Superhalogen oxidizers capable of ionizing water molecules

Chemical Physics Letters 574 (2013) 13–17

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Superhalogen oxidizers capable of ionizing water molecules Marzena Marchaj, Sylwia Freza, Olimpia Rybacka, Piotr Skurski ⇑ ´ sk, Sobieskiego 18, 80-952 Gdan ´ sk, Poland Department of Chemistry, University of Gdan

a r t i c l e

i n f o

Article history: Available online 16 May 2013

a b s t r a c t The ability of ionizing water molecules by chosen strong oxidizing agents of superhalogen nature is demonstrated. It is shown that two selected Gutsev–Boldyrev superhalogens, BF4 and AlF4 (whose corresponding daughter anions BF4  and AlF4  are known to strongly bind an excess electron), might be employed to ionize single water molecule and small water clusters ((H2O)n, n = 2–4) which results in forming [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] stable species (n = 1–4). The [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] molecules are characterized by large values of binding (interaction) energy (28–73 kcal/mol) and substantial charge flow (0.5–0.8 au) between the components which confirms their ionic nature. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Superhalogens are inorganic compounds exhibiting enormously large electron affinities (significantly exceeding that of the Cl atom). The existence of these systems was postulated in 1981 by Gutsev and Boldyrev who employed various theoretical chemistry tools to support their hypothesis [1]. In fact, this prediction not only allowed for introducing novel class of molecular anions (formed by an excess electron binding to superhalogen molecules) but also greatly extended the range of the electronic stability the negatively charged systems might be characterized with. Namely, it is now well established that the molecular anions may exhibit even very large electron binding energies (manifested by their vertical electron detachment energies, VDE) [2–4] approaching 14 eV in certain cases [5]. In addition, a large number of novel superhalogen systems and their corresponding anionic daughters (i.e., superhalogen anions) have been proposed and studied by various theoretical and experimental groups thus far [6–19]. Even though the most common superhalogen systems (MLk+1) contain one central atom M decorated with k + 1 halogen ligands L, where k stands for the maximal formal valence of atom M (NaCl2, BeBr3, AlCl4, SiF5, and PF6 are representative examples [1,6,7,9,18]), many larger polynuclear MnLnk+1 species (containing n central atoms) were also investigated [6,9,15]. It was also demonstrated by Anusiewicz that electrophilic and acidic functional groups might be used as ligands (e.g., Mg(NO2)3, Na(HSO4)2) [20,21], whereas the Jena group proposed and investigated an alternative class of strong electron acceptors containing non-modified superhalogens as ligands coordinated to the central atom M (e.g., Al(BF4)4, Mg(BH4)3) [13,16,17,19]. ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (P. Skurski). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.05.009

Recently we pointed out that one might also distinguish a ‘superhalogen fragment’ in certain well-known molecules which allowed for more thorough analysis of their stabilities [22]. This observation was followed by the formulation of the conclusion that the stability of some molecular systems (such as HAlCl4 acid and its salts [22], MgBF5, and MgBCl5 [23]) might be explained by the proper balance between the ionization potential (IP) and electron affinity (EA) of the fragments these systems consist of. For example, the stability of NaAlCl4 and KAlCl4 salts might be viewed as caused by the ability of AlCl4 ‘superhalogen fragment’ (whose anionic daughter binds an excess electron by 7.02 eV) [24] to ionize Na and K alkali metal atoms (whose IPs read 5.14 and 4.34 eV, respectively [25]), whereas the instability of the corresponding parent HAlCl4 acid might be explained by the inability of AlCl4 to ionize the hydrogen atom (whose IP is much larger and reads 13.60 eV [24]). The extension of this concept resulted in a successful utilizing selected Gutsev–Boldyrev superhalogens as strong oxidizing agents capable of forming stable ionic compounds with moderately reactive and exhibiting relatively large IPs molecules (such as silicon dioxide (SiO2), chloroform (CHCl3), and dichlorodifluoromethane (Freon-12, CCl2F2)) [12]. In this Letter we report our attempts to ionize small water clusters ((H2O)n, n = 1–4) by using two well-known superhalogens (BF4 and AlF4). Since the IP of an isolated single water molecule is large (12.6 eV) [26] while the IPs of water dimer, trimer, and tetramer are only slightly smaller (spanning the 11.8–12.3 eV range) [27] we decided to verify whether the BF4 and AlF4 superhalogens (whose usefulness to oxidize molecules exhibiting similar IPs was previously demonstrated) are capable of ionizing these systems. Knowing that the ionization potentials of even larger H2O oligomers (up to the hexamer) are in similar range (not exceeding 12.3 eV) [27] we believe that our findings should be valid also for water clusters of practically any size.

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2. Methods The equilibrium geometrical structures of the species studied and the corresponding harmonic vibrational frequencies were calculated by applying the second-order Møller-Plesset (MP2) perturbational method with the 6-31++G(d,p) basis set [28,29]. Using such obtained equilibrium structures we applied the coupled-cluster method with single, double, and non-iterative triple excitations (CCSD(T)) [30,31] and the same 6-31++G(d,p) basis while estimating the binding (i.e., interaction) energy (BE) between each superhalogen molecule (acting as an oxidizer) and various water clusters ((H2O)n, n = 1–4). Hence, the binding energies for the resulting (H2O)nX systems (where n = 1–4 whereas X stands for BF4, AlF4, BCl4, and AlCl4) were calculated as BE = E((H2O)n) + E(X)  E((H2O)nX), where E stands for the CCSD(T)/6-31++G(d,p) energy of a given species calculated for its equilibrium MP2/6-31++G(d,p) geometry. It is worth noting that such defined binding energies involve the energies of the isolated water clusters having their hydrogen bonds formed, thus the reported BE values are expected to represent only the strength of the interaction between the superhalogen systems and water clusters. The partial atomic charges (qESP) were fitted to the electrostatic potential according to the Merz–Singh–Kollman scheme [32]. On the basis of such calculated partial atomic charges we estimated the charge flow (DqESP) between the water clusters and superhalogen oxidizers in each (H2O)nX system (by summing up the qESP values for all atoms belonging to either (H2O)n (n = 1–4) or X (X = BF4, AlF4, BCl4, and AlCl4) component). Since the methods we used for the odd-electron systems (i.e., isolated neutral superhalogen molecules BF4, AlF4, BCl4, AlCl4, and the resulting (H2O)nX systems) are based on an unrestricted Hartree–Fock starting point (using the single-determinant reference wavefunction) it is important to ensure that little, if any, artificial spin contamination enters into the final wavefunctions. We computed the expectation value hS2i for the odd-electron species studied in this Letter and found values of 0.756-0.770 in all doublet cases, hence, we are certain that spin contamination is not large enough to affect our findings significantly. All calculations were performed with the GAUSSIAN09 (Rev.A.02) software package [33]. In order to avoid erroneous results from the default direct SCF calculations, the keyword SCF = NoVarAcc was used and the two-electron integrals were evaluated (without prescreening) to a tolerance of 1020 au. The optimizations of the geometries were performed using relatively tight convergence thresholds (i.e., 105 hartree/bohr (or radian) for the root mean square first derivative).

3. Results 3.1. The choice of superhalogen oxidizers and water clusters In order to demonstrate the ability of selected superhalogens to ionize water molecules we have chosen two such systems (BF4 and AlF4) because of their large electron affinities (6.66 and 7.88 eV, respectively). Our choice was also dictated by the fact that their anionic daughters (BF4  and AlF4  ) are known to possess significant VDEs (8.98 and 9.79 eV, respectively) [24] which indicates that the electron density, once abstracted from other system, should be tightly held in the vicinity of a superhalogen counterpart. In addition, both BF4 and AlF4 seem to be of particular interest due to their small molecular size and structural simplicity (most superhalogen anions characterized by such large VDEs are substantially larger systems). On the basis of our previous studies [12,23] we assumed that employing the superhalogen oxidizers with electron binding energies of 9 eV (or larger) might allow for effective

ionization of small water clusters whose vertical IPs are known to read 11.8–12.6 eV [26,27]. Since water molecules tend to form clusters of various sizes stabilized by the presence of hydrogen bonds, we decided to investigate the interaction of a given superhalogen system separately with water monomer, dimer, trimer, and tetramer. It is important to notice that the largest IP among these clusters corresponds to the H2O monomer (IP = 12.6 eV) [26], while the ionization potentials of larger clusters ((H2O)n, n = 2–4) are slightly smaller (with the smallest IP value of 11.8 eV for the dimer) [27]. Hence, one may anticipate that the conclusions we formulate for the ((H2O)n, n = 1–4) species (considering the possibility of oxidizing these systems) should also be valid for larger water oligomers (and therefore water in general). This prediction is supported by the fact that the IP values reported for water pentamer and hexamer (whose interaction with superhalogen oxidizers is not considered in this Letter) are similar to those for smaller clusters, namely, the IP of water pentamer reads 12.1 eV whereas the IP values found for various stable structures of water hexamer (i.e., ring, cage, book, and prism isomers of (H2O)6) span the 11.7-12.1 eV range [27]. Moreover, recent findings indicate that the vertical IPs of even larger H2O clusters (up to 20 monomers) decreases with the cluster size [34]. In this Letter we do not report detailed geometrical parameters (nor the corresponding vibrational frequencies) of the (H2O)nX (n = 1–4, X = BF4, AlF4) systems (they are available upon request). Instead, we limit our discussion to graphical presentation of all equilibrium structures together with providing numerical values of selected bond lengths and angles that are necessary to quantitatively demonstrate the deformations both interacting components (i.e., superhalogen and water cluster) are subjected to. In addition, we discuss the binding energies of the interacting systems and the amount of charge flow (corresponding to the amount of the electron density transferred from a given water cluster to superhalogen system). Having these arguments at hand, we provide the conclusions about the ability of BF4 and AlF4 oxidizers to ionize water clusters. 3.2. Structural deformation of superhalogens and water clusters upon ionization It seems justified to assume that the initial structures of both systems (i.e., the structures of superhalogen oxidizer and water (either H2O or (H2O)n, n = 2–4) before any interaction between them occurs) correspond to their neutral ground state equilibrium geometries. Hence, it is important to recall that in the case of the superhalogens considered: (i) the equilibrium structure of the neutral BF4 system resembles a slightly deformed triangular planar BF3 fragment (with 1° deviation from pure planarity) with the fourth fluorine atom tethered to the central boron atom through the elongated (2.809 Å) bond; similarly (ii) the structure of the neutral AlF4 corresponds to the AlF3 triangular subunit (with 10° deviation from planarity) and the fourth F atom linked to it through the Al–F bond (2.235 Å), see Figure 1(top). The most stable structures of the neutral water clusters (excluding the monomer) are also depicted in Figure 1(top) and resemble those established recently by Segarra-Martí and co-workers at analogous theory level [27]. In particular, the lowest energy structure of the water dimer corresponds to the trans orientation of the monomers (connected via one hydrogen bond) whereas the ring-type structures are the equilibrium geometries for both (H2O)3 and (H2O)4 (stabilized by three and four H-bonds, respectively). One should also notice that each of the H2O monomers assembled into (H2O)n (n = 2–4) neutral clusters approximately preserves its original structure (i.e., typical for isolated H2O); moreover, subunits of different types (e.g., H3O, OH) are absent. This last observation is particularly important because

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Figure 1. The equilibrium structures of the neutral (top) BF4, AlF4, (H2O)2, (H2O)3, (H2O)4, and ionic (bottom) BF4  , AlF4  , [(H2O)2]+, [(H2O)3]+, [(H2O)4]+ species.

it allows to distinguish such neutral water clusters from their corresponding cationic daughters (depicted in Figure 1(bottom)), as we demonstrate later in this section. Having these results in mind we investigated the structural changes that occur when such water clusters interact with either BF4 or AlF4 superhalogen. The resulting structures of the (H2O)nBF4 and (H2O)nAlF4 (n = 1–4) are depicted in Figure 2. In each of these systems one may easily notice the presence of H3O fragment, presumably positively charged, which suggests the cationic character of water cluster (the H3O subunit is always present in the cationic [(H2O)n]+ (n = 2–4) clusters, as shown in Figure 1(bottom)). In the case of a single H2O interacting either with BF4 or AlF4, one of the hydrogen atoms is located in the vicinity of a superhalogen, yet remains connected to the oxygen atom via an elongated bond (presumably H-bond) whose length reads 1.735 Å for (H2O)BF4 and 1.378 Å for (H2O)AlF4. The monomers in all water clusters involved in (H2O)nBF4 and (H2O)nAlF4 systems are oriented in a way that enables stabilizing interaction of local water dipoles with electronegative fluorine atoms (due to the formation of hydrogen bonds). In particular, the lengths of the H-bonds involving fluorine atoms span the 1.321–1.765, 1.472–1.841, and 1.533–1.813 Å range, for (H2O)2BF4, (H2O)3BF4, and (H2O)4BF4, respectively, whereas the analogous ranges for (H2O)2AlF4, (H2O)3AlF4, and (H2O)4AlF4 were found to read 1.557–1.751, 1.487–1.712, and

Figure 2. The equilibrium structures [(H2O)n]+[AlF4] molecules (n = 1–4).

of

the

ionic

[(H2O)n]+[BF4]

and

1.481–1.720 Å, respectively (see Figure 2 where these H  F hydrogen bonds are shown). As far as the structures of superhalogen fragments in (H2O)nBF4 and (H2O)nAlF4 (n = 1–4) are concerned, we also observe significant geometrical changes upon their interaction with water clusters. Likely the most important observation is the quasi-tetrahedral orientation of four F ligands around the central atom (B or Al) in the resulting systems, see Figure 2. Namely, the F–Al–F–F dihedral angles in (H2O)nAlF4 are deviated from the 120° (i.e., the value typical for perfect tetrahedron) by only ca. 3° (in (H2O)AlF4), 4° (in (H2O)2AlF4), 5° (in (H2O)3AlF4), and 1° (in (H2O)4AlF4), whereas the corresponding deviations in (H2O)nBF4 read 4° (in (H2O)2BF4), 1° (in (H2O)3BF4), and 3° (in (H2O)4BF4). The only resulting system in which the superhalogen fragment does not adopt the tetrahedral structure is the (H2O)BF4 (the corresponding F–B–F–F dihedral angle reads 168°). However, even in that latter case, the BF3 fragment became less planar (by 11°) in comparison to its structure in the isolated BF4. Recalling that the BF4  and AlF4  anions are tetrahedral (Td-symmetry) species, we conclude that the superhalogens assembled into the (H2O)nBF4 and (H2O)nAlF4 (n = 1–4) tend to adopt the geometrical structures typical for their anionic daughters.

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Our analysis of the structural deformations in the superhalogen and water interacting systems indicate that the water clusters resemble their cationic daughters (which is confirmed by the presence of the H3O fragment) while the superhalogens adopt their typical anionic structures. Therefore, we conclude that these structural changes might be treated as the first evidence of the effective ionization process that occurs in all (H2O)nBF4 and (H2O)nAlF4 (n = 1–4) systems considered. We are aware of the fact that such a geometrical evidence might not be sufficient to confirm the ionic nature of the (H2O)nBF4 and (H2O)nAlF4 molecules, thus later in this section we provide the analysis of the interaction (i.e., binding) energy and the charge flow. However, since the first evidence of the ionic nature has already been offered, in the following sections we use the [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] notation while referring to those systems. 3.3. Binding energy and charge flow in the superhalogen/water systems The binding energy seems one of the most important criterion that allows to quantitatively judge whether the nature of the interaction between the fragments may be considered as ionic. The estimated BE values (between the superhalogen and water in superhalogen/water systems considered) are collected in Table 1. Recalling that our binding energies involve the energies of the isolated water clusters having their hydrogen bonds already formed, we treat these reported BE values as representing only the strength of the interaction between the superhalogen systems and water clusters (in the resulting [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] molecules). The BE value estimated for [H2O]+[BF4] reads 27.5 kcal/mol while the corresponding binding energies for larger water clusters interacting with the BF4 superhalogen are even larger (38.7 kcal/ mol for [(H2O)2]+[BF4], 46.3 kcal/mol for [(H2O)3]+[BF4] and 47.7 kcal/mol for [(H2O)4]+[BF4]), see Table 1. The smallest BE value found in the case of the water monomer might be explained by the fact that the IP of H2O (12.6 eV) is larger than the IPs of small water clusters, and thus water monomer is clearly less susceptible to ionization. In general, such substantial values of the binding energy (27–48 kcal/mol) support our assumption considering the ionic character of these species. Similar situation we observe for [(H2O)n]+[AlF4] systems investigated. Namely, the smallest BE of 46.3 eV we found for the AlF4 interacting with water monomer, whereas the binding energies for [(H2O)2]+[AlF4], [(H2O)3]+[AlF4], and [(H2O)4]+[AlF4] were estimated as equal to 65.5, 73.4 and 73.3 kcal/mol, respectively, see Table 1. As indicated in the previous case, such significant BE values (spanning the 46–73 kcal/mol range) strongly support our

Table 1 The binding energies (BE in kcal/mol, see text for definition) and the charge flow values (DqEPS in au) calculated for the ionic [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] molecules (n = 1–4). Species

Binding energy (BE) kcal/mol

Charge flow (DqEPS) au

[(H2O)]+[BF4] [(H2O)2]+[BF4] [(H2O)3]+[BF4] [(H2O)4]+[BF4]

27.53 38.70 46.27 47.71

0.53 0.81 0.82 0.83

[(H2O)]+[AlF4] [(H2O)2]+[AlF4] [(H2O)3]+[AlF4] [(H2O)4]+[AlF4]

46.32 65.50 73.38 73.27

0.63 0.83 0.78 0.82

The vertical ionization potentials of water clusters are: 12.6 eV (H2O), 11.79 eV ((H2O)2), 12.27 eV ((H2O)3), 12.27 eV ((H2O)4) (see Refs. [26,27]). The vertical electron binding energies of BF4  and AlF4  are 8.975 and 9.789 eV (see Ref. [24]).

conclusion about the ionic nature of the [(H2O)n]+[AlF4] (n = 1– 4) systems (one may also refer to our previous studies on the formation of [SiO2]+[AlF4], [CHCl3]+[AlF4], [NH3]+[AlF4], [CCl2F2]+[AlF4], [SiO2]+[AlCl4], and [NH3]+[AlCl4] molecules whose nature were also classified as ionic mainly due to large binding energies of 21–93 kcal/mol) [12]. The third evidence we present is based on the population analysis performed for each of the systems studied. It seems clear that the ionic (cationic) character of the water cluster assembled into the water/superhalogen system (and thus the ionic (anionic) character of the superhalogen component) has to be the result of a significant electron density flow from (H2O)n to the associated superhalogen counterpart molecule. Indeed, the population analysis (based on the partial atomic charges fitted according to the Merz–Singh–Kollman scheme [32] to reproduce the electrostatic potential) indicates the substantial charge flow (DqEPS) occurring when the [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] (n = 1–4) molecules are formed. Namely, the DqEPS values were found to span the 0.530.83 au range for [(H2O)n]+[BF4] and the 0.63-0.83 au range for [(H2O)n]+[AlF4] (n = 1–4, see Table 1). One may also recall that that the analogous charge flow values for ‘classic’ ionic components, such as LiF, LiCl, NaF, and NaCl, are similar (0.76-0.89 au, as determined at analogous theory level). Since the DqEPS value might be interpreted as the amount of the electron density transferred from the electron donor (water cluster) to the electron acceptor (superhalogen molecule), we believe that the conclusion about the ionic character of all [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] (n = 1–4) molecules is strongly supported and confirmed. 3.4. The comparison with weaker superhalogen oxidizers In order to demonstrate that the ionic nature of [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] (n = 1–4) molecules is the result of strong oxidizing ability of both BF4 and AlF4 superhalogens, we performed analogous calculations with selected water clusters and two different superhalogens, BCl4 and AlCl4. These particular superhalogens were chosen because of their structural similarity to previously investigated BF4 and AlF4 and due to their weaker oxidizing ability. Indeed, the EAs of BF4 and AlF4 are equal to 6.66 and 7.88 eV, respectively, whereas those of BCl4 and AlCl4 are substantially smaller (5.35 and 5.92 eV, respectively). The VDE values of the BF4  (8.975 eV) and AlF4  (9.789 eV) are also significantly larger than those of the BCl4 (6.218 eV) and AlCl4 (7.016 eV) anions [24]. Hence, the BCl4 and AlCl4 superhalogens, albeit characterized by relatively large electron affinities, are not expected to oxidize water clusters because of large ionization potentials of (H2O)n (n = 1-4). We demonstrate this by discussing the properties of two such molecules: (H2O)AlCl4 and (H2O)3BCl4. First of all, one may notice (see Figure 3) that the water molecules in both (H2O)AlCl4 and (H2O)3BCl4 seem intact, i.e., H3O fragments are absent, which indicates that water counterparts (either monomer in (H2O)AlCl4 or trimer in (H2O)3BCl4) are not ionized (the absence of H3O fragments is typical for neutral small water clusters, see Figure 1). The binding energies estimated for these molecules are rather small (comparing to those gathered in Table 1 and discussed in the previous section), namely, the BE predicted for (H2O)AlCl4 is 5.8 kcal/mol whereas that for (H2O)3BCl4 is 7.4 kcal/mol. Such small binding energies are likely related to the hydrogen bonds formed between the components rather than to any stronger interactions. The analysis of the charge flow (DqEPS) between the water and superhalogen counterparts supports our predictions as the DqEPS values estimated for (H2O)AlCl4 and (H2O)3BCl4 read 0.18 and 0.29 au, respectively. Hence, we conclude that neither BCl4 nor AlCl4 is capable of ionizing water clusters due to limited oxidizing strength of those superhalogens. However, we believe that providing these two ‘unsuccessful’ examples in this

M. Marchaj et al. / Chemical Physics Letters 574 (2013) 13–17

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the large estimated binding energies (28–73 kcal/mol) between the water and superhalogen components, and (iii) the charge-flow evidence consisting in the calculated substantial amount of electron density flow (0.53–0.83 au) from (H2O)n to the associated superhalogen counterpart molecule. Despite our results are limited to small water clusters ((H2O)n, n = 1–4), we believe that the BF4 and AlF4 superhalogens ability of oxidizing (ionizing) such systems might be extended to cover water clusters of practically any size (due to the fact that the ionization potential of water cluster tends to decrease with increasing cluster size, hence the ionization of larger water systems is not expected to require stronger oxidizers). Acknowledgments The authors thank Prof. A.I. Boldyrev for fruitful conversations that inspired this Letter. This research was supported by the Polish Ministry of Science and Higher Education grant No. BMN/5388371-B019-13 (to M. Marchaj) and by the Polish Ministry of Science and Higher Education grant No. 530-8371-D191-12. The computer time provided by the Academic Computer Center in Gdan´sk (TASK) is also gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Figure 3. The equilibrium structures of the non-ionic (H2O)AlCl4 and (H2O)3BCl4 complexes.

Letter (in a sense that the water clusters remain non-ionized while interacting with BCl4 or AlCl4) is helpful because it allows to better understand how various oxidizers affect (H2O)n (n = 1–4) systems. 4. Summary On the basis of ab initio CCSD(T)/6-31++G(d,p)//MP2/631++G(d,p) calculations we demonstrated that BF4 and AlF4 superhalogen molecules are capable of ionizing single H2O molecule and small water clusters (up to the tetramer). The ability of ionizing (H2O)n (n = 1–4) systems by these superhalogens is related to very large electron affinities (6.7–7.9 eV) characterizing BF4 and AlF4 molecules and to the significant vertical electron binding energies (8.98–9.79 eV) of their anionic daughters (BF4  and AlF4  ). We provided three evidences of the ionic nature of [(H2O)n]+[BF4] and [(H2O)n]+[AlF4] (n = 1–4) systems: (i) the geometrical evidence that mainly consists in the presence of H3O fragment (typical for the cationic water oligomers) in all water clusters assembled into the resulting systems, (ii) the energetic evidence that is based on

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