Proton and protonic entities in solid heteropoly compounds: An ab initio calculation of the environmental effect on the H5O2+ ion

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Journal of Molecular Structure 885 (2008) 131–138 www.elsevier.com/locate/molstruc

Proton and protonic entities in solid heteropoly compounds: An ab initio calculation of the environmental effect on the H5O2þ ion Ubavka B. Miocˇ a,*, Milena Petkovic´ a, Milorad Davidovic´ b, Miljenko Peric´ a, Tyno Abdul-Redah c a

Phaculty of Physical Chemistry, University of Belgrade, P.O. Box 276, 11001 Belgrade, PAC: 111101, Serbia b Institute Gosˇa, Research and Development, Milana Rakic´a 35, 11000 Belgrade, Serbia c Institute of Chemistry, Stranski Laboratory, Technical University of Berlin, D-10623 Berlin, Germany Received 20 June 2007; received in revised form 11 October 2007; accepted 18 October 2007 Available online 4 November 2007

Abstract Systematic experimental investigations of heteropoly compounds, particularly their structure and activity, led to the conclusion that most of their characteristics are governed by the presence of protons and protonic entities. Special attention has been paid to two forms of 12-tungstophosphoric acid: hexahydrate (WPA-6) and dehydrated phase (WPA-0). It was postulated that in WPA-6 dynamic equilibrium of protonic entities exists, and that dehydrated phase is stabilized by protons. To confirm the role of the ‘‘free protons” or ‘‘proton gas” derived on the basis of thermal, structural and spectroscopic experimental studies, we carried out also ab initio calculations on a number of systems containing H5 O2 þ ion. We were not able to perform direct calculations on the real systems investigated experimentally since the structure of heteropoly compounds is too complex. However, it has been found that H5 O2 þ ion in WPA-6 definitely is not planar and the results obtained indirectly support the postulated dynamic equilibrium, i.e. possibility of existing of free protons. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Heteropoly compounds; Proton motion; Infrared spectroscopy; Ab initio

1. Introduction More than a century heteropoly compounds (HPCs) of Keggin type [PM12O40]3 (M = W or Mo) have been the subject of extensive investigations focused on both fundamental research and technical applications [1]. The applications, mainly in analytical chemistry, biochemistry and catalysis are connected to the high redox properties, polarity, surface charge distribution, electron and proton transfer/storage ability, high charges and high ionic weights of heteropolyions. After discovery of superionic-proton conductivity [r = (1  100)  103 S/cm] by Nakamura [2,3] in 1979, chemical and electrochemical-conductive properties of selected compounds have been outlined and these

*

Corresponding author. Tel.: +381 11 3336 692; fax: +381 11 187 133. E-mail address: ubavka@ffh.bg.ac.yu (U.B. Miocˇ).

0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.10.033

compounds have become very interesting as new materials potentially useful for different solid state devices as sensors, electrolytes in H2/O2 fuel cells, electrochromic display, ionselective membranes and so on. Proton species, as charge carriers and hydrogen bonding have fundamental importance for fast protons transport and in explaining the mechanism of proton conductivity in HPCs. Keggin type HPCs have Td symmetry and the anion is built by tetrahedral XO4 group (X = P or Si) surrounded by 12 MO6 octahedra (Cs symmetry) which shared corners and sides, and form four linked M3O13 units (C3v symmetry). In such a structure 40 oxygen atoms form four groups: four Oa (C3v) linking a XO4 tetrahedron and each of four M3O13 groups, and 12 Ob atoms (Cs) in M–O–M bridges within the same M3O13 groups, 12 Oc atoms (Cs) in M– O–M bridging two different M3O13 groups, and 12 Od atoms (Cs) in terminal, unshared positions, Fig. 1 [4,5]. This structure is well known as primary structure of the

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Fig. 1. Primary structure of Keggin anion.

Keggin anion. The anions are interconnected by water molecules and/or other protonic entities forming the secondary structure [1,4,6]. Solid HPCs are known as tertiary structure and in this case their properties are governed by laws of solid state physics. It was interesting and necessary to see how these compounds change in the process of calcinations/dehydration and to determine structural phase transformations, region of their stability, to identify all protonic entities present in each phase and to follow their dynamics as a function of temperature. Although much experimental [1,6–8] as well as theoretical [4,5,9] effort has been spent on the investigation of the secondary structure of HPCs, many problems remain controversial. This concerns the details of the secondary structure, particularly that related to protons, and a better understanding of protonic entities behavior is thus desirable. Addressing this, in the present paper we are trying, on the basis of previous experimental results and ab initio calculation, to contribute to better understanding of this problem. 2. Review of experimental results Spectroscopic methods are very useful for investigation of water and protonic entities, hydrogen bonding and bond length or polarity. The results of thermal, spectral analysis have been correlated with structural ones (XRPD, X-ray powder diffraction), with the aim to complete the picture about processes in the secondary structure of Keggin ion caused by the process of calcinations/dehydration. In the process of calcinations the lower hydrates (n = 21, 14, and 6) are easily formed [6,8,10]. From DTA, DSC and TGA diagrams and XRPD analysis, all hydrate forms and structural phase transition were determined: 29-, 21-, 14-, and 6-hydrates, as well as dehydrated forms (WPA-0 and D-phase). The process of dehydration is finished at 250 °C for WPA (12-tungstenphosphoric acid) and for almost all the HPCs. Besides the phase transitions to different crystal hydrates, the Tg anomaly [8] at about 400 °C (broad peak)

is evident from the DTA curves. It corresponds to exothermic transformation, which is followed by the loss of mass (TGA curves) corresponding to a half or one water molecule. This process is finished at about 440 °C when the process of dehydration is completed. The denuded Keggin anion, the D-phase, is then formed [8], which could be related to those given by Fournier [11]. XRPD data confirm that the Keggin structure is preserved up to 602 °C, temperature corresponding to exothermic structural phase transition, i.e., solid–solid recrystallization of Keggin anions to phosphorus doped bronzes (PW8O26) [8]. To get an idea about proton and proton species in secondary structure, we studied the HPCs by IR, Raman and IINS spectroscopy. In the IR and Raman spectra of HPCs the bands belonging to the host lattice and those from proton species are easily distinguished. The bands above 1300 cm1 are mostly characteristic for proton species and those below 1300 cm1 for host lattice [4– 6]. Since in the present paper, special attention is paid to water molecules, hydrated proton species (H(H2O)n)+ and protons, the bands characteristic for these entities will be discussed. From the solid state spectroscopic data it is possible to distinguish: (i) protons in highly hydrated samples (n P 6); (ii) protonated water which is hydrogenbonded to terminal oxygen W = O  H+(H2O); (iii) protons which are directly bonded to bridging oxygen W– O(H)–W [9]. However, this division has to be understood conditionally, because in high hydrated samples few protonic species were identified: OH groups, H2O molecules, H3O+ and H5 O2 þ ions, being in dynamic equilibrium. IR and Raman spectra taken in situ at different temperatures (from room temperature down to 77 K), and the spectra of deuterated compounds [6,10] and neutron IINS spectra [12,13], showed the existence of various interactions (OH  H2O, H2O  H2O and H3O+  H2O) which were confirmed. The band at 3490 cm1 appears to be the strongest in the IR and Raman spectra and should originate from the most frequent species interaction, H2O  H2O. The band corresponding to m4 H2O vibration is found at about 1625 cm1 as a well defined medium intensity band. H5 O2 þ ion is identified on the bases of d(OHO) band at about 1100 cm1, and the OH groups by the band at about 3550 cm1. It can be summarized that oxonium ion forms the strongest hydrogen bonds with the neighboring water molecules and/or rigid framework oxygen atoms. The bands at 3292, 3225, 3140 and 1730 cm1 can be assigned to the OH stretching (m1 and m3), 2 (m2) and m2 vibrations of H3O+ ion, respectively. This assignment is supported by the fact that the relative intensity of these bands increases with temperature lowering similarly as that of the H3O+ band near 1720 cm1 (m4). The RO  O distances were calculated from the IR spectra, based on the empirical relationship between m(OH) frequency and RO  O distances given by Novak [14]. They are between 294 and 263 pm, and it is evident that the hydrogen bonds belong to weak and medium.

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Existence of different protonic species in HPCs was confirmed by the data obtained from dielectric measurements, i.e. from the Cole–Cole diagrams [e00 = f(e0 )]. The relaxation time for 21- and 14-WPA found to be 1010 s could be assigned to the relaxation of protonic species: H2O, H3O+, H5 O2 þ and/or H+ jump. For WPA-6 (frequency = 10 MHz) the relaxation time of s = 1.6  108 s was ascribed to reorientation of the oxonium ion and/or the water molecule. Expected times for the H2O reorientation and the H3O+/H2O jumps are 109 and 108 s, respectively [10,13]. All these results confirm the existence of different proton species in the structure of HPCs being in dynamic equilibrium. On the basis of thermal, XRPD and spectral data the equilibrium between different proton species and existence of free protons was assumed to be: þ þ H5 Oþ 2 () H3 O þ H2 O () H þ 2H2 O "

and correspond to the mentioned transitions of an almost isotropic oscillator. Similar spectra have also been observed in intermetallic hydrides [19,20]. A broad background in the IINS spectra was taken as evidence for mobile protons, ‘‘free protons” or ‘‘proton gas”. The broad, quasi-continuous intense absorption, which also appears in many liquid and solid systems, demands further careful studies by spectroscopic methods as well as theoretical calculations. Our IINS results were discussed by Ratajczak et al. [9] and compared with the following structure, postulated by Kozhevnikov [21] on the basis of 17O NMR spectroscopy:

ð1Þ

when the process of dehydration is finished, at about 250 °C, and it has been postulated that free protons stabilize the framework of HPCs. Hexahydrate (WPA-6) is the most convenient form for investigations of dynamic equilibrium between protonic entities: its structure is stable in the range 60–175 °C [8,13] and this structure is well known [15]. Crystallographic results for WPA-6 show that the structure of the single crystal WPA-6 hydrate is stabilized by the dioxonium ion, which is found to be rigid and almost planar with the proton about 4° above the mentioned surface. However, we have proven by IR spectra the existence of bands characteristic for (H5 O2 þ , H3O+and H2O). On the basis of these experimental results we conclude the existence of the above given dynamic equilibrium, Eq. (1) [6,10,12]. To confirm definitely the proposed equilibrium, neutron IINS spectra of WPA-6 were recorded in the process of calcination. In IINS spectra of WPA-6 [12] and other hydrates all bands characteristic for mentioned proton species are evident and our data are in accordance with the results of previous studies [16,17]. Changes in IINS spectra, in the process of dehydration are evident. At higher temperatures, the proposed equilibrium is displaced to the right. Analysis of IINS spectra in detail has been discussed in refs [12]. The spectrum WPA-0 recorded when 0.2 water molecules dehydrate (WPA-0.2 phase) 300 °C is very similar to that observed for manganese oxide (c-MnO2), where Fillaux [18] identified ‘‘free protons” or ‘‘proton gas” for the first time. The single strong band in the spectrum of (WPA-0.2) was assigned to a proton centered within a symmetric (octahedral) environment of oxygen atoms, without being preferentially bound to any particular oxygen. Three main peaks are at: 1150 cm1 0 ? 1 2290 cm1 0 ? 2 3300 cm1 0 ? 3

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The IINS spectra and recent neutron Compton scattering measurements on the VESUVIO instrument at ISIS (the spallation neutron source of the Rutherford Appleton Laboratory, UK), using high-energy neutrons (in excess of 1 eV), have confirmed our conclusions. The remaining ‘‘free protons” present in host lattice of WPA-0 phase have a different single-proton momentum distribution from those in WPA-6. The results show that the protons are more mobile in WPA-0 than in the WPA-6 phase on the time scale of 1015 s [22]. The existence of the dynamic equilibrium between proton entities in WPA-6 could be followed indirectly through the changes of the chemical bonds and the influence of the H+/H2O ratio on the lattice framework. The recent highresolution XRPD measurements and Raman spectra in situ of WPA-6 [23] confirm such behavior. These studies have shown how the H5 O2 þ ions can be reorganized and different protonic species be formed. They react with the Keggin anion and change both its crystal structure and microstructure. Dissociation of the H5 O2 þ ions, which interconnect Keggin anions via oxygen atoms (Od), influences the angles and interatomic distances. The changes are mostly pronounced at about 47 °C (Tc). We defined this temperature as the phase transition. The change in the ratio of the protonic entities modifies the electron density around the Keggin anions what causes the partially reversible reduction/oxidation of tungsten (W6+ M W5+) near Tc. This occurs very easily since tungsten is a mixed valence ion. At the same time, the color changes from white to blue. The Keggin ion, particularly the PO4 tetrahedron, shrinks by approximately 10% near Tc. At the same time, the space group of the crystal structure is preserved during the phase transition. The phase transition near 47 °C is thus defined as non-convergent. The process of reduction/oxidation is more expressed in the case of copper salt of WPA because in this salt two ions with mixed valence (copper and tungsten) exist [24].

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These results were also confirmed by Raman spectra where it is evident that the local symmetry of the PO4 group changes significantly near Tc. At the same temperature, a broadening of m1(PO4) vibration band at 1000 cm1 occurs, indicating a charge transfer in WPA-6 [23]. Additional HRNPD (high resolution neutron powder diffraction) data on the structure of WPA-6 have been obtained at different temperatures in the range 10–358 K [25]. The phase transition at ca. 47 °C was confirmed. An abrupt decrease of the P–Oa bond length and an increase in the planarity of H5 O2 þ ion was observed. Four H5 O2 þ conformations have been found, which is in agreement with ab initio calculation [26–28]. Nowadays the idea of ‘‘free proton” and ‘‘proton gas” seems to be generally accepted. The results of Colomban and Tomkinson [29] and those by Essayem [30] have shown that various protonic species and free protons are also present in deuterated WPA-n and its cesium salt Cs1.9H1.1PW12O40. Their relative amount is dependent on temperature. 3. Ab initio calculations As pointed out in the previous section, the experimental results suggest that the presence of the dioxonium ion, but not in the rigid form, is responsible for the stability of the WPA-6 structure. In order to get a better insight into the behavior of the bridging proton in the H5 O2 þ ion, we carried out a set of ab initio calculations. The investigations are based on the optimization of the structure and subsequent vibrational analysis, which is expected to reveal the dynamic properties of this species, i.e., we primarily aimed at describing the origin of the experimentally observed transition at 1150 cm1, which was assigned to ‘‘free proton” defined by Eq. (1). All quantum chemistry calculations were performed with the Gaussian program package [31]. First, we optimized the structure of an isolated H5 O2 þ ion in order to get an insight into how the system behaves in the gas phase. The optimization was performed with the B3LYP functional and 6-31+G(d,p) basis set. Although the ion is not too large and the computations could have been carried out with more sophisticated methods, we relied on the density functional theory that is less demanding and at the same time generally gives reliable results. Also, the presence of the diffuse and polarization functions should capture the most significant features of hydrogen bonded species The quality of the results obtained with the B3LYP functional are described in the work by Tauterman et al. [32], who studied a prototype hydrogen bonded system, malonealdehyde, and showed that the energy for the proton transfer computed at the B3LYP/6-31+G(d) level was in good agreement with the one calculated using CCSD(T) method and the aug-cc-pVDZ basis set. Further, they investigated double proton transfer in the formic acid dimmer [33] with the same functional, but they applied 6311G(2d,2p) basis, and the electronic energy barrier was

in this case quite low. This means that B3LYP gives satisfactory results in combination with smaller basis sets probably as a consequence of cancellation of errors. Starting from the planar structure (in the single crystal XRD experiments, it was namely found, that the hydrogen atom is presumably located 4° above the surface defined by the water molecules) but with no restrictions to the symmetry, the ion bent during the optimization, ending with a non-planar conformation shown in Fig. 2. This is not surprising having in mind the fact that the lone electron pairs of the oxygen atoms repel each other and it is that repulsion that leads to the twisted structure. In order to model the motion of the proton between two water molecules in heteropoly acids, we attempted to create an environment that would resemble Keggin units: Due to the restricted space and the presence of hydrogen bonds, the H5 O2 þ fragment is nearly planar in heteropoly acids. Keggin cages are far too large for handling with quantum-chemical methods, even if one restricts to the simplest method and the smallest basis set. For this reason, we were looking for another frame that would force the structure to remain planar. An option was to”iron” the ion with a crown compound C30H44O8 by forming hydrogen bonds between the oxygen atoms of the crown and the hydrogen atoms of the water molecules, like in heteropoly acids. It would have been hard (or most likely impossible) to design an identical structure, that is to reproduce all bond lengths and angles, and it is evident that the applied approach yields a qualitatively good picture. Due to the size of the ring, we limited the calculations to the HF/STO-3G level of theory, which should provide qualitatively reliable results. Nevertheless, though the initial structure of the system was chosen to be planar, some of the hydrogen bonds were broken during the optimization and the final structure was twisted, like in the gas phase (Fig. 3A). This showed that it seems to be impossible to keep the ion planar with this type of cage, since it was difficult to keep control over the whole system. Hence we turned to a simpler frame consisting of two CHO(CH)6CHO chains, each of them forming hydrogen bonds with one water molecule. This was supposed to make the system more flexible and prevent the hydrogen bonds of the original (planar) structure from breaking. However, the system of interest twisted again, which meant that the chain was not a good solution either (Fig. 3B). Therefore, we focused on burdening each of the four hydrogen atoms by hydrogen bonding with heavy systems. How heavy the burden was supposed to be was hard to guess, so we started with something simple, CO2 molecules Fig. 4A. This was another failure and we continued by adding more massive load: CH3CHO (acetaldehyde) (Fig. 4B), CH3COCH3 (acetone) (Fig. 4C), C6H4O2 (p-Benzoquinone) (Fig. 4D), C6H2 Cl2O2 (2,5-Cyclohexadiene-1,4dione,2,6-dichloro) (Fig. 4E), and C6H2I2O2 (2,5-Cyclohexadiene-1,4-dione,2,6-diiodine). Since the weight of the applied fragments was obviously not sufficient to keep the investigated ion planar, we turned to metal oxides:

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Fig. 2. (A) Planar and (B) Non-planar structure of the H5 O2 þ ion.

Fig. 3. An attempt to mimic the environment within the heteropoly acids using (A) a crown compound and (B) hydro-carbon chains.

MgO, CaO, SrO, and finally to WO3 (with the CEP-4G basis). However, none of the mentioned candidates led to success. Each time the planarity got lost. That is why we decided to apply another strategy, namely to focus on the system of interest, H5 O2 þ itself, and to try to elucidate its behavior in the heteropoly acids indirectly. More precisely, we tried to enquire whether the change of its geometry from the planar structure, as in the complete system considered (i.e. when it is surrounded by Keggin units), to the non-planar one (when it is isolated in the gas phase) has a crucial influence of the behavior of the key vibrational mode observed at 1150 cm1. Thus, we considered both the planar, and the non-planar structure of the isolated H5 O2 þ species. Both of them were treated at the same level of theory [B3LYP/631+G(d,p)]. The results for vibrational frequencies are presented in Table 1. By forcing all the atoms to lie in the same plane, i.e. by carrying out the computations in the framework of the Cs point group, one must deal with three imaginary frequencies (223, 268 and 332 cm1) all of which refer to out-of-plane motion, i.e., they try to bend the system. By examining the harmonic frequencies, we recognized for both the planar and the non-planar case a similar type of motion: A normal mode that is characterized by an almost pure motion of the hydrogen species that

connects the two oxygen atoms, Fig. 5. It moves back and forth between the two oxygen atoms. This kind of motion was actually observed in the experiments with the heteropoly acids. The harmonic values of those frequencies are 918 and 1004 cm1 for the planar and the non-planar structure, respectively. The anharmonic frequencies are 1432 and 1456 cm1. It is important to note the following: First, both the harmonic and anharmonic values are comparable (having in mind the usual accuracy of ab initio calculations of the vibrational frequencies at the present level of sophistication) for the two structures. This means that this type of motion is virtually independent of the angle between the planes of the two water molecules. Secondly, the discrepancy between the harmonic and the anharmonic values shows that the anharmonicity is rather pronounced. This is also obvious from the shape of the potential energy curve shown in Fig. 6. One should keep in mind that the anharmonic frequencies are generated in the calculations corresponding to a one-dimensional model and that the inclusion of other degrees of freedom would somewhat alter the obtained values. Nevertheless, the fact that the planar and the non-planar ion exhibit the same kind of motion leads to a conclusion that this sort of vibration is characteristic for the H5 O2 þ ion irrespectively of the mutual orientation of the water molecules, and consequently of the presence of the species surrounding the H5 O2 þ fragment. The calculated frequencies differ from the experimental value, the harmonic values underestimating and the anharmonic ones overestimating the corresponding experimental finding. The reason for this discrepancy is twofold: First, the ‘‘real” structure is not ‘‘completely” planar. Secondly, more sophisticated quantum-chemical methods, than the one used in this study, would most likely lead to more accurate results (which would, on the other hand, be hardly feasible for such a large system). Nevertheless, the computed data give qualitatively satisfactory picture. Moreover, the experimental value lies between the results of computations carried out in the framework of the harmonic approximation (i.e. in the calculations that take into account the mutual coupling of different internal vibrational coordinates but neglect the anharmonic effects) and those obtained in the one-dimensional anharmonic computations. This indicates that a treatment that would include both the coupling and anharmonic effects would lead to an even better agreement between the theoretical and experimental results. The

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U.B. Miocˇ et al. / Journal of Molecular Structure 885 (2008) 131–138 Table 1 Harmonic frequencies of the planar and the non-planar structure given in cm1, with the emphasized mode of interest Normal mode

Planar structure

Non-planar structure

Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Q11 Q12 Q13 Q14 Q15

332 268 223 432 621 672 918 1007 1656 1772 1789 3764 3775 3876 3878

191 303 373 485 500 618 1004 1471 1525 1678 1763 3742 3751 3849 3849

Fig. 5. Mode Q7.

Fig. 6. Potential energy surface along the mode of interest, Q7, for the non-planar and the planar structures (overlapped). Fig. 4. Utilization of different burdens: (A) CO2, (B) CH3CHO, (C) CH3COCH3, (D) C6H4O2, (E) C6H2Cl2O2.

strength of interaction of this mode to the other degrees of freedom can be investigated through higher-order derivatives of the potential at the equilibrium position. The lowest order anharmonic terms give the highest contribution to the anharmonicity, so we computed the cubic anharmonic force fields, which involve the mode of interest (Q7) by means of finite difference procedure from analytic second derivatives [34]. We followed the procedure given in [35].

The absolute value of the most significant term for the planar and the non-planar structure amounts to 1070 and 985 cm1, respectively. In both cases, it is a Kiij mode with i = Q7, and j is a mode which includes the distance change between the oxygen atoms. This shows that the coupling of mode Q7 to the other degrees of freedom is rather pronounced, and an adequate picture can only be obtained by considering a multidimensional model. Such approach was used by Vener et al. [36] who constructed a four dimensional model – three degrees of free-

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dom are the Cartesian coordinates of the bridging proton, and the fourth coordinate was the distance between the oxygen atoms. They described our mode of interest as an asymmetric vibration of the O  H  O fragment. The harmonic frequency computed with the second order Møller– Plesset perturbation theory and the anharmonic value of the 4D model amount to 1054 and 1158 cm1, respectively. The latter value is in excellent agreement with the experimental one. Further, the effective (Mulliken atomic) charge of the ‘‘free” bridging particle is calculated to be 0.60 a.u. when it is equidistant from the oxygen atoms, and 0.56 a.u. when its distance from the oxygen is the smallest. This means that the particle involved in the vibrational mode considered is neither an atom (with no net charge) nor a pure proton (charge equal to 1.00 a.u.) but has properties intermediate between these two entities. Furthermore, by substitution of the bridging hydrogen with deuterium in the planar structure, the harmonic value of the frequency of interest to 687 cm1, which matches quite well pamounts ffiffiffi with the 2 rule (expected 710 cm1). Those facts confirm that the active atom is actually the middle hydrogen atom, which would imply that the H5 O2 þ ion might in a sense be considered as two water molecules with the proton moving between them. Similar conclusion has also been drawn from the experiment (cf. Section 2). This conclusion is in accordance with the results presented in Ref. [37] for hydrogen-bonded systems by classical infrared spectroscopy approach concerning WPA-6 sample. But if one is focused on WPA-0 (where as we postulated on the basis of experimental results, the structure of Keggin anions is stabilized by protons), time-resolved infrared spectroscopy, particularly coherent anti-Stokes Raman (CARS) can be useful. This method is well known as one of the best laser spectroscopic methods for studying short lived molecular systems and also molecular dynamics of hydrogen-bonded liquids on the pico- and femtosecond time scale. Some theoreticians foresee the hydrogen bond on such time scale, especially in liquid water and ice, but theory is still far from maturity!” [37]. 4. Conclusion Theoretical ab initio computations were carried out in order to confirm previous experimental data and proposed dynamic equilibrium relation. The crystal structure of WPA-6 is a bcc packing of PW12 O40 3 and oxonium ion, but our experimental results have shown that there exists temperature dependent dynamic equilibrium of different protonic species. The existence of different protonic species influences many physical/chemical characteristics of HPCs. The present study predicts the existence of the freehydrogen/proton motion between the oxygen atoms in the dioxonium ion. Since we were not able to carry out the calculations on the complete system of interest, we applied another strategy: First, we investigated a number of systems involving the dioxonium ion and different frag-

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ments surrounding it. It was shown that none of the applied substitutes for the Keggin cage leads to a planar structure of the H5 O2 þ ion. Secondly, we showed that the crucial vibrational mode, namely the motion of the partially free proton between the two oxygen atoms of the dioxonium ion was virtually independent of the presence and/or kind of the surrounding fragments. Thus, although we cannot claim that the present results completely and finally confirm the experimental findings, but is was found that obtained results strongly support the model proposed on the basis of previous experimental investigations. Let us notice that such a behavior of proton species has been found in a number of related systems and represents thus a very general feature. Acknowledgment This work was financially supported by the Ministry of Science and Environmental Protection of Republic Serbia, within the framework of the projects 142047, 142055 and 141030 G. References [1] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983. [2] O. Nakamura, T. Ogino, T. Kodoma, Z. Migaka, Chem. Lett. (1979) 17. [3] O. Nakamura, T. Ogino, T. Kodoma, Solid State Ionics 3–4 (1981) 347. [4] M.J. Janik, A.C. Kimberly, B.B. Bardin, R.J. Davis, M. Neurock, Appl. Catal. A: General 256 (2003) 51. [5] A.J. Bridgeman, Chem. Phys. 287 (2003) 55. [6] U. Miocˇ, Ph. Colomban, A. Novak, J. Mol. Struct. 218 (1990) 123. [7] Ph. Colomban (Ed.), Protonic Conductors, Solid Membranes and Gels, Cambridge University Press, Cambridge, 1992. ˇ . Dimitrijevic´, M. Davidovic´, Z.P. Nedic´, M.M. [8] U.B. Miocˇ, R.Z Mitrovic´, Ph. Colomban, J. Mat. Sci. 29 (1994) 3705. [9] H.R. Ratajczak, A.J. Barnes, A. Bielanski, H.D. Lutz, A. Mu¨ller, M.T. Pope, in: M.T. Pope, A. Mu¨ller (Eds.), Polyoxometalate Chemistry from Topology via Self-Assembly to Applications, Kluwer Academic Publishers, Dordrecht/Boston/London, 2001, pp. 100–117. [10] U.B. Miocˇ, M. Davidovic´, N. Tjapkin, Ph. Colomban, A. Novak, Solid State Ionics 46 (1991) 103. [11] M. Fournier, Ch. Feumi-Jantaou, Ch. Rabia, G. Herve, S. Launay, J. Mater. Chem. 2 (1992) 971. [12] U.B. Miocˇ, Ph. Colomban, M. Davidovic´, J. Tomkinson, J. Mol. Struct. 326 (1994) 99. [13] U.B. Miocˇ, M.R. Todorovic´, M. Davidovic´, Ph. Colomban, I. Holclajtner-Antunovic´, Solid State Ionics 176 (2005) 3005. [14] A. Novak, Structure Bonding (Berlin) 18 (1947) 177. [15] G.M. Brown, M.-R. Noe-Spirlet, W.R. Busing, H.A. Levy, Acta Cryst. B 33 (1977) 1038. [16] D.J. Jones, J. Penfold, J. Tomkinson, J. Roziere, J. Mol. Struct. 179 (1989) 113. [17] G.J. Kearley, R.P. White, C. Forano, R.C.T. Slade, Spectrochim. Acta 46A (1990) 419. [18] F. Fillaux, C.H. Cachet, H. Ouboumout, J. Tomkinson, C. LevyClement, L.T. Yu, J. Electrochem. Soc. 140 (1993) 585. [19] R. Hempelmann, D. Rrichter, O. Hartmann, E. Karlsson, R. Wappling, J. Chem. Phys. 90 (1989) 1935. [20] B. Dorner, I.T. Belash, E.L. Bokhenkov, E.G. Ponyatovsky, V.E. Antonov, L.N. Pronona, Solid State Commun. 69 (1989) 121.

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