-24 Aubriet and Muller measured a mass spectrum of chromium oxide cluster cations,

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CrmOn+, up to m = 6 and reported that Cr2O4+, Cr3O6+, and Cr4O10+ were mainly observed in the mass spectrum.12 Castleman and co-workers found two different compositions of chromium oxide cluster anions of oxygen-rich (CrO3)m− and oxygen-deficient, CrmO2m+2− with controlling cluster generating condition.13 They also optimized structures of (CrO3)m0/− and CrmO2m+20/− with the range m = 3-6 by quantum chemical calculations. Duncan and co-workers carried out

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photodissociation of CrmOn+ (m = 2-13).14 They found stable compositions of m, n = (1,1), (2,4), (3,6), (3,7), (4,9) and (4,10) as fragment cations. In theoretical studies,17,18 the difference of vertical detachment energies (VDEs) of structural isomers of small chromium oxide cluster



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anions were discussed. About this issue, Wang and co-workers reported experimental VDEs of several compositions of chromium oxide cluster anions.19-23 For example, they measured photoelectron spectra (PES) of Cr1-2O1-7− and compared these spectra with theoretical PES simulated by quantum chemical calculations.19,20 Also they compared experimental PES of 5

Cr4O10− with theoretical one of tetrahedral Cr4O10−.22 Moreover, they determined the structures of (CrO3)m−, m = 1-5, as monocyclic structures from the PES measurements.23 These cluster studies were applied only on the limited cluster sizes so far because of the difficulty of applying spectroscopic measurements after size selection. Therefore, relationship between compositions and structures of chromium oxide clusters has not been revealed

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comprehensively. Now, ion mobility spectrometry is one of the powerful methods to investigate the structures of gas phase clusters. Collision cross sections (CCSs) of cluster ions are measured by this technique, from which structural information can be deduced. This method is relatively easily applied to wide range of cluster sizes. Although detailed structural parameters cannot be measured even with this method, structural assignment can be achieved by the combination with

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quantum chemical calculations. In this study, structures of chromium oxide cluster anions were investigated by ion mobility mass spectrometry (IM-MS). By comparing experimentally measured CCSs and theoretically calculated CCSs of structures optimized by quantum chemical calculations, geometrical structures were assigned. Additionally, we examined the effect of collision induced

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dissociations at the inlet of the ion mobility cell (ion drift cell) on the mass spectrum on , so as to reveal stable compositions with the respect to collisions with He buffer gas.


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2. Experimental and Computational Methods The IM-MS experiments were performed with a home-built vacuum apparatus which was reported in detail elsewhere.25-32 This apparatus consists of a cluster ion source, an ion drift cell and a reflectron time-of-flight (TOF) mass spectrometer. Chromium oxide cluster anions, 5

CrmOn−, were produced by combination of laser vaporization on a chromium rod and subsequent reaction with O2 molecules which were introduced by supersonic expansion of 5 % O2/He gas mixture. Stagnation pressure of the O2/He mixture gas was around 3 atm. The generated cluster ions were injected into the ion drift cell with a kinetic energy (injection energy, Einj) of 50-450 eV by a pulsed electric field. The ion drift cell was 100 mm long, and it was filled with 0.80

10

Torr of He buffer gas. The cell was further cooled to 180 K with liquid nitrogen, and 10 V/cm of electrostatic field was applied in the cell. The velocity of the cluster ions injected into the cell was firstly decreased by collisions with He buffer gas atoms at the inlet of the cell. Then, because of a balance between deceleration by buffer gas collisions and acceleration by the applied electric field, E, inside the cell, drift

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velocity of the ions, vd, becomes a constant value that is proportional to E, that is, vd = KE,

(1)

in which the coefficient K is known as ion mobility.33 An equation of the ion mobility K was given from the kinetic theory as K=

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3 e  2π  16 N  k B µ Teff

12

  

1 Ω

(1,1)

,

(2)

where e is the elementary charge, N is the number density of the buffer gas, kB is the Boltzmann constant, µ is the reduced mass of the ion and the buffer gas atom, and Ω(1,1) is a collision



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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 5

mass of the buffer gas atom. After exiting the ion drift cell, the ions were reaccelerated by another pulsed electric field in source of the TOF mass spectrometer. The accelerated ions were detected after mass analyzed in a reflectron TOF mass spectrometer. We denote the time difference between the injection pulse into the ion drift cell and the reacceleration pulse as “arrival time”. In the IM-MS

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measurement, we obtained a series of TOF mass spectra sequentially by scanning the arrival time which was operated by LabVIEW software. The obtained TOF data were shown in a two-dimensional (2D) plot of mass number vs. arrival time. An arrival time distribution (ATD), in which the ion intensity of a certain TOF peak was shown as a function of the arrival time, was also obtained for each cluster ion from the 2D plot. The drift velocity of a given isomer ion

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was calculated from the ATD peak. The ATD peak was determined by the fitting with a Gaussian function. Details of the fitting procedures were shown in Supporting Information. Finally, CCS of each ion was deduced from the ion mobility K using equations (1) and (2). A mass spectrum were also obtained by summing up all the mass spectra configuring the 2D plot, since ions exited the ion drift cell have spatial distribution depending on their mobilities.

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The structures of chromium oxide cluster anions, CrmOn− (m = 1-7), were optimized by B3LYP/6-31+G(d) level with Gaussian 09.34 The bond length of CrO− (X 6Σ+) calculated in this level was 1.68 Å, which agreed with the value of 1.63 Å calculated in the previous report with


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~

BPW91/6-311+G* level.19 Moreover, the bond length of Cr=O in CrO3− ( X 2A’1) was calculated to be 1.63 Å in this study, which almost coincides with the values of 1.64 Å with BPW91/6-311+G* level19 and 1.63 Å in average with four types of calculation level.18 Also in a larger sized cluster, the bond length between Cr and a terminal oxygen was obtained to be 1.57 5

Å in this study for a Cr4O10− isomer, in which the Cr-atom framework has a tetrahedral geometry. This value showed good agreement with the reported value of 1.58 Å with B3LYP functional using aug-cc-pVTZ basis set for oxygen atom and aug-cc-pVTZ-pp set based on relativistic effective core potential for the Cr atom.22 Therefore, difference on structural parameters derived by calculation level should not cause much influence on the CCS calculation

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here. Theoretical CCSs of the cluster ions were calculated from the optimized ion structures by using the projection approximation method35 within MOBCAL program.36 In this approximation method, the orientation-averaged projected area of each structure was assumed as the CCS of the cluster ion. Bowers and coworkers have confirmed that the averaged CCS was converged to

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better than 1% by using several hundred random orientation.35 Therefore, the error in theoretical CCSs did not cause a problem in the present discussion. This method requires the hard sphere atomic radius of each atom in the ion. The hard sphere atomic radii of chromium and oxygen atoms in the cluster anions were determined so that theoretical CCS of tetrahedral Cr4O10− coincides with experimental CCS of Cr4O10− (81 ± 2 Å2). This estimation of atomic radii was

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rationalized because the tetrahedral structure of Cr4O10− was assigned as the most probable structure in the several previous reports.13,14,22 The radius ratio of chromium and oxygen atoms was fixed to account for the radius relation of crystal chromium oxide (Cr6+:O2− = 0.40:1.21).37



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As a result, optimized hard sphere atomic radii were 0.53 Å for chromium and 1.61 Å for oxygen. In addition, a radius of 1.15 Å for neutral helium atom was used in the calculation.35

3. Results and Discussion 5

3.1. 2D plot of mass number vs. arrival time In Figure 1, typical 2D plot of chromium oxide cluster anions, CrmOn− (m = 1-7), is shown, in which horizontal axis is mass number and vertical axis is arrival time corresponding to CCS of each cluster ion. In this measurement, cluster ions were injected with the lowest energy of 50 eV at the inlet of the drift cell in order to minimize the effect of this injection energy on CCS

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estimation. We confirmed that the CCS determined for fullerene C60+ ion at this injection energy reproduced the value reported by Jarrold and his coworkers.38,

39

In order to deduce the

experimental CCSs of the cluster ions, ATDs were plotted from this 2D plot. The peak top values of the arrival time were determined by fitting the ATD with a Gaussian function (see supporting information, Figures S1-S7). 15

As shown in Figure 1, observed cluster ions had wide range of number of oxygen atoms for each Crm group. The number of oxygen atom increased with increasing m; for example, there were two species of Cr1O3− and Cr1O4− for Cr1 series, and six species from Cr7O16− to Cr7O21 − for Cr7 series. In general, the wide range of compositions from CrmO2m+2− to (CrO3)m− were mainly observed in the mass spectra, as already reported by Castleman and his coworkers.13 The

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(CrO3)m− series, which was discussed in several previous studies,13,14,23 was one of the predominant ions up to m = 4, whereas it began to be weak for m ≥ 5. However, there were no significant magic numbers of compositions in Figure 1. This feature was quite different from the


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features observed in other metal oxide cluster ions, for not only early transition metal group, Ti30 and V31,32, but also for late transition metals, Fe27, Co26, Ni29 and Zn25. The cluster compositions (TiO2)m

+/−

, (V2O5)m/2+/− and (MO)m+ (M = Fe, Co, Ni, Zn) had considerable

intensities in the mass spectra of those metal oxide cluster ions. 5

3.2. Structural assignments of CrmOn− Calculated structures of chromium oxide cluster anions with the compositions observed in Figure 1 were shown in Figures 2-4. Experimental CCSs of each composition, theoretical CCSs of each structure, and relative energies of isomers were also shown in the same figures. The 10

experimental error for each CCS was obtained from the standard deviation of the results of seven independent measurements. The optimized structures were shown with the order of their energies from the left side of Figures 2-4. All of the calculated isomers not presented in these figures were shown in supporting information (Figures S8-S10). Spin multiplicity of (CrO3)m− were assumed to be doublet which was the same as the previous reports,13,23 and it was also

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assumed to increase by 2 with decreasing one oxygen atom from (CrO3)m−. Spin multiplicities of oxygen richer compositions than (CrO3)m− were assumed to be doublet. In Figure 5, experimental CCSs with errors and assigned theoretical CCSs of each composition were plotted. Fitting parameters (atomic radii) for estimating of theoretical CCSs were scaled with tetrahedral Cr4O10− as mentioned in Section 2. Although the secondly stable structure in Cr4O10−

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was found at only 0.1 eV difference with close CCS to tetrahedron as shown in Figure 3, we concluded that tetrahedral structure was always more stable than the another one with several calculation level (Table S1). Therefore, we discuss the theoretical CCSs using parameters fitted



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with tetrahedral structure of Cr4O10− hereafter. The theoretical CCSs thus determined for the low-lying structures were found to be in good agreement with those determined experimentally as shown in Figure 5. In particular, the ion series with the different number of oxygen atoms n for each Crm series (m = 1-7) had almost the 5

same CCS changes in both theoretical and experimental ones. The detailed structures of these clusters were discussed in the next sections.

3.2.1 Small sized anions Cr1-3On− 10

For the smallest m sized clusters of CrmOn−, CrO3− and CrO4− were strongly observed in the IM-MS measurement. For both clusters, theoretical CCSs of well-known structures, planar structure (C3v) of CrO3−,15,18,19,23 and methane-like three-dimensional structure (C2v) of CrO4−,19 were calculated as shown in Figure 2. These theoretical CCSs almost coincided with experimental values of 43 Å2 for CrO3− and 48 Å2 for CrO4−.

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For the Cr2 series, calculated structures of Cr2O5−, Cr2O6− and Cr2O7−, which were predominant compositions in the mass spectra, were listed in Figure 2. They did not have so much number of structural candidates, and the CCSs of the most stable structures had always close values with experimental CCSs within errors. The most stable structure of Cr2O6− had a D2h symmetry which was the same with previous studies.17,18,23 Assigned structures of Cr2O5−

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and Cr2O7− were derived structures from Cr2O6−, and the structure of Cr2O5− corresponded with the previous one.17 For m ≥ 3, much more structural candidates were optimized than those for m = 1 and 2. For


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the Cr3 series, the obtained isomers can be in general labeled by two types. One is triangle monocyclic shape of three chromium atoms, and another is linear type of three chromium atoms. Monocyclic structures were always more stable than linear structures, and these most stable monocyclic structures were assigned for every m = 3 series with comparisons of the CCSs. A 5

monocyclic structure of Cr3O9− corresponded to that reported in previous studies,12,23,24 and also the structure of Cr3O8− corresponded to the low lying structure in a previous paper.21

3.2.2 Large sized anions Cr4-7On− Structural assignments of the Cr4 series are discussed here firstly. Since the parameters for 10

theoretical CCS calculation were fitted with the tetrahedral structure of Cr4O10− as mentioned above, the CCS of this anion structure corresponded to the experimental CCS. This tetrahedral structure was the same type of structure reported in vanadium oxide clusters.31,32,40 It consisted of framework structure of tetrahedral Cr4, bridging 6 oxygen atoms between each Cr atom, and terminal 4 oxygen atoms binding to each top of tetrahedral framework of Cr atom. Then, the

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most stable structure of Cr4O11− (left side one in Figure 3) was derived from this tetrahedral Cr4O10−, and the CCS of this structure, 87 Å2, was the closest to that of the experimental CCS, 88 Å2, among the optimized structures. This Cr4O11− structure was constructed by the following steps; one of the edges of the bridging Cr-O-Cr bond in the tetrahedral Cr4O10− were firstly broken into Cr-O and Cr, and then two Cr-O terminals were formed by adding a new oxygen

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atom to the Cr atom having a dangling bond. The structure of Cr4O11− thus produced can be regarded as bicyclic structure. This structural change procedure is illustrated in the top of Figure 6, and the same structural changing scheme was adopted in many cases of structural



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assignments for Cr4-7 series in this study, as shown in the figure. The monocyclic structure of Cr4O12− can also be constructed by following this scheme from the bicyclic structure of Cr4O11−: An additional oxygen atom was inserted between central Cr-O-Cr bonding, and consequently the bicyclic ring opened to a monocyclic ring structure (Figure 6). This monocyclic structure of 5

Cr4O12− was calculated to be the most stable and the closest CCS structure in this size. Moreover, addition of one more oxygen atom by opening this monocycle with following the scheme resulted in a linear structure of Cr4O13− as shown in the right side of the Cr4O13− structures in Figure 3. However, the CCS of this linear structure, 106 Å2, was quite larger than the experimental value, 98 Å2. Instead of this structure, the CCS of the monocyclic structure with

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one terminal O2 of Cr4O13−, 95 Å2, agreed better with the experimental CCS. For the Cr5 series, cluster anions with the different number of oxygen atoms from Cr5O11− to Cr5O15− were observed in this IM-MS measurement. Here the discussion starts from the largest sized Cr5O15− anion in this series, since the structure of this anion was already discussed in a previous report.23 In the present study, the most stable Cr5O15− was concluded to have a

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monocyclic structure (left side structure in Figure 3) which was the same with that determined in the previous paper.23 This structure was also assigned to be the most probable one in the present study, because it had a CCS, 109 Å2, which was close to the CCS obtained in the IM-MS experiment, 108 Å2. However, the secondary stable structure (bicyclic structure) of Cr5O15− could not be excluded, since it lay at only 0.01 eV higher in energy than the most stable

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one, and it had very close CCS with the most stable structure. In the same way as the Cr4 series, the structural change scheme can be applied to the oxygen atom decrease from Cr5O15− to Cr5O14−. The most stable and probable bicyclic structure of Cr5O14− was constructed by


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removing one of the terminal oxygen atom from the monocyclic structure of Cr5O15− and by reconnecting between the Cr atom having a dangling bond and a terminal oxygen at the counter side of the ring (Figure 6). Furthermore, the most stable and probable bridged structure of Cr5O13− was obtained by removing one terminal oxygen from a triangle part of the bicyclic 5

structure of Cr5O14− and by reconnecting the Cr atom with a dangling bond to two terminal oxygen atoms at the counter side square (Figure 6). The similar bridged structure was also found in V5O13− cluster.31,40 However, the most stable and probable structure of Cr5O12− did not follow the above scheme of structural change. The assigned structure for Cr5O12− had a three-coordinated oxygen atom, which was not seen in the other chromium oxide cluster

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compositions explained so far. Moreover, by removing one bridging oxygen atom (it was not a terminal oxygen atom), the most stable and probable structure of Cr5O11− as shown in the left structure in Figure 3 was found. This structure also had three-coordinated oxygen atoms. In the Cr6 series, structures of Cr6O14− to Cr6O18− were assigned from the comparison of CCSs between experiment and theory in this study. The Cr6On− ions had broader ATDs for even

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n clusters than those for odd n as shown in Figure S6. Therefore multiple isomers may exist in Cr6On− with even n. However, it was impossible to resolve isomers due to the weak ion intensity and data scattering. For Cr6O14−, two structures were found as probable structures. One was a one-terminal-oxygen-lacked triangular prism structure which had 100 Å2 of theoretical CCS, and another structure had 96 Å2 of theoretical CCS. Both of the theoretical CCSs were within an

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error of experimental CCS of 99±3 Å2, and the latter structure was a little bit more stable than the former by 0.03 eV. The coexistence of two isomers corresponds to the broader feature of the ATD of Cr6O14- shown in Fig. S6 than that of Cr6O15-. However, the secondary stable structure



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can be transformed to an assigned Cr6O15− structure by following the structural change scheme, and

this

scheme

continues

up

to

Cr6O17−

(see

Figure

6).

Therefore,

a

one-terminal-oxygen-lacked triangular prism structure of Cr6O14− (middle structure in Figure 4) can be sequentially changed to a triangular prism structure of Cr6O15− (also left side one), an 5

opened triangular prism (tricycle in other words) of Cr6O16− (left side), and finally a bicyclic structure of Cr6O17− (left side one). By adding one oxygen atom at a Cr atom in a Cr-O-Cr diagonal line in the bicyclic ring of Cr6O17−, and by breaking that bond to two Cr-O terminals, we could further construct a monocyclic structure of Cr6O18−. This monocyclic structure of Cr6O18− was secondary stable, and it had a CCS of 125 Å2, which was considerably larger than

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that obtained from the IM-MS experiment, 120 Å2. The most stable and thus assigned structure for Cr6O18− was a bicyclic structure with a five-coordinated Cr atom at the link of the two rings. This result was different from other smaller (CrO3)m− series like Cr3O9−, Cr4O12− and Cr5O15−, in which monocyclic structures were the most stable. The most stable structures of Cr7O16− and Cr7O17− were also assigned to be the observed

15

structures in the present experiment from the comparison of CCSs. Both of the structures had a three-coordinated oxygen atom like the most stable structure of Cr5O11− and Cr5O12−. For Cr7O18−, the ATD was fitted with two Gaussian functions because the width of the ATD was broader than those of others with moderate ion intensity. The bridged structure of Cr7O18− was the most stable, and it was assigned as an observed structure in this IM-MS study. However,

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observed ATD could be fitted with two Gaussian functions. The calculated CCS (117 Å2) was the intermediate value of two observed CCSs (112±4 and 122±4 Å2). The reason for this discrepancy between calculated and experimental CCSs was unclear at present. The most stable


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Cr7O19− and Cr7O20− structures, which had consistent CCSs with experimental CCSs, were obtained by sequential addition of oxygen atoms to Cr7O18− following with the structural change scheme noted above. However, the assigned structure of Cr7O21− from the present IM-MS experiment was a bicyclic structure, which was similar to the most stable structure of Cr6O18− 5

noted in the last paragraph. For the assigned structures of all Crm series (m = 1-7), almost all of the Cr atom tend to have four-coordination numbers with O atoms. This should be the reason why the structures changed from three-dimensional structures to cyclic structures with increasing number of oxygen atom. These four-coordination numbers of metal atoms were also found in vanadium oxide cluster

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ions31,32,40 and part of titanium oxide cluster ions.30 However, bicyclic structures of Cr5O15−, Cr6O18− and Cr7O21− had five-coordinated Cr atom which was different character with the other chromium oxide cluster anions. We will discuss the difference of monocyclic and bicyclic structures of Cr5O15−, Cr6O18− and Cr7O21− with respect to their molecular orbitals in the next section.

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3.2.3 Electronic structures and stability of monocyclic and bicyclic structures As noted in the preceding section, the Cr5O15− ion has two structural candidates of monocyclic and bicyclic rings. Both of these structures have comparable energies and CCSs, and thus it was difficult to assign the probable structure from the two isomers. Here we discuss 20

these structures from the consideration of electronic structures which were observed in the PES study by Wang and his co-workers. They obtained PESs of (CrO3)m− series such as Cr3O9−, Cr4O12− and Cr5O15−,23 and they concluded that all of these ions have monocyclic structures



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by comparisons between the experimental and theoretical PESs. In this section, we compared molecular orbital (MO) energies of the monocyclic and bicyclic structures. These energies were obtained by shifting all orbital energies so as to the energy of singly occupied molecular orbital (SOMO) corresponds to energy difference between optimized anion and non-optimized 5

neutral which took same structure with the corresponding anion.41 First, we verified the MOs of monocyclic and bicyclic structures of Cr5O15− (in Figure 7 and Table 1), which had close stabilities and CCSs. As a result, there were large energy gap between X band and A band in the monocyclic structure, which was similar with the band gap observed in the previous report.23 On the other hand, large band gap was not obtained in the bicyclic

10

structure as shown in Figure 7. Therefore, we can conclude that the monocyclic structure was more probable for Cr5O15− than the bicyclic one. However, we cannot exclude the possibility of superposition of these two spectra in their experiment, indicating coexistence of the two isomers. We also verified the MOs of monocyclic and bicyclic structures of Cr6O18− and Cr7O21−

15

(Figure S11). Similarly to Cr5O15−, large band gaps were obtained for both monocyclic structures of Cr6O18− and Cr7O21−, and small gaps were observed in bicyclic structures. Unfortunately, we cannot assign the structures from the MOs since experimental PES of Cr6O18− and Cr7O21− were not reported. However, according to our CCS comparisons, bicyclic structures were more assignable for Cr6O18− and Cr7O21−.

20

Finally, it should be noted that the intensities of Cr6O18− and Cr7O21− were very weak in the 2D plot. Moreover, larger sized (CrO3)m− series were not observed in m = 8 and 9. One of the reason of this weak intensity of (CrO3)m− series may be the instability of the cyclic structures


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with respect to collisions with He buffer gas which occur at the inlet of the ion drift cell. Because of the large distortion in the monocyclic structures for m = 6 and 7, they may easily isomerize to bicyclic structures. The bicyclic structures may also easily dissociate to smaller monocyclic structures by collisions. 5

3.3. Abundant ions generated by collision induced dissociation The high energy collision experiment at the inlet of the cell was applied in order to discuss the relative stability of each composition of CrmOn− cluster anions. The mass spectra obtained 10

with two different injection energies Einj = 50 eV and 450 eV were shown in Figure 8. These mass spectra were made by summing up all the mass spectra configuring the 2D plot at each injection energy. Under the Einj condition of 50 eV, the CrmOn− cluster ions with wide range of the number of oxygen atoms around n = 3m were observed for each Crm series. Under the condition of 450 eV,

15

the cluster anions collided with He gas at the inlet of the cell much harder than at Einj = 50 eV. As a result, CrmO2m+2− series began to be strongly observed through dissociation from the larger sized ions such as (CrO3)m− series. Mass spectra with the injection energies between 50 eV and 450 eV were shown in Figure S12. It was clearly seen that mass distribution shifted from the composition of (CrO3)m− series to around CrmO2m+2− series with increasing Einj. However, both of

20

the exact (CrO3)m− series at low Einj condition and CrmO2m+2− series at high Einj were not observed as specially abundant compositions. Their neighboring compositions, which had one more oxygen atom or one less oxygen atom, had similar intensities with those series. As



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mentioned in section 3.1, metal oxide clusters with other transition metal elements often exhibited specific compositions.25-27,29-32 Therefore, the observed trend of mass distribution of chromium oxide cluster ions was much different from those of other first raw transition metal oxide clusters. 5

From the above observation it was concluded that the (CrO3)m− series can easily be dissociated by high energy collisions with He gas. This phenomenon was also observed even at low Einj condition as explained in section 3.1. Under Einj = 450 eV condition, the (CrO3)m− ions were not observed any more at m > 4 sizes. This behavior of (CrO3)m− series can partly be explained from the comparison of stable oxidation state of chromium oxide in the bulk: While

10

the oxidation state of (CrO3)m− series were almost +VI for all of the Cr atoms in the cluster, the most stable oxidation state of chromium oxide in the bulk was +III. Thus, it can be concluded that the instability of large sized (CrO3)m− series in the gas phase cluster started to represent the instability of CrO3 in the bulk. However, the compositions which correspond to +III oxidation state, Cr2O3 series, were not observed in this study. The oxidation state of chromium atoms in

15

the CrmO2m+2− series vary with the cluster size m; around +IV to +VI in our observed size region (m = 1-7). The Cr2O3 composition was seen not only in the bulk but also as nano-particles of the chromium oxide with 10-100 nm diameter scale.42,43 Therefore it can be deduced that the size region where the (Cr2O3)m/2− series become predominant may exist at much larger size region than the sizes examined in this study.

20

4. Conclusion Geometrical structures of gas phase chromium oxide cluster anions, CrmOn− (m = 1-7), were


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investigated by the ion mobility mass spectrometry. Specific compositions were not predominantly observed in the mass spectra, while wide range of compositions from CrmO2m+2− to (CrO3)m− were observed. Almost all of the most stable structures of CrmOn− were assigned to be the structures observed in this study by comparisons between experimental collision cross 5

sections measured in the ion mobility mass spectrometry and theoretical cross sections for optimized structures by quantum chemical calculations. In these structures, oxygen atoms tended to bind three dimensionally with chromium atoms even for the smallest sized Cr1 series. Structures of (CrO3)m− were assigned as monocyclic or bicyclic structures for m = 3-7, and structures of CrmO2m+2− were assigned to be three-dimensional framework structures for m = 4-7.

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The molecular orbitals of monocyclic and bicyclic structures of Cr5O15− were calculated and compared with the result of previous photoelectron spectroscopy. We concluded that the monocyclic structure was more probable for Cr5O15− and bicyclic for Cr6O18− and Cr7O21− than the other structure from the consideration of collision cross sections, relative energies and calculated molecular orbitals. The intensities of the larger sized (CrO3)m− cluster anions in the

15

mass spectra was found to decrease with increasing collision energy. This observation indicated that the larger sized (CrO3)m− were easily dissociated by the collision induced dissociations occurred at the inlet of the ion drift cell. It was concluded that instability of +VI oxidation state of chromium atoms in (CrO3)m− clusters had a correlation with the instability of +VI oxidation state of bulk chromium oxide.

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Associated content Supporting Information



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Arrival time distributions and optimized structures of chromium oxide cluster anions. Calculated relative energies of monocyclic and bicyclical structures of Cr4O10−. Energy levels of molecular orbitals of monocyclic and bicyclic structures of Cr5O15−, Cr6O18− and Cr7O21−. Mass spectra of CrmOn− obtained with different injection energies from 50 eV to 450 eV. 5

Acknowledgements This work was supported by the Research Seeds Quest Program (JST) and The Murata Science Foundation. Theoretical calculations were partly performed with the help of the Research Center for Computational Science, Okazaki, Japan. 10

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Fig. 1 2D plot of arrival time vs. mass number of chromium oxide cluster anions, CrmOn− (around m = 1-7), taken under 50-eV injection energy condition. The He buffer-gas pressure, applied electric field, and cell temperature were 0.80 Torr, 10 V/cm and 180 K, respectively.


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Fig.2 Optimized structures of chromium oxide cluster anions, CrmOn− (m = 1-3). Spin multiplicity, experimental CCSs (Ωexp), calculated CCSs (Ωcalc) of each structure and relative energy are also shown. The most stable structures are shown at the left side. Other low-lying structures are shown in the middle and the right side. The structural and energy calculations were done with B3LYP/6-31+G(d) level.



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Fig.3 Optimized structures of chromium oxide cluster anions, CrmOn− (m = 4 and 5). Spin multiplicity, experimental CCSs, calculated CCSs of each structure and relative energy are also shown. The most stable structures are shown at the left side. Other low-lying structures are shown in the middle and the right side. The structural and energy calculations were done with B3LYP/6-31+G(d) level.


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Fig.4 Optimized structures of chromium oxide cluster anions, CrmOn− (m = 6 and 7). Spin multiplicity, experimental CCSs, calculated CCSs of each structure and relative energy are also shown. The most stable structures are shown at the left side. Other low-lying structures are shown in the middle and the right side. The structural and energy calculations were done with B3LYP/6-31+G(d) level.



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Fig.5 Plot of experimental CCSs (blue filled circle) and theoretical CCSs (red triangle) of CrmOn− (m = 1-7). Theoretical CCSs of the most stable structures shown in Figures 2-4 are plotted except Cr6O14−, for which the CCS of secondary stable structure is plotted. The experimental CCSs were measured in 50-eV injection energy conditions. Buffer-gas pressure, applied electric field and cell temperature were 0.80 Torr, 10 V/cm and 180 K, respectively.

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Fig.6 Schematic illustration of structural changing scheme of CrmOn− (m = 4-7).



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Fig.7 Energy diagrams of molecular orbitals for monocyclic and bicyclic structures of Cr5O15−. The calculations were done with B3LYP/6-31+G(d) level. 5


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Fig.8 Mass spectra of chromium oxide cluster anions, CrmOn−, with injection energies of (a) 50 eV and (b) 450 eV. 5



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Table 1 Calculated energies of SOMO (ESOMO) and energy gaps between SOMO and SOMO−1 (EGAP) of monocyclic and bicyclic structures of Cr5O15−. All energies are shown in eV. Vertical detachment energy (VDE) and energy gap between X band and A band, which were determined by a 5

previous PES experiment,23 are also shown. The calculations were performed with B3LYP/6-31+G(d) level.

Present calculation Cr5O15−

monocyclic bicyclic

ESOMO 5.80 7.06

EGAP 1.43 0.31

Previous PES experiment23 VDE EGAP 5.17(7)

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Geometrical Structures of Gas Phase Chromium Oxide Cluster Anions

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