Isomer Separation of Iron Oxide Cluster Cations by Ion Mobility

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Isomer Separation of Iron Oxide Cluster Cations by Ion Mobility Mass Spectrometry Keijiro Ohshimo, Tatsuya Komukai, Ryoichi Moriyama, and Fuminori Misaizu* Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan S Supporting Information *

ABSTRACT: Geometrical structures of iron oxide cluster cations have been analyzed by ion mobility mass spectrometry. The series of (FeO)n+ and FenOn + 1+ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into a drift cell. Arrival time distributions in the ion mobility spectrometry indicate that two structural isomers coexist for the (FeO)n+ clusters at n ≥ 5. By comparison of experimental collision cross sections determined from the arrival times with theoretical ones, two-dimensional ring and sheet structures were assignable for (FeO)n+ (n = 3−8). In addition to these isomers, compact three-dimensional structures were also found to be stable at (FeO)n+ (n ≥ 6). Thus, the two-dimensional and three-dimensional structural isomers coexist for (FeO)n+ (n = 6−8).

(FeO)n+ have not been studied experimentally probably due to experimental difficulties. Ion mobility mass spectrometry (IM-MS), which is a combination of ion mobility spectrometry (IMS) and mass spectrometry (MS), is a powerful method to elucidate the geometrical structures of gas-phase ions. In the IMS experiment, a swarm of ions is injected into a gas cell (ion-drift cell) in which an electric field is applied. Due to a balance of acceleration of ions by the electric field, E, and deceleration by collisions with buffer gas atoms in the cell, a drift velocity of the ions, vd, becomes a constant value proportional to E, which is represented by the following

1. INTRODUCTION Iron oxide nanoparticles have been intensively investigated not only for their fundamental scientific interests but also for their potential use in many technological applications such as catalysis,1 magnetic storage media,2 and biological applications.3 Recently, geometrical structures of iron oxide clusters with less than 100 atoms have been studied by gas-phase spectrometric experiments and quantum-chemical calculations. The study of gas-phase metal oxide clusters has been stimulated by the increasing need to obtain atomic level models of the formation and evolution of nanoparticulate oxides.4 Iron oxides in bulk phases are expressed by chemical formulas FeO, Fe2O3, and Fe3O4. In a photoionization mass spectrometric study of iron oxide clusters, strong ion intensities were found for the oxygen-equivalent clusters [(FeO)n+] and oxygen-rich clusters (FenOn + 1+, FenOn + 2+, and so on).5,6 Multiphoton dissociation experiments of iron oxide cluster cations showed that stable (FeO)n+ cluster cations were produced from several parent ions.7 The (Fe2O3)n+ and (Fe2O3)nFeO+ cluster ions were also observed in the mass spectra of iron oxide cluster cations generated in several clusterformation conditions.8 Quantum chemical calculations of neutral (FeO)n clusters showed that small (FeO)n (n = 2−5) clusters form stable monocyclic ring structures. For (FeO)6 and larger clusters, these ring elements are assembled into nano columnar structures to form (FeO)n towers.9 These theoretical results for (FeO)n (n = 2−5) were quite different from the bulk FeO structure which has the NaCl rock-salt type geometry. In order to determine the geometrical structures of iron oxide clusters experimentally, infrared multiphoton dissociation spectra were measured for oxygen-rich clusters.10 However, structures of oxygen-equivalent iron oxide cluster cations © 2014 American Chemical Society

vd = KE

(1) 25

in which the coefficient K is called to be an ion mobility. An equation of the ion mobility K of thermalized atomic ions drifting through the buffer gas atoms in the electric field was given from the kinetic theory as 1/2 3e ⎛ 2π ⎞ 1 K= ⎜ ⎟ (1,1) 16N ⎝ kBμTeff ⎠ Ω

(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 integral representing an average over collision energy, orientations, and inelastic collisions.25 When we treat the ion and neutral as a hard sphere without internal states, the collision integral reduces to the hard-sphere collision cross Received: February 13, 2014 Revised: May 12, 2014 Published: May 14, 2014 3899

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section, Ω. Although eq 2 has been derived for the collisions of atomic ions with atoms, this equation has been widely applied to the system of polyatomic ions in the buffer gas atoms. The term Teff, the effective temperature of the ions, is given by TBG + mBvd2/3kB, where TBG is the buffer gas temperature and mB is the mass of buffer gas. The time that the ion spends in the iondrift cell is thus inversely proportional to K and directly proportional to Ω, as is known from eqs 1 and 2.26 Therefore, the collision cross section of the ion can be evaluated by measuring the mobility of the ion in the ion-drift cell. In order to identify structures of cluster ions, IM-MS was applied to various cluster ions, such as carbon clusters,11−16 silicon clusters,17 alkali halide clusters,18,19 gold clusters,20 and boron clusters.21 Recently, by using the home-built IM-MS apparatus, we investigated the structures of metal oxide cluster cations, (ZnO)n+ (ref 22) and (CoO)n+ (ref 23). Structural transitions from two-dimensional (2D) rings to three-dimensional (3D) compact structures were commonly observed in these clusters. In this study, we have investigated the structures of iron oxide cluster cations, (FeO)n+ and FenOn+1+, by IM-MS. Experimental collision cross sections of these clusters were obtained by the measurements of ion mobility. On the other hand, theoretical collision cross sections were calculated by the MOBCAL program24 for stable structures calculated by quantum-chemical calculations. By comparison between experimental cross sections and theoretical ones, we have determined structures of iron oxide cluster cations.

were separately detected at different arrival times in a twodimensional plot of TOF versus arrival time. We also obtained a plot of arrival time distribution (ATD), in which the total ion intensity of a certain TOF peak was shown as a function of the arrival time. In IMS, the ratio of the drift electric field, E, to the number density of buffer gas, N, is an important parameter.26 Typical other experiments on isomer-separation of carbon clusters, the E/N values were 1.5−10 Td (1 Td = 10−17 V cm2) .15,24,27 It is desirable to keep E/N low for obtaining sufficient collision frequency between the cluster ions and the buffer gas to separate different structure ions. On the other hand, the amount of the cluster ions after the separation decreases at low E/N conditions due to 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 present experiments, conditions were optimized with E/N = 22 Td. Although this E/N value may not be as low as in most IMS measurements, the deviation of the obtained ion mobility from the low-field limit is probably no more than 10%, based on examination of available data in http://nl.lxcat.net/home/. For example, the reduced mobility K0 of oxygen atomic anion (O−) in 22 Td is ∼3% larger than that in a weak electric field (E/N < 20 Td).28 The E/N dependence of ion mobility was not considered in the present study. Quantum chemical calculations for the geometry optimization of iron oxide cluster ions, (FeO)n+ and FenOn + 1+ (n = 2− 9), were performed prior to the calculations of the collision cross sections of the cluster ions. Present calculations were carried out by using B3LYP/6-31+G(d) level in Gaussian 09.33 In a benchmark calculation, the bond length of FeO+ molecule (X6Σ+) obtained by B3LYP/6-31+G(d) level (1.636 Å) reproduced the experimental value (1.641 Å).34 Charge distributions in optimized structures of cluster ions were estimated by a natural population analysis.35 In this analysis of the (FeO)3+ ring isomer, atomic charges on Fe and O atoms were estimated to be about +1.2 and −0.9, respectively. Similarly it was found that all cluster cations in this study are formed by ionic bonds. In principle, all possible magnetic states (spin multiplicities) should be examined for all geometrical structures. For (FeO)2+ and (FeO)3+ cluster ions, we have calculated the stable geometrical structures with all possible magnetic states. Consequently, collision cross sections of ions with all magnetic states are calculated to be almost the same values within ±0.7% error. In addition, spin contaminations were found to be negligible for high magnetic states. Because the calculations of stable structures with all possible magnetic states need large computer resources at larger cluster sizes, we have calculated the structures with ferromagnetic states which were used for calculations of collision cross sections. Orientation-averaged collision cross sections of the cluster ions were calculated by using the projection approximation,16 which is included in the MOBCAL program.24 Although the trajectory method in the MOBCAL program is the most reliable to calculate collision cross sections, the trajectory method requires parameters for Lennard-Jones interaction potentials. Because it is difficult to determine these parameters for interaction potentials including iron atoms, we used the simplest projection approximation method in the present study. With a structure calculated by quantum chemistry, only adjustable parameters in this approximation are the hard sphere atomic radii of the constituent iron, oxygen, and helium

2. EXPERIMENTAL AND COMPUTATIONAL METHODS IM-MS experiments were performed using a home-built vacuum apparatus composed of a cluster ion source, an iondrift cell for IMS,29 and a reflectron time-of-flight mass spectrometer (TOFMS). Details of the experimental setup and procedures for IM-MS were already reported elsewhere.22,23,30−32 Iron oxide cluster cations, FemOn+, were generated by a combination of laser vaporization of an iron rod and supersonic expansion of 0.5% O2/He mixture gas. Stagnation pressure of the mixture gas was 2 atm. The FemOn+ ions were injected into the ion-drift cell with kinetic energies of 100−250 eV by a pulsed electric field at a given time (t = t0). The cell was 100 mm long and was filled with He buffer gas with a pressure of 0.90 Torr. The drift electric field in the cell was E = 10 V/cm. The cell temperature was cooled down to 190 K by liquid N2 circulation. After running through the cell, the ions were reaccelerated to ∼1.8 keV by another pulsed electric fields in an acceleration region of the TOFMS at a given time later from the first pulse, t = t0 + Δt. We hereafter denote this delay time, Δt, as “arrival time”. This arrival time consists of TOFs in three regions: (i) between the first acceleration region and the entrance of the cell, (ii) inside of the cell, and (iii) between the exit of the cell and the acceleration region of TOFMS. By seeking the value of drift velocity for satisfying the measured arrival time, we have calculated these TOFs in three regions by solving the equations of motion of cluster ions. Therefore, we can obtain the time that an ion spends in the cell from the measured arrival time. The time that an ion spends in the cell depends on the collision cross sections between ions and He atoms. Therefore, cluster ions with different cross sections reach the acceleration region of the TOFMS at different arrival times. Finally, the ions were mass-analyzed by the reflectron TOFMS. In the IMS measurement, we obtained a series of TOF mass spectra sequentially by scanning the arrival time. As a result, cluster ions with different cross sections 3900

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atoms. In the present calculations of collision cross sections, we used atomic radii of 1.02, 1.40, and 1.15 Å for iron, oxygen, and helium atoms, respectively. These radii of iron and oxygen were determined by scaling of crystal radii of Fe2+ and O2− ions (0.92 and 1.26 Å, respectively),36 so as to reproduce the experimental cross section of the (FeO)4+ ring isomer. In order to decrease flexibility of our scaling, we fixed the ratio of the hard sphere atomic radii at the ratio of the known crystal radii (0.92 Å/1.26 Å = 0.73). Similar scaling procedure was already applied to Aun+ and Bn+ cluster cations by Kappes and his co-workers,20,21 and also to (ZnO)n+ and (CoO)n+ by the authors’ group.22,23

3. RESULTS AND DISCUSSION 3.1. Measured Mass Spectra of Cluster Ions after Collisions in the Ion-Drift Cell. Figure 1a shows a typical

Figure 3. Arrival time distributions of (FeO)n+ (n = 2−9) measured at an injection energy of 250 eV. Red solid curves are Gaussian functions with a full width at half-maximum of 10 μs, which are used for fitting the experimental plots (Black circles). For n ≥ 5, blue solid curves are the sum of two Gaussian functions.

reaching the reacceleration region of TOFMS. Therefore, the TOF mass spectra shown in Figure 1 were obtained by summing up all the TOF spectra measured at every arrival time because of the spread of arrival time distribution of an ion packet. A series of oxygen-equivalent (FeO)n+ cluster cations was observed for n ≤ 10, in addition to a variety of oxygen-rich iron oxide cluster cations, FenOn + m+. This tendency resembles to the photoionization mass spectra of FenOn + m neutrals5,6 and the mass spectra of FenOn + m+ cations generated in laservaporization cluster sources.7,8 Figure 1b,c show TOF mass spectra measured under the condition that the ion-drift cell was filled with He buffer gas with a pressure of 0.90 Torr. At the ion-injection energy of 100 eV (Figure 1b), we observed prominent ion peaks of (FeO)n+ cluster cations at n ≤ 4 along with oxygen-rich FenOn + 1+ and FenOn + 2+. By contrast, in the mass spectrum obtained at a higher ion-injection energy of 250 eV (Figure 1c), only (FeO)n+ and FenOn + 1+ cluster cations were predominantly observed. Except for n = 3, FenOn + m+ (m ≥ 2) cluster cations were not observed in Figure 1c. Hence, the intensities of the oxygen-rich FenOn + m+ (m ≥ 2) clusters decrease relative to those of (FeO)n+ and FenOn + 1+ cluster cations at high ion-injection energies. Previous study of collision-induced dissociation (CID) of small iron oxide cluster cations showed that Fe2O4+ and Fe2O5+ ions have O2-solvated Fe2O2+ and Fe2O3+ cores, respectively, because the elimination of O2 molecules can easily occur from oxygen-rich Fe2On + m+ (m ≥ 2) cluster cations.37 Although

Figure 1. Mass spectra of iron oxide cluster cations: (a) without He buffer gas in the ion-drift cell, (b) buffer-gas pressure and ion-injection energy were 0.90 Torr and 100 eV, and (c) 0.90 Torr and 250 eV, respectively. Temperature in the cell was 190 K.

Figure 2. Two-dimensional plot of TOF vs arrival time of (FeO)n+ and FenOn + 1+ (▲) cluster cations at an ion-injection energy of 250 eV.

TOF mass spectrum of iron oxide cluster cations which exited from the ion-drift cell without He gas. In the present apparatus, after injection into the ion-drift cell by a pulsed electric field, cluster ions diffuse spatially in the flight of ∼200 mm before 3901

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Figure 4. Examples of optimized structures of (FeO)n+ (n = 2−9) cluster cations calculated with B3LYP/6-31+G(d) level. Brown and red balls represent Fe and O atoms, respectively. Calculated collision cross sections (Ωcalc/Å2) and relative energies from the most stable isomers (ΔE/eV, in parentheses) are also shown. For comparison, experimental collision cross sections (Ωexp/Å2) of each n are also shown. Isomers which are indicated by underlines are assigned to observed isomers from the comparison between Ωexp and Ωcalc.

cations just after entering into the ion-drift cell at high ioninjection energies (Figure 1c). It is worth noting that these tendencies in mass spectra of iron oxide cluster cations showed marked contrast with those of cobalt oxide cluster cations in our previous IM-MS study: (CoO)n+ were predominantly observed in addition to ConOn − 1+ in the mass spectrum obtained at an ion-injection energy of 250 eV.23 This difference appears to have a relationship with the number of electrons in cluster ions, because Fe n O n + 1 − and Fe n O n + 2 − are predominantly observed in the mass spectrum of iron oxide cluster anions obtained by the same procedure (Figure S1). 3.2. Arrival Time Distributions (ATDs) and Structural Assignments. Figure 2 shows a two-dimensional plot of TOF versus arrival time of iron oxide cluster cations at the ioninjection energy of 250 eV. ATDs of (FeO)n+ of n = 2−9 are also shown in Figure 3. An ATD of an ion packet depends on the spatial diffusion that occurs during the traveling of ions in the drift cell. Assuming that all the ions enter the drift cell at the same time, an ATD, I(t), is given by eq 3

Figure 5. Collision cross sections of (FeO)n+ cluster cations obtained by ion-mobility experiments (Ωexp) and theoretical calculations (Ωcalc). Structures used in the theoretical calculations are one-, two- and threedimensional (1D, 2D, and 3D) structures, as shown in Figure 4.

CIDs of larger iron oxide cluster cations were not studied so far, multiphoton dissociation of these cluster cations was investigated using the second and the third harmonic of a Nd:YAG laser by Duncan and co-workers.7 In their study, FenOn + 2+ was also dissociated preferentially to form (FeO)n+ cluster cations through the loss of two oxygen atoms (or an O2 molecule). Therefore, in the present study, it is concluded that oxygen-equivalent (FeO)n+ and oxygen-rich FenOn + 1+ are formed by CIDs of the oxygen-rich FenOn + m+ (m ≥ 2) cluster

I(t ) =

⎛ r 2 ⎞⎤ ⎡ (z − v t )2 ⎤ Ce−αt ⎛⎜ z ⎞⎟⎡⎢ d ⎥ ⎜− 0 ⎟⎥exp⎢− v 1 exp + − d t ⎠⎢⎣ 4DL t ⎦ 4(πDL t )1/2 ⎝ ⎝ 4DTt ⎠⎥⎦ ⎣

(3) 3902

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Figure 6. Arrival time distributions of FenOn + 1+ (n = 2−9) measured at the injection energy of 250 eV. Red solid curves were Gaussian functions with a full width at half-maximum of 10 μs which were used for the fitting of the experimental plots (Black circles). For n ≥ 5, blue solid curves were the sum of two Gaussian functions.

Figure 7. Examples of optimized structures of FenOn + 1+ (n = 2−9) cluster cations calculated with B3LYP/6-31+G(d) level. Brown and red balls represent Fe and O atoms, respectively. Calculated collision cross sections (Ωcalc/Å2) and relative energies from the most stable isomers (ΔE/eV, in parentheses) are also shown. For comparison, experimental collision cross sections (Ωexp/Å2) of each n are also shown. Isomers which are indicated by underlines are assigned to observed isomers from the comparison between Ωexp and Ωcalc.

where z is the length of the cell, r0 is the size of the exit aperture, C is a scaling factor, α is the reaction rate constant, and DL and DT are the longitudinal and transversal diffusion coefficients.25 In the present experiment, α is zero because no reaction gas is mixed with the He gas. The diffusion coefficients can be estimated by the modified Wannier equations.25 In the case of relatively slow drift velocity (vd ∼1000 m/s), the functional form of eq 3 is almost the same with the Gaussian function. Because the drift velocities of the cluster ions are about 500−900 m/s, the measured ATD in the present study could be fitted by a Gaussian function. We thus analyzed the experimental ATD plot by fitting a Gaussian function with a fixed width of 10 μs (fwhm). For n = 2−4, observed ATDs were fitted well with one Gaussian function. For n ≥ 5, two Gaussian functions were necessary for the fittings. Therefore, it is suggested that two isomers which have different collision cross sections exist for n ≥ 5 in the present experimental condition. For n = 2−5, observed bands of ATDs gradually shifted to longer arrival time with increasing cluster size. However, the arrival time of the major ATD band of n = 6 was shorter than that of n = 5. Because an ion with a small collision cross section exits the ion-drift cell before the ion with a large cross section,11 the structure of the major isomer of (FeO)6+ is found to be more compact than that of (FeO)5+. Experimental collision cross sections, Ωexp, of (FeO)n+ cluster cations determined from these ATDs are summarized in Figures 4 and 5. Ωexp was determined by averaging data of four

independent measurements, and standard deviations were shown by error bars in Figure 5. In order to evaluate the collision cross sections theoretically, optimized structures of (FeO)n+ cluster cations have been calculated by quantum-chemical calculations with B3LYP/631+G(d) level. As shown in Figure 4, various isomers [onedimensional (1D), 2D, and 3D structures] were obtained for n = 2−9. Calculated relative energies (ΔE) of these isomers are also shown in Figure 4. In previous theoretical studies of neutral (FeO)n clusters, 2D ring isomers were calculated for n = 2−5, and 3D tower isomers were obtained for n = 6−12.9 For these optimized structures, we next calculated collision cross sections, Ωcalc, using the MOBCAL program.24 This calculation was based on the projection approximation method which is included in MOBCAL. The values of Ωcalc of (FeO)n+ cluster cations are also summarized in Figures 4 and 5. For (FeO)2+, Ωcalc of the 1D linear isomer 2a (37.6 Å2) was found to be in good agreement with Ωexp (38.4 ± 0.7 Å2). However, in the present quantum-chemical calculations by 3903

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structure was found as a stable structure of the FenOn + 1+ cluster ion, although the isomer 2B is nearly a linear structure. For n = 2 and 5, Ωcalc of 2D structures (isomer 2A and 5A) are in good agreement with Ωexp. Except n = 2 and 5, Ωexp were well reproduced by Ωcalc of 3D structures for n = 3−9. Therefore, the smallest cluster size of 3D structures is n = 3 for FenOn + 1+ ions. This is quite different from the (FeO)n+ ions, in which the smallest size of 3D structures is n = 6.

B3LYP/6-31+G(d) level, the ring isomer 2b was the most stable isomers in (FeO)2+. This is not consistent with the experimental result; the ring isomer of (FeO)2+ was not observed at least at high injection energies in the present experiment. However, calculations of (FeO)2+ ions by B3LYP/ aug-cc-pVTZ level, the linear isomer is found to be 0.078 eV more stable than the cyclic isomer. This result suggests that larger basis set is needed in order to determine the most stable isomer of the cluster ions containing transition metal atoms. Because it is difficult to perform the calculations of larger cluster ions with larger basis set, we assigned the observed isomers on the basis of comparison between observed collision cross sections and theoretical ones. For (FeO)3−8+, Ωcalc of 2D isomers (monocyclic-ring, polycyclic-ring, and sheet) all agreed well with Ωexp. In addition, for n ≥ 6, Ωcalc of 3D isomers (tower and/or cube) were also in good agreement with Ωexp along with the 2D sheet isomers. The most stable 3D isomers of (FeO)n+ for n = 6−9 have tower-like structures which consist of plural rings. For example, the tower structures for n = 6 and 9 (isomers 6d and 9b) are made from two and three stacked six-membered rings, respectively. Also for n = 8, the tower isomer 8b consists of two stacked eight-membered rings. These tendencies of stacking rings are similar to the results of the previous theoretical study of neutral iron oxide clusters.9 In addition to the tower structure, the cube structure (isomer 9d) was found to be stable for (FeO)9+. This isomer 9d has a 3 × 3 × 2 cuboid structure which is similar to bulk rock-salt type structure of FeO. Although the Ωcalc of this cube isomer (71.3 Å2) coincides well with the Ωexp of (FeO)9+ (71.9 ± 1.1 Å2), this cube isomer 9d is 1.40 eV unstable than the tower isomer 9b. Thus, the major isomer of (FeO)9+ is expected to be the tower isomer 9b from the relative energies estimated by the present density functional calculations. However, it is difficult to rule out the possibility of existence of cube isomer 9d, because it is known that densityfunctional calculations sometimes fail to predict the most stable isomer of transition metal oxide clusters.22,23,38 From the present results, the smallest cluster size of 3D structure is n = 6 for (FeO)n+ cluster cations. This conclusion is similar to that concluded earlier in our recent IM-MS study of (CoO)n+ cluster cations.23 However, in the present study of (FeO)n+, 2D sheet isomers were also found for n = 5−8 by curve-fittings of ATDs with Gaussian functions, as shown in Figure 3. The 2D sheet isomers were not found in the previous study of (CoO)n+ cluster cations.23 In Figure 4, Ωcalc of sheet isomers (isomers 5d, 6a, 7a, and 8a) are in good agreement with Ωexp of (FeO)n+ (n = 5−8). However, for (FeO)9+, Ωcalc of the sheet structure (isomer 9a) is ∼7 Å larger than Ωexp. Instead, Ωexp for (FeO)9+ are reproduced by Ωcalc of 3D structures (isomers 9b, 9c, and 9d). Therefore, we have concluded that 2D sheet and 3D structures coexist in (FeO)n+ (n = 6−8). In Figure 6, ATDs of oxygen-rich FenOn + 1+ cluster ions are shown. For n = 2−4, each ATD can be fitted with only one Gaussian function. As with (FeO)n+, two Gaussian functions were necessary for the fitting of ATDs of FenOn + 1+ (n = 5−9). Collision cross sections Ωexp derived from these ATDs are shown in Figure 7. At the same n, FenOn + 1+ have larger cross sections than (FeO)n+. This difference in cross sections is reasonable because there are more oxygen atoms in FenOn + 1+ than (FeO)n+. Structures of FenOn + 1+ calculated by B3LYP/631+G(d) level are shown in Figure 7 along with Ωcalc and ΔE. In the present quantum-chemical calculations, no 1D linear

4. CONCLUSION In this study, we have investigated the structures of iron oxide cluster cations by ion mobility mass spectrometry. In the mass spectrum measured at high ion-injection energy of 250 eV, (FeO)n+ and FenOn + 1+ cluster cations were predominantly observed. We deduced experimental collision cross sections of these clusters from the measurements of arrival time distributions. Theoretical collision cross sections were also calculated for stable structures optimized by quantum-chemical calculations. We determined structures of iron oxide cluster cations by comparison between experimental cross sections and theoretical ones. For (FeO)n+ (n ≥ 5), two Gaussian functions were necessary for the fitting of the arrival time distributions. As a result, the (FeO)n+ ions were found to have monocyclicring structures for n = 3 and 4, and polycyclic-ring for n = 5. For n ≥ 6, three-dimensional tower and/or cube structures were found. These results are somewhat similar to those found in our previous IM-MS study of (CoO)n+ cluster cations.23 However, sheet isomers were also observed in the present study of (FeO)n+, whereas no such isomers were obtained in (CoO)n+. In conclusion, we have found the coexistence of twodimensional (sheet) and three-dimensional (tower and/or cube) structures in (FeO)n+ for n = 6−8. For oxygen-rich FenOn + 1+ cluster cations, three-dimensional structures were assigned for n ≥ 3, which is quite different from the result for (FeO)n+.

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ASSOCIATED CONTENT

S Supporting Information *

The measured mass spectrum of iron oxide cluster anions after collisions in the ion-drift cell. Geometrical coordinates of structures in Figures 4 and 7. The complete author list of ref 33. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81 22 795 6580. Tel.: +81 22 795 6577. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors are grateful to Prof. Kiichirou Koyasu for his helpful comments. This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS) and in part by the Research Seeds Quest Program (JST), as well as The Murata Science Foundation. Theoretical calculations were partly performed using the Research Center for Computational Science, Okazaki, Japan. 3904

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dx.doi.org/10.1021/jp5015687 | J. Phys. Chem. A 2014, 118, 3899−3905