Oxidizing Metal Oxides with Polynuclear Superhalogen: An ab Initio

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Oxidizing Metal Oxides with Polynuclear Superhalogen: An Ab Initio Study Celina Sikorska J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05095 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Oxidizing Metal Oxides with Polynuclear Superhalogen: An Ab Initio Study Celina Sikorska1, * 1

Laboratory of Molecular Modeling, Department of Theoretical Chemistry, Faculty of Chemistry, University of Gdansk Wita Stwosza 63, 80-308 Gdansk, Poland

* Corresponding author. E-mail address: [email protected], telephone number: (+48) 58 523 5351. ACS Paragon Plus Environment

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Abstract The ability of metal oxides (CoO, CuO, MgO, MnO2, NiO, SiO2, TiO2, and ZnO) to form stable systems with polynuclear superhalogen (i.e. Mg3F7) is examined on the basis of theoretical considerations supported by ab initio calculations. It is demonstrated that the MeOn (n=1, 2) molecules (such as CoO, CuO, MgO, MnO2, NiO, TiO2, ZnO) should form stable and strongly bound (MeOn)+(superhalogen)− salts when combined with the Mg3F7 superhalogen radical (acting as an oxidizing agent). This conclusion is supported by providing: (i) structural deformation of superhalogen upon ionization, (ii) predicted charge flow between each MeOn and superhalogen (which allows estimating the amount of electron density withdrawn from MeOn molecule during the ionization process), (iii) the localization of the spin density distribution, and (iv) the interaction energies for the compounds obtained at the CCSD(T)/6311+G(d) level of theory. On the other hand, the Mg3F7 superhalogen was found to be incapable of ionizing molecules whose adiabatic ionization potentials (AIPs) exceed 12 eV (e.g. SiO2). I believe that the results provided in this contribution may likely be of prospective relevance in the future studies on the issue of binding and preventing metal oxide nanoparticles aggregation in the environment before they occur harmful to human health and environment.

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1. Introduction The reduced sizes of nanoscale materials result in their usefulness in a novel area of materials science where materials with features of interest can be manufactured. A fundamental understanding of how their features evolve one atom and/or one electron at a time can be finest investigated with atomic clusters. A plenty of studies of clusters over the last three decades have demonstrated their unusual properties, which can be custom-tailored by fixing their size and composition. One of the essential properties of molecular clusters is that they exhibit enhanced stability at a specific size and composition. These stable clusters are often referred to as magic clusters and includes (i) clusters of simple metals (e.g. Na8, Na20, Na40, Na58, Na92)1 and (ii) clusters of noble gas atoms (e.g. Xe13, Xe55, Xe147)2. Very recently, I have investigated a magic cluster with molecular composition (Mg3F7)−.3 On the basis of an ab initio calculations, I confirmed that this magic cluster owes its unusual stability to the superhalogen character of its corresponding neutral parent (Mg3F7). The Mg3F7− superhalogen anion, owing to its enlarged stability, have the potential to serve as building blocks of a new class of salts with superoxidizing properties. 3-5 Superhalogens are inorganic compounds capable of exceeding the 3.62 eV (chlorine atom)6 atomic electron affinity’s limit and have been theoretically as well as experimentally studied since the 1980s (see Refs.

7-23

and references cited therein). One class of these

compounds is describing by the MXk+1 formula according to which superhalogen is a neutral system containing the central atom M (the main or transition group metal atom) decorated with k+1 halogen atoms (where k is the maximal formal valence of the atom M).24 The original MXk+1 formula, however, turned out to be much more general than it appeared. Namely, various functional groups (e.g., fluoroxyl groups8 or superhalogens themselves14) were demonstrated to act as suitable ligands, whereas certain non-metal atoms (e.g., noble gas atoms)25 were found to be capable of playing the central atom role in superhalogen anions. Another important extension of the superhalogen formula led to the definition of the ACS Paragon Plus Environment

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polynuclear MnXn·k+1 superhalogens and their corresponding (MnXn·k+1)− anions, such as Xe2F13−, Rn2F13−, and Mg3F7−, whose promising properties are constantly being discovered.3, 25 The discovery of these species conduces to the development of oxidizing agents of a new type. Due to their tremendous ability of an excess electron binding, superhalogens may act as strong oxidizing agents when interacting with other molecules.26-28 As it was demonstrated, the stability (or its lack) of some molecular systems (such as [C60][superhalogen])3, 29 might be explained by the proper balance between the ionization potential (IP) and the electron affinity (EA) of the fragments these systems consist of. For instance, the stability of the C60Mg3F7 salt might be viewed as caused by the ability of superhalogen component (Mg3F7 whose adiabatic electron affinity is equal to 7.93 eV)3 to ionize C60 fullerene (with IP of 7.58 eV)30, whereas the instability of the C60LiI2 system29 might be explained by the inability of the LiI2 (whose excess electron binding energy, manifested by the vertical electron detachment energy (VDE) of its daughter anion, is much lower and reads 4.57 eV)31 to ionize the C60 nanoparticle. Successful utilizing superhalogens as oxidizing agents, capable of forming stable ionic compounds with the moderately reactive C60 nanoparticle, turn me to an extension of this concept for metal oxide nanoparticles. In this work, to further examine the behavior of the Mg3F7 cluster in [NP+][Mg3F7–] design, a series of MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) and MeO2/Mg3F7 (Me=Mn, Si, Ti) systems were theoretically constructed and studied. In particular, the main goal was to demonstrate that metal oxides might be assembled into a stable, strongly bound compounds when combined with a properly designed polynuclear superhalogen system. Moreover, it is shown that such resulting compounds exhibit partially ionic character as their stability is a consequence of the substantial electron density flow to the oxidizing agent (i.e., Mg3F7).


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2. Methods The equilibrium geometrical structures of the MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) and MeO2/Mg3F7 (Me=Mn, Si, Ti) systems and the corresponding harmonic vibrational frequencies were calculated by applying the second-order Møller−Plesset (MP2) perturbational method with the 6-311+G(d) basis sets.32, 33 The coupled-cluster method with single, double, and noniterative triple excitations (CCSD(T)) with the same 6-311+G(d) basis set was used to calculate the vertical ionization potentials (VIPs) and the final energies of the species at their geometries obtained with the MP2 method. Explicitly, the binding energy (BE) between the superhalogen fragment and metal oxide was calculated as BE = E(MeOn) + E(Mg3F7) – E(MeOn/Mg3F7), where E(species) stands for the CCSD(T)/6-311+G(d) electronic energy of a given species obtained for its MP2/6-311+G(d) geometry. In the BE computations, the basis set superposition error, BSSE, was corrected by applying the counterpoise (CP) approach.34 The adiabatic ionization potentials (AIPs) for the MeOn systems considered (i.e., CoO, CuO, MgO, MnO2, NiO, SiO2, TiO2, ZnO) were calculated by employing the coupled-cluster method (CCSD(T)) and 6-311+G(3df) basis sets. The methods which were used herein for the odd-electron systems are based on an unrestricted Hartree-Fock starting point (using the single determinant reference wave function), hence it is important to ensure that little, if any, artificial spin contamination enters into the final wave functions. I computed the expectation value for the species studied in this contribution and found values of 0.750-0.809 in all radical (doublet) cases; thus, I am certain that spin contamination is not large enough to affect my findings considerably. The partial atomic charges (required for estimating the charge flow values) were fitted to the electrostatic potential according to the Merz-Sigh-Kollman scheme35, whereas spin density distribution in MeOn/Mg3F7 species was estimated with using the Mulliken population analysis. All computations were accomplished with the GAUSSIAN09 (Rev.E.01) software package.36 ACS Paragon Plus Environment

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3. Results and Discussion My previous experience in ionizing nanoparticles with superhalogens3, 29 indicates that the most important factor ensuring the effectiveness of this process is the proper balance between the IP of the species that is supposed to be oxidized and the superhalogen’s tendency to bind an additional electron. Since the ionization potential values of studied metal oxides span the 8.695-12.581 eV range, I decided to choose the superhalogen whose excess electron binding energy (manifested by the vertical electron detachment energy (VDE) of its corresponding daughter anion) reads 10.479 eV.3 Explicitly, I chose the Mg3F7 molecule, as it is structurally simple polynuclear system and its electron affinity appeared to be suitable.

3.1. The Equilibrium Structures and Stabilities of the MeO/Mg3F7 Species (Me=Co, Cu, Mg, Ni, Zn) The equilibrium structures of the lowest energy MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) systems (labeled 1 for each MeO/Mg3F7 compound) and their higher energy isomers characterized by the CCSD(T)/6-311+G(d) relative zero-point corrected energies (denoted as ER) are depicted in Figures 1–4. In general, each set of isomers consists of the structures that differ by (i) structural deformation of the superhalogen from its isolated neutral structure, (ii) mutual orientation of the superhalogen and metal oxide, and (iii) the number the bridging –Mg– F–Me– fragments (double-bridged (DB) versus triple-bridged (TB) forms) connecting metal oxide (MeO) and superhalogen (Mg3F7) fragments. The large HOMO–LUMO gaps ranging from 10.787 to 13.941 eV obtaining for the MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) isomers (see Tables 1 and 2) indicate that they all may be thermodynamically stable. The precise values of the geometrical parameters characterizing the species studied are reporting in Tables S1–S3 of the supplementary material, whereas the qualitative discussion of the low-energy isomers structures is provided in the following paragraphs. The lowest energy structure for the CoO/Mg3F7 molecule is double-bridged (DB) form


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with Co atom located between two terminal fluorine (interacting only with magnesium atom in triangular superhalogen fragment) atoms (F6, F7); see 1 CoO/Mg3F7 in Figure 1. Similar DB structure possesses only slightly higher in energy 2 CoO/Mg3F7 conformer (Er=0.01 kcal/mol), while the 3 CoO/Mg3F7 (ER=0.97 kcal/mol) isomer is of Cs symmetry structure in which cobalt and three magnesium atoms lie in a straight line and CoO moiety is bound with superhalogen fragment through two fluorine (F6, F7) atoms. The TB isomers (4 and 5 in Figure 1) having higher but similar energies with respect to the 1 CoO/Mg3F7 (within 4 kcal/mol) form three Co– F bonds between CoO and quasi-triangular superhalogen counterparts. The 6 CoO/Mg3F7 (Er=10.03 kcal/mol) and the 9 CoO/Mg3F7 (Er=24.58 kcal/mol) structures contain the ring F (linking one given pair of Mg atoms in triangular superhalogen fragment) atoms binding with Co atoms, whereas in the 8 isomer (Er=13.72 kcal/mol) Co atom is bonded with two bridging fluorine (linking to each of the three magnesium ions and forming altogether the umbrella-like structure in superhalogen fragment; F3). In turn, the 7 CoO/Mg3F7 isomer (Er=12.02 kcal/mol) is Cs symmetry structure in which one Mg atom (Mg2) is decorated with four fluorine ligands (F2, F4, F5, F6) localized in a square manner (i.e. valence F2–F6–F5 and dihedral F2–F4–F5–F6 angles read 91.7 and 0.0°, respectively), whereas Co atom forms bonds with three fluorine (F1, F3, F7) atoms and Co–F3 and Co–F1,7 bonds are equal to 1.873 and 1.852 Å, respectively. The Mg–F bond lengths in the superhalogen subunits of low energy 1–5 CoO/Mg3F7 structures spanned the 1.789–1.991 Å range excluding the distances for the Mg1–F3(Mg2)–Mg3 bridging fragments of the 1, 2, 4, 5 systems, in which a 2.000–2.105 Å magnesium-fluorine separations were predicted; see Table S1. It turns out that the 1–8 structures of CoO/Mg3F7 system should be treated as the strongly bound systems, because of the short Co–F6,7 distances (1.859–1.956 Å, Table S1).


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F2

F1 Mg1

F1 Mg1

Mg2

F3

O

Mg3

F6 1.55Å

(C1) ER= 0.01 F4

Mg2

F3

Mg3

F5

F7

3 CoO/Mg 3F7 (CS) ER= 0.97

O

F1 F1 Mg1

F2 F3

4 CoO/Mg 3F7 (C1) ER= 1.18

F2

F1 Mg1 F4

Mg1

F6

Mg2

F3

Co

F5

5 CoO/Mg 3F7 (C1) ER= 4.26

O Mg2

F2

6 CoO/Mg 3F7 (C1) ER= 10.03

1.53Å Co

Mg3 F6

1.61Å F7

F6

F1

ER= 12.02

9 CoO/Mg 3F7 (CS) ER= 24.58

F7 Mg1

Co F2

8 CoO/Mg 3F7 (C1) ER= 13.72

F4 O Mg3

F3 F5 F7

O

Mg3

O

F5

(CS)

Co

Mg2 F4

O

Mg1 F1 7 CoO/Mg 3 F7 F4

F5

F3

Co F6

F3

1.54Å

F6

1.67Å Mg3

F2

F7

Mg3

F1 Mg1

O

Co

F5

F4

1.54Å

Mg3

F7

F3

F7

F5

F4

Mg2

F2

Mg2

O

O

1.56Å

F6 Co

Mg1

Co

Mg3

1.55Å 2 CoO/Mg 3 F7

F6

F2

F1

F7 F5

F4

Co O

F5

1 CoO/Mg 3F7 (C1) ER= 0.00

Mg2

F3

F7

F4

F2

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Mg2

F6

R(CoO)=1.51Å

Figure 1. The CoO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level; selected interatomic distances in Å. The CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for 1-9 CoO/Mg3F7 isomers with respect to the global minimum (1). The analogous set of isomers was found for the NiO/Mg3F7 system. The 1–3 NiO/Mg3F7 structures are very close in energy (within 5 kcal/mol) thus the presence of all three should be expected in gas phase, whereas the formation of the remaining six NiO/Mg3F7 isomers (4–9 in Figure 2) should be considered less likely (as ER always exceeds 8 kcal/mol). Each of the 1–3 NiO/Mg3F7 equilibrium C1-symmetry structures corresponds to the TB form with the nickel atom linked to the quasi-triangular Mg3F7 moiety through three fluorine (F5, F6, ACS Paragon Plus Environment

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F7) atoms (see Figure 2). In these compounds (1–3 NiO/Mg3F7), the presence of chemical bonds between the Mg3F7 and NiO moieties was confirmed by relatively short interatomic distances. Explicitly, the terminal F atoms (F6, F7) are separated by 2.0 Å from the Ni atom and F5–Ni distance does not exceed 2.2 Å; see 1–3 structures in Figure 2 and Table S2. The Ni–O bond lengths were evaluated to be ca. 1.8 Å, whereas the Mg–F bonds in the Mg3F7 spanned the 1.797–1.996 Å range (excepting the bonds forming the bridging Mg1–F3(Mg2)–Mg3 fragment whose lengths were 1.984–1.997 Å (Mg2,3–F3) and ca. 2.12 Å (Mg1–F3), see Figure 2 and Table S2). Hence the NiO system seems to form covalent bonds with the quasi-triangular Mg3F7 superhalogen. 1 NiO/Mg 3F7 (C1) ER= 0.00 F2

F1 Mg1

Mg2

F3

F2

F1 Mg1

Ni

1.82Å O

Mg1

F1 Mg1

Mg2

Mg3

F4

Mg3

Ni

1.68Å O

Mg1 F1

1.71Å F7

Mg3

O

Ni

ER= 8.07 O Ni

F1

O

ER= 22.29

F7

F2 Mg1

F6

9 NiO/Mg 3F7 (C1) F4

Ni F3

Mg2 F7

F5

Mg3

Mg1 F2

Mg2

F3

F4

7 NiO/Mg 3F7 (CS) ER= 19.14 F1

8 NiO/Mg 3F7 Ni (Cs)

Ni 1.78Å O

1.64Å 4 NiO/Mg 3 F7 (C2V)

F7

F7

F6

Mg2 F3 F5 F6

F6

F5

ER= 16.29

ER= 13.94

R(NiO)=1.75Å

F3

F2

F5

F7

Mg3

6 NiO/Mg 3F7 (CS) Mg2

F4

Mg2 F5

F6

F4

F2

F2 F4

1.78Å O

F2 F3

5 NiO/Mg 3F7 (C1)

F1 Mg1 Ni

F6

1.82Å

Ni

O

F3

Mg3

3 NiO/Mg 3F7 (C1) ER= 5.18

F1

F6

Mg3

F5

F4

F7

F5

Mg2 F7

F3

Mg2

F3 F4

F6

Mg3

Mg1

F7

F5

F4

2 NiO/Mg 3F7 (C1) ER= 1.3 ˙10 -4 F2

F1

O F5

R(NiO)=1.80Å

ER= 24.57 Mg3 F6

Figure 2. The NiO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in ACS Paragon Plus Environment

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kcal/mol) estimated for 1-9 NiO/Mg3F7 isomers with respect to the global minimum (1) are also provided. As far as CuO/Mg3F7 system is concerned, the lowest energy 1 CuO/Mg3F7 structure (of Cs symmetry) corresponds to DB form with the Cu atom linked to the triangular superhalogen fragment through terminal fluorine (F6, F7) atoms (with Cu–F6,7 bonds of 1.855 Å; see Table S2). The copper and three magnesium atoms in the isomer 2 structure lie in a straight line (C2V symmetry structure) and CuO moiety is connected with superhalogen through two terminal fluorine (F6, F7) atoms (with Cu–F6,7 bonds equal to 1.848 Å). This latter structure (2) is only 2.64 kcal/mol higher in energy than the former (1) hence the presence of both should be expected in gas-phase. The four other CuO/Mg3F7 isomers (labeled 3, 4, 5, 6 in Figure 3) found are much higher in energy (by 12 kcal/mol or more) than the lowest energy structure (1), which indicates that the formation of such isomer should be regarded as less unlikely. 1 CuO/Mg 3 F7 (CS)

F2

F1 Mg1 F4

Mg2

F3

F7 Cu

F5 F6

ER= 0.00

Mg2

F3

Mg2 F7 F3

1.63Å

F6

Mg3

5 CuO/Mg 3 F7 (CS) ER= 21.64

O1.63Å

4 CuO/Mg 3 F7 (C1)

Cu F2

F1 Mg1

Mg1

F7

Mg2

Mg3

F5

F1

F2 F4

Mg2 F3

F4

O ER= 2.64

1.62Å

F7

O

ER= 12.00 2 CuO/Mg3 F7 (C2V)

Cu

F6

Cu

F5

Mg3

O

F5

3 CuO/Mg 3 F7 (C1)

F2

F4

F4

F2 Mg1

Mg1

1.65Å

Mg3

F1

F1

F6

ER= 12.93

F5

O F3 Cu Mg3

F6

1.63Å F7

F1

6 CuO/Mg3 F7 (C1) ER= 31.76

Mg1 F2 Mg2 F7

Cu F3 F5

F4 r(CuO)=1.62Å

O Mg3

F6


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Figure 3. The CuO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for CuO/Mg3F7 isomers with respect to the global minimum (1) are also provided.

In case of the MgO/Mg3F7 system, the lowest energy isomer (1) corresponds to TB structure with Mg atom (in MgO fragment, Mg4) bound with three fluorine (F5, F6, F7) atoms (each of which is involved in forming the Mg–F–Mg bridging fragment connecting the Mg of MgO with superhalogen moiety; see Figure 4). The 2 MgO/Mg3F7 isomer is DB system with the Mg atom (Mg4) of metal oxide bound to two terminal fluorine (F6, F7) atoms of a quasitriangular superhalogen fragment. This later structure (2 MgO/Mg3F7) is only 1.46 kcal/mol higher in energy than the former (1 MgO/Mg3F7) which indicates that interchange between the structures is probably rapid at the temperatures used experimentally. Next isomeric form (3 MgO/Mg3F7, ER=11.16 kcal/mol) resembles double rhombic Mg3F7 system interacting with MgO moiety through two terminal fluorine atoms (F6, F7). Due to short Mg4–F6,7 separations (1.891–1.980 Å, Table S3), the 1-3 MgO/Mg3F7 structures should be treated as strongly bound systems. In the 4 MgO/Mg3F7 isomeric form (ER=16.19 kcal/mol) MgO moiety is connected through three Mg–F bonds with a quasi-square pyramidal Mg3F7 fragment (which mimics higher energy structure of Mg3F7– anion).3 Finally, the less stable MgO/Mg3F7 isomer found corresponds to compact Cs symmetry structure, 5 MgO/Mg3F7 in Figure 4, with the Mg atom (Mg4) of MgO bound to three ring fluorine (F2, F4, F5) atoms and its energy exceeds that of the corresponding global minimum (1 MgO/Mg3F7) by 29.88 kcal/mol hence its formation is not likely in the gas phase (despite their geometrical stability).


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1 MgO/Mg 3 F7 (Cs) ER= 0.00

F2

F1 Mg1

Mg2

F3

F4

F5

Mg3

Mg1

Mg4 1.90Å F3

F5 F6

Mg3

O

Mg1

Mg2

F3

F1

Zn

Mg3 F6

ER= 5.94

F1

O

F3

Mg1

Mg2

Mg3 F6

2 ZnO/Mg 3 F7 (C1) ER= 0.52

F3

F5

Mg2

F5

1.82Å

Zn

F6 Mg3

F7

O 1.81Å

5 ZnO/Mg 3F7 (CS) ER= 32.19 Mg2 F7

F3

ER= 16.63

F7 Zn

O

F6

F1 Mg1

(Cs)

F6

F2

F2

F5 4 ZnO/Mg 3 F7

F4

Mg3

F5

Mg2

F4

O 1.80Å Zn F2 F7 Mg2 F3

F1

O

Mg3

F2 Mg1

F4

Mg4

F4

1.83Å

3 ZnO/Mg 3 F7 (C2v)

R(Mg4O)=1.51Å Mg1

Mg1

F7

F5

F4

ER= 11.16

F3

1 ZnO/Mg 3F7 (Cs) ER= 0.00

F1

O

F1

F2

F7

F2

3 MgO/Mg 3F7 (C2v)

5 MgO/Mg 3 F7 (Cs) ER= 29.88

4 MgO/Mg 3 F7 F7 (CS) ER= 16.19

F7

O F5 1.90Å F6

F7 Mg4 1.89Å

Mg3

Mg1 F1

Mg2

Mg4

F3

F6

F2 F4

Mg2

Mg3

F4 F5

2 MgO/Mg 3 F7 (Cs) ER= 1.46

F4

Mg4 O F6 1.90Å

Mg2

Mg1 F3

F1

F7

F2

F1

F2

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F5

F4 Mg3

Zn F6 1.80Å

O

Figure 4. The equilibrium structures of the MgO/Mg3F7 and ZnO/Mg3F7 species obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6311+G(d) relative energies (ER, in kcal/mol) estimated for MeO/Mg3F7 isomers (Me=Mg, Zn) with respect to the corresponding global minimum (1) are also provided. I found five different isomeric forms for the ZnO/Mg3F7 system (within ER energy range of 32 kcal/mol) which are depicted in Figure 4. The structures of the isomers 1-3 of ZnO/Mg3F7 are very close in energy (within 6 kcal/mol) thus the presence of each of them should be expected in the gas phase (the remaining structures 4 and 5 ZnO/Mg3F7 possess significantly higher energies and the differences exceed 16 kcal/mol in both cases). The 1 ZnO/Mg3F7 and 2


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ZnO/Mg3F7 equilibrium structures correspond to the triple-bridged (1) and double-bridged (2) forms with the zinc atom linked to a quasi-triangular Mg3F7 moiety through respectively three (F5, F6, F7) and two (F6, F7) fluorine atoms (Figure 4). On the contrary, the 3 ZnO/Mg3F7 structure mimics double rhombic-like Mg3F7 fragment interacting with ZnO moiety through two F–Zn bonds. The presence of chemical bonds between the Mg3F7 and ZnO moieties (1-3 isomers of ZnO/Mg3F7) was confirmed by relatively short interatomic distances as the terminal F atoms (F6, F7) in Mg3F7 fragments are separated by 1.984 Å (1 ZnO/Mg3F7), 1.941 Å (2 ZnO/Mg3F7), and 1.939 Å (3 ZnO/Mg3F7) from the Zn metal atom. The Zn–O bond lengths were evaluated to be ca. 1.8 Å, whereas the Mg–F bonds in the Mg3F7 spanned the 1.798–1.966 Å range (excluding the triple-bridged Mg–F3(Mg)–Mg fragment in which Mg–F distances were 1.994–2.114 Å, see Figure 4 and Table S3). Thus the ZnO oxide seems to form covalent bonds with the Mg3F7 fragment. To conclude, the Mg3F7 cluster is able to form stable and strongly bound compounds with each of CoO, CuO, MgO, NiO, ZnO oxides. Obtained results clearly reveal that the Mg3F7 fragment (i) adopts a quasi-triangular configuration, (ii) resembles two rhombic parts perpendicular to each other or (iii) possesses one Mg atom decorated with five fluorine ligands localized in a square pyramidal manner. In each case, the lowest energy isomer (labeled 1) adopts the quasi-triangular localization of three Mg atoms and the importance of this observation will be discussed in the following sections.

3.2. Competition between MeO/Mg3F7 and FMeO/Mg3F6 Formation The MeO/Mg3F7 molecules (Me=Co, Cu, Mg, Ni, Zn) might be considered as complexes between a Mg3F6 cluster and the corresponding FMeO system (Me=Co, Cu, Mg, Ni, Zn). However, such a treatment would not be justified because of the structure of the overall MeO/Mg3F7 system in which the metal atom Me (of the MeO oxide) forms two or three (depending on the structure) bonds with the fluorine atoms (Figures 1–4). The lengths of these


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Me–F bonds (Me=Co, Cu, Mg, Ni, Zn) are larger (by 0.11–0.28 Å) than those calculated for the isolated FMeO molecules (at the same level of theory), and those elongations are clearly the result of the fact that the Me metal atoms are involved in two or three F–Me bonding interactions in the MeO/Mg3F7 salts instead of one in FMeO species. Moreover, it needs to be pointed out that the isolated Mg3F6 molecule is D2d symmetry structure with a linear arrangement of three magnesium atoms while the Mg3F7− anion possesses a triangular (C3v symmetry) structure.3, 4 The analysis of the MeO/Mg3F7 geometrical structures (in their lowest energy structures, 1) clearly reveals the quasi-triangular configuration of the F3 and three Mg atoms (Figures 1–4). Thus, I assume that the structural changes the superhalogen moiety undergo upon its interaction with metal oxide molecules convert its geometry toward the Mg3F7− anion. This observation indicates the electron density withdrawal that takes place when a superhalogen combines with metal oxides and is yet to be discussed in section 3.4. 3.3. Structural Deformation of Superhalogens upon Metal Monoxides Ionization In order to further analyze the relaxation of the superhalogen moiety observed when it is assembled into MeO/superhalogen products, the knowledge about the equilibrium structures of the isolated (non-interacting) species is required. The Mg3F7 structure possesses C2v symmetry and resembles two rhombic parts perpendicular to each other, with one terminal F atom at the end of one side of the molecule and two out-of-plane F atoms at the other side of the system. The Mg–F bond lengths in Mg3F7 superhalogen are in the 1.788–1.929 Å range and the three Mg atoms lie in a straight line (with valence Mg1-Mg2-Mg3 and dihedral Mg1-F3-Mg3-Mg2 angles equal to 180.0° and 0.0°, respectively) 3. From the other hand, the Mg3F7− anion is C3v symmetry structure in which the three Mg ions lie in a triangle arrangement (valence Mg1– Mg2–Mg3 and dihedral Mg1–F3–Mg3–Mg2 angles are equal to 51.3° and 94.9°, respectively). As far as geometrical localization of ligands in Mg3F7− molecule is concerned, (i) three terminal F atoms are linked to only one Mg atom each, through a single 1.821 Å bond, (ii) three ring fluorine atoms play the same role of bridging ligand which links one given pair of Mg ions ACS Paragon Plus Environment

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(through a 1.936 Å bond), and (iii) one fluorine atom is linked to three central metal atoms (through a 2.061 Å bonds) and localized on the top of the ‘umbrella’ formed by all three Mg atoms present in the system.3 Structure of the Mg3F7 fragment in the MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) compounds (at their lowest energy structures, labeled 1 for each MeO/Mg3F7 molecule) adopts an umbrella-like configuration with the Mg1–F3(Mg2)–Mg3 bridging fragment (as depicted in Figures 1–4) with dihedral Mg1–F3–Mg3–Mg2 angles of 95.8°. The Mg–F bond lengths in Mg3F7 fragments are in the 1.797–2.128 Å range and the longest bonds (of 1.984–2.128 Å, Tables S1–S3) are formed by the bridging F3 atom connecting all three Mg atoms. Having in mind that the isolated Mg3F7 neutral system adopts the C2V-symmetry structure (with the Mg1– Mg2–Mg3 angle of 180.0°) and corresponds to two rhombic parts, perpendicular to each other, whereas the corresponding Mg3F7– anion is umbrella-like (C3V symmetry) structure3, I conclude that the Mg3F7 fragment adopts an anionic rather than a neutral geometrical configuration while assembled into 1 MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) species. As I demonstrated above, superhalogen moiety undergoes significant geometry relaxation when it is brought together with the MeO to form the resulting MeO/Mg3F7 compounds. In particular, the structural changes the superhalogen moiety undergo upon its interaction with each of metal oxide molecule converts its geometry toward its anionic daughter. Albeit the final anionic structure of superhalogen is not completely achieved in MeO/Mg3F7 products (Me=Co, Cu, Mg, Ni, Zn), this observation indicates the electron density withdrawal between reactants (i.e. MeO and Mg3F7) when they are assembled into the MeO/Mg3F7 products. 3.4. The Charge Flow, Spin Density, and Binding Energy Values in MeO/Mg3F7 Systems Since the results described in the preceding section seemed to indicate an electron density withdrawal that takes place when a superhalogen combines with each of metal oxide


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studied, there is a need to verify this observation by the analysis of changes in electron density distribution caused by the mutual interactions of the reactants in MeO/superhalogen systems. These changes manifest themselves in the charge flow (∆qESP) values calculated for each pair of components (i.e. MeO and Mg3F7). Namely, the positive ∆qESP values estimated for each system (see Table 1 and Figure 5) indicate that the electron density was transferred from the MeO molecule to the superhalogen moiety in all cases considered. The largest charge flow value (0.71–0.76 a.u.) was predicted for the MgO/Mg3F7 system and substantial ∆qESP were also calculated for CuO/Mg3F7 (0.60–0.63 a.u.), NiO/Mg3F7 (0.64–0.67 a.u.), and ZnO/Mg3F7 (0.68–0.71 a.u.) systems. Since such a large charge flow value is typical for an ionic compound, there is a strong indication that the CuO, MgO, NiO, and ZnO form the ionic products when combined with the Mg3F7 superhalogen. The similar conclusion appears when the charge flow in the CoO/Mg3F7 is considered as the ∆qESP in the 0.47–0.57 range was estimated. Clearly, the amount of the electron density withdrawn from CoO is smaller, yet approaching 0.6 a.u., hence, the CoO/Mg3F7 species might also exhibit ionic character. Table 1. The binding energies (BE in eV), the relative energies (ER in kcal/mol, see text for definition), the vertical ionization potentials (VIP in eV), the HOMO-LUMO gaps (GAP in eV), the MK charge localized on the metal atom (of MeO fragment, qMe in a.u.), the MK charge flow values (∆qESP and εSOMO in a.u.), and the spin density of the unpaired electron (localized over MeO molecules) calculated for the MeO/Mg3F7 systems. The BE and VIP values were calculated at the CCSD(T)/6-311+G(d) for the MP2/6-311+G(d) structures (M=Ni, Mg, Zn). The spin densities are the sums of the spin density population on the atoms of MeO molecule.

Species (Symmetry)

∆qESP Spin density GAP BE (MeO) 0.66 0.969 11.530 6.93 0.67 0.969 11.530 7.02

ER

qMe

1 NiO/Mg3F7 (C1) 2 NiO/Mg3F7 (C1)

0.00 1.3·10-4

1.32 1.33

3 NiO/Mg3F7 (C1)

5.18

1.27

0.64

1.007

13.140

6.67

10.636

4 NiO/Mg3F7 (C2v)

8.07

0.94

0.46

1.008

13.918

6.78

9.729

5 NiO/Mg3F7 (C1)

13.94

1.27

0.67

0.972

12.522

6.38

6 NiO/Mg3F7 (Cs)

16.29

1.14

0.52

0.961

12.562

6.17

10.637

7 NiO/Mg3F7 (Cs)

19.14

1.39

0.77

0.980

13.922

6.37

11.795

8 NiO/Mg3F7 (Cs)

22.29

1.01

0.59

0.997

12.115

5.79

8.902

9 NiO/Mg3F7 (C1)

24.57

1.30

0.67

0.965

12.750

5.73

11.551

1 MgO/Mg3F7 (Cs)

0.00

1.44

0.71

1.001

12.221

7.94

10.127


VIP 10.219 10.220

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2 MgO/Mg3F7 (Cs)

1.46

1.50

0.76

1.001

12.020

7.88

10.043

3 MgO/Mg3F7 (C2v)

11.16

1.43

0.71

1.010

13.118

7.48

10.709

4 MgO/Mg3F7 (Cs)

16.19

1.45

0.70

0.998

11.965

7.24

9.669

5 MgO/Mg3F7 (Cs)

29.88

1.41

0.70

0.995

13.267

6.64

11.244

1 ZnO/Mg3F7 (Cs)

0.00

1.37

0.71

1.010

13.148

6.38

10.811

2 ZnO/Mg3F7 (C1)

0.52

1.35

0.70

1.011

13.442

6.38

11.128

3 ZnO/Mg3F7 (C2v)

5.94

1.32

0.68

1.023

13.941

6.17

11.310

4 ZnO/Mg3F7 (Cs)

16.63

1.38

0.76

1.012

13.790

5.71

11.701

5 ZnO/Mg3F7 (Cs)

32.19

1.42

0.71

0.983

12.622

6.33

10.371

In order to further examine the charge flow issue, the analysis of the localization of the spin density of the unpaired electron was performed. The analysis of the spin density distribution is basically in line with the magnitudes of the charge flow for MeO/Mg3F7 systems. Explicitly, for the pair of the non-interacting reactants (closed-shell metal oxides (i.e. MgO, NiO, ZnO) and superhalogen), the whole spin density coming from the unpaired electron is localized on the Mg3F7 superhalogen component (as its ground electronic state is doublet). Hence, when the MeO/Mg3F7 product is considered, the significant amount of the spin density (of the unpaired electron) localized on the MeO moiety (MgO, NiO, ZnO) would indicate that the previously unpaired electron on the superhalogen became paired, whereas the MeO valence shell became open. Such a situation is clearly the case for all closed-shell metal oxides (MgO, NiO, ZnO) considered. Namely, the spin density localized on MeO approaches 1.000 (0.969– 1.010, see Table 1 and Figure 5) for MgO/Mg3F7, NiO/Mg3F7, and ZnO/Mg3F7 systems (at their lowest-energy structures). It implies that closed-shell MeO (Me=Mg, Ni, Zn) molecules lose an electron to become MeO•+ radical cations and the superhalogen Mg3F7• radical captures the electron to become the Mg3F7− anion. Therefore, the localization of the spin density of the unpaired electron is in accordance with the conclusion about the ionic character of the MeO/Mg3F7 (Me=Ni, Zn, Mg) systems. Due to the ionic bond character between MeO and Mg3F7, the MeO/Mg3F7 species can


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be seen as a superhalogen anion Mg3F7− in combination with a counterion MeO+. On the whole, the charge distribution integrity of Mg3F7− is maintained in the MeO/Mg3F7 systems. Compared with the recently reported 3 atomic charge distribution of Mg3F7−, only a slight influence from the counterion MeO+ on the Mg3F7− subunit is discerned. On the one hand, upon direct interaction with MeO+, the negative atomic charges on fluorine atoms (qF) are found to increase. On the other hand, the counterion indirectly influences the atomic charges on magnesium atoms (qMg) and leads to a larger qMg charges as compared to those in an isolated Mg3F7− anion. In particular, it was reported a positive charge of 1.50 a.u. on each Mg atom and the atomic charges on the terminal (F1, F6, F7), ring (F2, F4, F5), bridging (F3) fluorine atoms read –0.81, –0.76, –0.77 a.u., respectively, for the Mg3F7− anion at the MP2/6-311+G(d) level.3 In present work, the qMg charges of the Mg3F7 subunits are in the range of 1.51–1.68 a.u., while the qF charges vary from –0.93 to –0.72 a.u with the same method; see Figure 5. In addition, the charge distortion of the Mg3F7 fragments is more in MgO/Mg3F7 than in NiO/Mg3F7 and ZnO/Mg3F7, suggesting that the influence of MgO on the Mg3F7 subunit is stronger than that of NiO and ZnO.


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Mg3 F7 − (C3V)

1 NiO/Mg3 F7 (C1) ER= 0.00

− 0.81 F7 − 0.76 1.50 − 0.81 Mg1 F1 F2 1.50 Mg2 − 0.77 F3 − 0.76 F5 − 0.76 F4 1.50 Mg3

− 0.78

− 0.75

1.52

− 0.84

− 0.77

1.64 (0.003) − 0.74 (0.007)

1.51

1.64 (0.003) − 0.74 (0.007) − 0.84

− 0.75

− 0.83 (0.005)

1.33 (2.022)

1.61 (0.003) − 0.66 − 0.72 (0.014) (−1.054)

− 0.77

1.52

1.63 (−0.002) − 0.72 (0.003) − 0.83

1.51 − 0.75

1.60 (−0.003)

1.64 (−0.002)

1.64 (−0.002) − 0.75

− 0.85 (0.002) − 0.75

− 0.78

1.52 (0.001) − 0.85 − 0.77

1.64 (−0.004) − 0.86 (0.001)

1.64 (−0.004) − 0.74 (−0.002)

− 0.71 (−0.015)

1.44 (−0.053) − 0.73 (1.054)

− 0.78

1.27 (−0.104) − 0.65 (1.111)

1.68 (−0.001)

− 0.78 1.55 (−0.002) − 0.93 (0.002)

− 0.78

1.50 (−0.051)

− 0.74 − 0.84 (1.052) − 0.78 1.68 (−0.001) − 0.78

1 ZnO/Mg 3F7 (Cs) ER= 0.00 − 0.77

− 0.81 (0.010)

2 MgO/Mg 3F7 (Cs) ER= 1.46

− 0.85 − 0.77

− 0.65 (−1.054)

− 0.77

− 0.78

1 MgO/Mg 3 F7 (Cs) ER= 0.00 − 0.79

1.32 (2.022)

3 NiO/Mg 3 F7 (C1) ER= 5.18

2 NiO/Mg 3 F7 (C1) ER= 1.3 ˙10-4 − 0.76

− 0.72 (0.014) − 0.82 (0.005)

F6 − 0.81

− 0.78

1.60 (0.003)

2 ZnO/Mg 3 F7 (C1) ER= 0.52 − 0.78 − 0.77 − 0.74 (−0.002)

1.52

1.37 (−0.068)

− 0.77

− 0.67 (1.078)

− 0.85 1.64 (−0.004)

1.64 (−0.004) − 0.73 (−0.002) − 0.84 1.35 (−0.073) − 0.66 − 0.73 (1.085) (−0.002)

Figure 5. The MK partial atomic charges and the atomic spin densities (in parenthesis) for the most probable MeO/Mg3F7 (Me=Ni, Mg, Zn) isomers (ER within 5 kcal/mol); the charge distribution of the isolated Mg3F7– anion is also provided for comparison.3

To support the discussion, the three-dimensional pictures of the singly occupied molecular orbitals (SOMOs) for the most probable isomers (ER within 6 kcal/mol) considered are presented in Figure 6. The highly negative SOMO eigenvalues (εSOMO, see Table 1) reflect that the unpaired electron is strongly bound for these species. The SOMO orbitals, shown in Figure 6, confirm the character of the unpaired electron density distribution in the ionic MgO•+/Mg3F7− (1, 2) species, in which the entire half-filled MO is localized on the MgO fragment (and dominated by the contributions from the oxygen p atomic orbital). Such a picture


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is consistent with the unpaired spin density value of the 1.0 (Table 1) estimated for the MgO/Mg3F7 species. As far as the transition metal compounds are concerned, the SOMO is localized on the MeO moieties (Me=Ni, Zn) and dominated by the contributions from the oxygen p and the transition metal d atomic orbitals; see structures 1–3 NiO/Mg3F7 and 1–3 ZnO/Mg3F7 in Figure 6. Hence, I assume that the most stable isomers of the MeO/Mg3F7 (Me=Mg, Ni, Zn) systems represent two interacting ionic fragments (i.e. MeO+ and Mg3F7−).

Figure 6. The singly occupied molecular orbitals (SOMOs) for the MeO/Mg3F7 (Me=Ni, Mg, Zn) species (ER within 6 kcal/mol).

Having discussed the charge flow issue, I move on to the analysis of the interaction ACS Paragon Plus Environment

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between the two reactants (i.e., metal oxide and the Mg3F7 superhalogen) when they are assembled into a resulting compound. The zero-point corrected binding energy (BE) values were obtained at the CCSD(T)/6-311+G(d) level and are listed in Table 1; in BE calculations, the counterpoise procedure was used to eliminate basis set superposition error (BSSE). The interaction between MeO and superhalogen is the largest (7.94 eV) in the 1 MgO/Mg3F7 system. Significantly large binding energies were also calculated for MeO/Mg3F7 compounds involving the NiO (6.93 eV for 1 NiO/Mg3F7), CoO (6.18 eV for 1 CoO/Mg3F7), ZnO (6.38 eV for 1 ZnO/Mg3F7), and the CuO (4.50 eV for 1 CuO/Mg3F7) oxides. Performing the same method, the binding energies for the NaF, NaCl, KF and KCl molecules were obtained to be 4.45, 3.71, 4.50, and 3.70 eV, respectively. Clearly, the binding energies of MeO/Mg3F7 are comparable to or even larger than those of traditional ionic BEs, indicating a strong interaction between MeO and the superhalogen Mg3F7, and also signifying the stability of the studied MeO/Mg3F7 species. Therefore, the CoO/Mg3F7, CuO/Mg3F7, MgO/Mg3F7, NiO/Mg3F7, and ZnO/Mg3F7 systems exhibit ionic nature and should be treating as composed of metal oxide cation

interacting

with

superhalogen

anion,

i.e.

(CoO)+(Mg3F7)−,

(CuO)+(Mg3F7)−,

(MgO)•+(Mg3F7)−, (NiO)•+(Mg3F7)−, and (ZnO)•+(Mg3F7)−. The ionic character of these systems is also confirmed by the structure relaxation (Figures 1–4), corresponding charge flow values, spin density distributions (Table 1, Figure 5), and the SOMOs analysis (Figure 6). In all studied MeO/Mg3F7 compounds (Me=Co, Cu, Mg, Ni, Zn) the superhalogen acts as an electron acceptor and the electron density withdrawn from metal oxide molecule is determined by the tendency to bind an excess electron exhibited by this acceptor. Explicitly, the Mg3F7 radical (whose adiabatic electron affinity reads 7.932 eV)3 effectively ionize MeO molecules which is manifested by large binding energies, and substantial charge flows predicted for the (CuO)+(Mg3F7)−, (MgO)•+(Mg3F7)−, (NiO)•+(Mg3F7)−, and (ZnO)•+(Mg3F7)− compounds. The ionic nature of (CoO)+(Mg3F7)− seems weaker yet also evident as the BE of the 5.80-6.87 eV range and the ∆qESP of 0.5 a.u. are predicted for the (CoO)+(Mg3F7)− species. ACS Paragon Plus Environment

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This is consistent with the larger ionization energy of the CoO molecule (its adiabatic ionization potential reads 10.608 eV). 3.5. Metal Dioxides (MnO2, SiO2, TiO2) as Interacting Counterparts In order to demonstrate that the ionic nature of metal oxide/superhalogen compounds is the result of strong oxidizing ability of superhalogen counterpart, I extended my studies to cover three additional compounds (i.e. MnO2/Mg3F7, SiO2/Mg3F7, and TiO2/Mg3F7) in which monoxides were replaced with dioxides systems with AIP values up to ca. 12.6 eV. As I calculated at the CCSD(T)/6-311+G(3df) level, adiabatic ionization potentials of MeO (Me=Co, Cu, Mg, Ni, Zn) are in the 8.695–10.885 eV range, whereas AIPs of considered dioxides read 9.837 (MnO2), 12.581 eV (SiO2), and 9.611 eV (TiO2). Hence, MeO2 molecules, as characterized by ionization potentials significantly exceeding AEA of Mg3F7 superhalogen, are expected to interact differently with the Mg3F7 than the MeO species (Me=Co, Cu, Mg, Ni, Zn) do. According to received findings, the Mg3F7 cluster forms a stable compounds with TiO2 and MnO2 molecules, and the resulting lowest-energy structures (1 TiO2/Mg3F7, 2 TiO2/Mg3F7 (ER=0.17 kcal/mol), and 1 MnO2/Mg3F7) correspond to the DB forms with the transition metal (TM) atom linked to the Mg3F7 moiety through terminal fluorine (F6, F7) atoms. The remaining structures (i.e. isomers 3–6 for TiO2/Mg3F7 and 2–6 for MnO2/Mg3F7 in Figure 7) are shifted up in energy by 9 kcal/mol or more with respect to their global minima and hence their formation is less likely in the gas phase (despite their geometrical stability). Obtained results clearly reveal that the Mg3F7 fragment adopts a quasi-triangular configuration in low energy 1–2 TiO2/Mg3F7 and 1 MnO2/Mg3F7 compounds. Since the isolated Mg3F7 neutral system is known to adopt the C2v-symmetry structure with the three magnesia atoms lying in a straight line, whereas the corresponding Mg3F7− anion is triangular (C3v symmetry), I conclude that the Mg3F7 fragment adopts an anionic rather than a neutral geometrical configuration while assembled into 1–2 TiO2/Mg3F7 and 1 MnO2/Mg3F7 molecules. This conclusion is additionally ACS Paragon Plus Environment

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supported by Mg–F bond lengths observed in each of these species; namely, the F atoms (F6, F7) localized in the vicinity of the metal oxide molecules remains connected to the nearest Mg atom via typical (although elongated) Mg–F bonds (the differences in the Mg2–F6 and Mg3–F7 distances in comparison to the Mg3F7− anion structure do not exceed 0.09 Å), whereas its separation from the metal atom approach 2 Å. Due to relatively short Me–F6,7 separations (1.914 Å for 1 MnO2/Mg3F7 and 2.005-2.018 Å for 1–2 TiO2/Mg3F7; Table S5), these MeO2/Mg3F7 (Me=Ti, Mn) structures should be treated as strongly bound systems. Clearly, the amount of charge flow between

MeO2 and Mg3F7 is smaller than those observed for

monoxides (∆qESP=0.53 a.u. for 1 TiO2/Mg3F7 and ∆qESP=0.45 a.u. for 1 MnO2/Mg3F7). The lower ability of the Mg3F7 system to ionize the MeO2 (Mn=Mn, Ti) oxides seems consistent with the fact that the IP values of the TiO2 (IP=9.611 eV) and MnO2 (IP=9.837 eV) considerably exceed the electron affinity of the Mg3F7 superhalogen (AEA=7.932 eV). Clearly, the ionic nature of [TiO2]•+[Mg3F7]− and [MnO2]+[Mg3F7]− species seems weaker (in comparison to [MeO]+[Mg3F7]− compounds) but yet evident as the BE exceeding 6 eV and the ∆qESP of ca. 0.5 a.u. are obtained for those compounds.


23


F2

F1 Mg1

F2 F1 Mg1

F6

F3 F4

F3

Mg2

F4

Mg3

F5

F6

1.87Å

O2

1.60Å

F5

Mg3

Ti F7 1.87Å O2

1 TiO2 /Mg 3 F7 (CS)

2 TiO2 /Mg 3F7 (CS) ER= 0.17

ER= 0.00

Mg2

F2

O1

F4

F1

Page 24 of 41

F6 1.87Å

Ti O2 1.60Å F7 O1

3 TiO2 /Mg 3 F7 (CS) F7 1.61Å O1 ER= 8.98 Ti

Mg2

Mg1

F1 F2

F5

1.87Å O1 Ti F6 1.61Å O2

Mg2

Mg3

F4

F7

Mg3

Mg2

F7

F2

F3 Mg3 F5

F7

1 MnO2 /Mg 3 F7 (CS) ER= 0.00

1.58Å

F2 F4 Mg1

Mg2

F5

F3

Mg1

Mn F4 1.58Å

F2

Mg3 F5

Mg1 F3

F2

3 MnO2 /Mg 3F7 (C2v) ER= 11.18 Mg2 F5

Mg3

F4

F6

O1 1.59Å

F6 1.59Å O2

F7

Mn

5 MnO2/Mg 3 F7 (C2v) ER= 17.09 O1 1.57Å O2 Mn

F1

Mg1

1.58Å F6

F2 Mg2

F4

F3 O2 Mg2

F1

F6 1.71Å O2 Mn Mg3 O1 F7 1.71Å

6 MnO2 /Mg3 F7 (CS) O1 1.57Å E = 33.13 R

F1

F7

O1 O2

1.58Å

F1

1.58Å O1 O2 1.58Å

Mn

F7

F7

Mn Mg2 F6

F6

F5 Mg3

2 MnO2 /Mg 3F7 (C1) ER= 10.66

F4

Mg2

F4

F1 Mg1

Ti 1.87Å 1.59Å O1 O2

F3

Mg3

F5

F6

F5

F2

O2 F6

Mg2

F1 Mg1

O1 1.59Å F4 Ti 1.86Å F3

Mg1

F2

F3

F4

F1

6 TiO2 /Mg 3F7 (CS) ER= 38.18

5 TiO2/Mg 3 F7 (C1) ER= 20.50

F2

F1 Mg1

4 TiO2 /Mg 3 F7 (CS) ER= 18.76

Mg1 F3

Mg3

F5

F3

F3

4 MnO2 /Mg3 F7 (C1) ER= 11.86

F5

Mg3 F7

Figure 7. The equilibrium structures of the TiO2/Mg3F7 and MnO2/Mg3F7 compounds obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for 1-6 MeO2/Mg3F7 isomers (Me=Ti, Mn) with respect to the corresponding global minimum (1) are also provided.


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Figure 8. The equilibrium structures of the SiO2/Mg3F7 species obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for SiO2/Mg3F7 isomers with respect to the global minimum (1) are also provided.

It turns out that the lowest energy structure of SiO2/Mg3F7 (labeled 1 in Figure 8) should be treated as the [Mg3F5]+[F2SiO2]•− ionic compound. Explicitly, the F2SiO2 subunit in the 1 SiO2/Mg3F7 system is nearly tetrahedral (with the dihedral O1–Si–F7–F6 angle of 114.98˚), the F–Si–O (98.0–121.9˚) and O–Si–O (116.5˚; Table S6) valence angles approached 120˚, whereas the Si–O (1.579–1.663 Å) and Si–F (1.579–1.663 Å) separations were typical for the silicon–oxygen and silicone–fluorine bonds. Hence, it is assumed that the F2SiO2 fragment with ACS Paragon Plus Environment

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the O1–Si–F7–F6 dihedral angle of 114.98˚ resembles tetrahedral F2SiO2− anion as the corresponding values for the isolated neutral (F2SiO2) and ionized (F2SiO2−) species read 143.07˚ and 121.43˚, respectively, as calculated at the same theory level. In turn, the Mg–F bond lengths in the Mg3F5 subunit of lowest energy structure (1 in Figure 8) spanned the 1.788– 1.929 Å range and resembles those in isolated C2v symmetry Mg3F5+ cation (1.777–2.006 Å). Finally, the conclusion on the ionic character of the 1 isomer is additionally supported by the fact that the ∆qESP value, representing the charge flow between F2SiO2 and Mg3F5 in 1 F2SiO2/Mg3F5, is equal to 0.73 a.u., whereas the spin density localized on F2SiO2 reads 1.000. Therefore, in opposition to the CoO, CuO, MgO, MnO2, NiO, SiO2, TiO2, and ZnO systems, the SiO2 dioxide does not reduce to its cationic form but form F2SiO2− anion when interacting with Mg3F7 cluster. As consequence, superhalogen and silicon oxide undergo significant structural deformation upon their interaction leading to form [Mg3F5]+[F2SiO2]•− ionic compound. In summary, MnO2 and TiO2 dioxides interact similarly with the Mg3F7 cluster than the MeO species (Me=Co, Cu, Mg, Ni, Zn) do and the most stable isomers of the MeO2/Mg3F7 (Me=Mn, Ti) systems represent two strongly interacting ionic fragments (i.e. MeO2+ and Mg3F7−). The ionization potential of SiO2 molecule, however, is substantially larger than those observed for CuO, CoO, MgO, MnO2, NiO, TiO2, ZnO, metal oxides which resulting in a Mg3F7 inability to its effective ionization. In other words, the inability of the Mg3F7 system to ionize SiO2 molecule seems consistent with the fact that the IP value of SiO2 system (AIP=12.581 eV) significantly exceed the vertical electron binding energy of the Mg3F7− anion (VDE=10.479 eV). In consequence, the SiO2 was found to form the [Mg3F5][F2SiO2]compound (as its lowest energy isomer) rather than the desired [SiO2][Mg3F7] product. Clearly, the SiO2/Mg3F7 system, unlike the CuO/Mg3F7, CoO/Mg3F7, MgO/Mg3F7, MnO2/Mg3F7, NiO/Mg3F7, SnO2/Mg3F7, TiO2/Mg3F7, and ZnO/Mg3F7 molecules, consist of reduced form of magnesium fluoride and should be viewed as the [Mg3F5]+[F2SiO2]•− ionic compound.


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3.6 Vertical Ionization Potentials (VIPs) and Counterion Effect The vertical ionization potentials (VIPs), i.e., the energy needed to ionize an electron from the neutral cluster without changing its geometry, of the MeO/Mg3F7 isomers , were calculated at the CCSD(T)/6-311+G(d) level of theory and are presented in Table 1. The usefulness of the CCSD(T) method for calculating the VIP of the superhalogen salts, was confirmed by Li group.37 From Table 1, the VIPs of the most probable MeO/Mg3F7 isomers (ER within 6 kcal/mol) span the 10.043-13.458 eV range, which are considerably larger than the earlier reported VDE of 9.515 eV of the Mg3F7− anion at the CCSD(T)/6-311+G(3df) level (VDE=9.255 eV of Mg3F7− at the same CCSD(T)/6-311+G(d) level).3 This indicates that the introduction of a counterion to the superhalogen anion Mg3F7− can produce more stable species. Above consideration of the charge-compensated oxide–superhalogen species is essential for the experimentalists.

The correlation between the VIPs and the M atomic size is not straightforward, however, some tendencies could be noticed. As it is well known, the atomic radii for transition metal (TM) atoms decrease in the order Mn, Ti (1.40 Å) > Co, Cu, Ni, Zn (1.35 Å).38 This is consistent with the calculated VIP values possessing the largest value for 1 MnO2/Mg3F7 (13.458 eV) and 1 TiO2/Mg3F7 (12.448 eV), whereas VIPs for the 1 NiO/Mg3F7 and 1 ZnO2/Mg3F7 are significantly smaller and read 10.219 and 10.811 eV, respectively. Moreover, the higher VIP values always possess salts with umbrella-like Mg3F7 structure. In other words, preserving Mg3F7 anion structure results in higher stability of the corresponding salts. Explicitly, the lowest VIP values were estimated for 5 CuO/Mg3F7 (6.481 eV), 8 NiO/Mg3F7 (8.902 eV), 4 NiO/Mg3F7 (9.729 eV) in which umbrella-like structure cannot be distinguished (see Figures 2 and 3). These low VIP values are accompanied by lowest among studied species qMe values (i.e. qMe=0.97, 1.01, 0.94 a.u. for 5 CuO/Mg3F7, 8 NiO/Mg3F7, and 4 NiO/Mg3F7, respectively. These tendencies show a dependence of the VIPs on the partial atomic positive charge localized on the TM atom (qMe). The larger qMe, the larger VIP values. ACS Paragon Plus Environment

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In general, because the bonding between the superhalogen and the metal oxide is of ionic character, where the outermost electron of the metal oxide is transferred to the superhalogen, both the size of the metal oxide and its IP should play a role in the strength of the bonding. On the one hand, the lower the IP of the metal oxide, the stronger the binding should be as it is less costly to transfer an electron from the metal oxide to the superhalogen. On the other hand, since ionic bonding would be inversely proportional to the distance separating the metal oxide from the superhalogen, the metal atom with the smaller size should be stronger bound to the superhalogen because it would come closer to the superhalogen. Thus, the IP and atomic size of the metal atoms (of the MeO counterparts) compete with each other as far as their bond strength to superhalogen is concerned. 4. Conclusions On the basis of an ab initio CCSD(T)/6-311+G(d) computations on the MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) and MeO2/Mg3F7 (Me=Mn, Ti, Si) molecules (whose geometries were optimized at the MP2/6-311+G(d) level of theory), I provided (i) the equilibrium geometrical structure of each MeO/Mg3F7 system, (ii) the interaction energy between metal oxide and superhalogen moieties for each MeOn/superhalogen system, and (iii) the charge flow between reactants (i.e. MeOn and Mg3F7) which emphatics the amount of electron density transferred from the MeOn molecule to the Mg3F7 electron acceptor. The analysis of these results reveals that the Mg3F7 system due to its large tendency to bind an extra electron (manifested by the large adiabatic electron affinity, AEA=7.93 eV), is capable of ionizing metal oxide

molecules

and

forming

strongly

bound

(CoO)+(Mg3F7)−,

(CuO)+(Mg3F7)−,

(MgO)•+(Mg3F7)−, (MnO2)•+(Mg3F7)−, (NiO)•+(Mg3F7)−, (TiO2)•+(Mg3F7)−, (ZnO)•+(Mg3F7)− salts. The ionic character of these compounds is confirmed by the predicted binding energies (at the CCSD(T)/6-311+G(d) theory level) spanning the 4.89-8.26 eV range and the substantial MeOn-to-superhalogen electron density flow (manifested by the calculated charge flow and unpaired spin density evaluation). On the other hand, the Mg3F7 superhalogen was found to be


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incapable of an ionizing molecule whose ionization potential exceeds 12 eV (i.e. SiO2). Instead [Mg3F5]+[F2SiO2]•− is formed which is consist of Mg3F5+ cation and radical F2SiO2•− anion. Since the adiabatic ionization potentials (AIPs) of CoO, CuO, MgO, MnO2, NiO, SiO2, TiO2, and ZnO read 10.61, 10.12, 8.70, 9.84, 9.38, 11.42, 12.58, 9.61, and 10.88 eV, respectively, and knowing that the each of them form ionic (MeOn)+(Mg3F7)− products when combined with Mg3F7 superhalogen while SiO2 does not, I postulate that Mg3F7 superhalogen should be capable of ionizing metal oxide molecules with ionization potentials approaching 11.4 eV. This observation not only reveals superhalogen’s ability to accept an excess electron as a key factor for predicting the stability of MeO/superhalogen species but also introduce the effective way for formation positively-charged magnesium fluorides with using silicon oxide. Supplementary Material See supplementary material for the geometrical parameters of the MeO/Mg3F7 (Me=Co, Cu, Mg, Ni, Zn) and MeO2/Mg3F7 (Me=Mn, Si, Ti) species studied as well as the binding energies, the charge flow values, and the spin density of the unpaired electron (localized over MeO molecules) calculated for the MeO/Mg3F7 systems (Me=Mg, Ni, Zn).

Acknowledgments This work was supported by the Polish National Science Centre (NCN) Grant No. NCN UMO2012/07/D/NZ7/04342. C.S. thanks the Foundation for Polish Science (START 2016 Programme) for granting her with a fellowship. Calculations have been carried out using resources provided by Wroclaw Centre for Networking and Supercomputing (http://wcss.pl), grant No. 378. References 1.

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Figure 1. The CoO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level; selected interatomic distances in Å. The CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for 1-9 CoO/Mg3F7 isomers with respect to the global minimum (1). 254x482mm (72 x 72 DPI)


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Figure 2. The NiO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for 1-9 NiO/Mg3F7 isomers with respect to the global minimum (1) are also provided. 254x443mm (72 x 72 DPI)



Figure 3. The CuO/Mg3F7 equilibrium structures obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for CuO/Mg3F7 isomers with respect to the global minimum (1) are also provided. 254x385mm (72 x 72 DPI)


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Figure 4. The equilibrium structures of the MgO/Mg3F7 and ZnO/Mg3F7 species obtained at the MP2/6311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for MeO/Mg3F7 isomers (Me=Mg, Zn) with respect to the corresponding global minimum (1) are also provided. 254x531mm (72 x 72 DPI)



Figure 5. The MK partial atomic charges and the atomic spin densities (in parenthesis) for the most probable MeO/Mg3F7 (Me=Ni, Mg, Zn) isomers (ER within 5 kcal/mol); the charge distribution of the isolated Mg3F7– anion is also provided for comparison.3 254x480mm (72 x 72 DPI)


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Figure 6. The singly occupied molecular orbitals (SOMOs) for the MeO/Mg3F7 (Me=Ni, Mg, Zn) species (ER within 6 kcal/mol). 254x416mm (72 x 72 DPI)



Figure 7. The equilibrium structures of the TiO2/Mg3F7 and MnO2/Mg3F7 compounds obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for 1-6 MeO2/Mg3F7 isomers (Me=Ti, Mn) with respect to the corresponding global minimum (1) are also provided. 254x597mm (72 x 72 DPI)


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Figure 8. The equilibrium structures of the SiO2/Mg3F7 species obtained at the MP2/6-311+G(d) level. Selected interatomic distances (in Å) and the CCSD(T)/6-311+G(d) relative energies (ER, in kcal/mol) estimated for SiO2/Mg3F7 isomers with respect to the global minimum (1) are also provided. 254x434mm (72 x 72 DPI)


Oxidizing Metal Oxides with Polynuclear Superhalogen: An ab Initio

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