# Ozone Activation on TiO\({}_{2}\) Studied by IR Spectroscopy and Quantum Chemistry
## 4 Discussion  
An unexpected result of our modeling was that for all the considered titania clusters we found two stable configurations, which we call monodentate and bidentate configurations. The real bidentate complex with two equivalent bonds, however, was formed only with the molecular TiO\({}_{2}\) cluster, which can be considered as a model of a highly unsaturated two-coordinated Ti\({}^{4+}\) center. The formation of two bonds with an energy value of 69.5 kJ/mol each made it energetically the most favorable. The only bond with the Ti atom in the monodentate complex is shorter and has higher adsorption energy of 92.7 kJ/mol. A greater adsorption energy as compared with bidentate complexes is typical of monodentate complexes with clusters of Ti\({}_{20}\)O\({}_{40}\). What we call here \"bidentate\" should rather be called \"tilted\" or even \"bridged\". In fact, the distance of the second terminal oxygen (O\({}^{3}\)) from the four-coordinated titanium ion is much greater than from the first one (3.090 against 2.277 A). Although the bond to the surface Ti\({}^{4+}\) ion is shorter than the only bond in the monodentate complex, the interaction energy is lower. This can probably be explained by the additional repulsion between ozone oxygen atoms and the surface oxygen anions. If we compare adsorption energies of ozone in monodentate complexes with four- and five-coordinated Ti\({}^{4+}\) ions, we find that interaction with the former is stronger. This is also reflected in the shorter O-Ti bond length and is a natural manifestation of the higher strength of coordinately more unsaturated centers. However, this explanation does not work for bidentate complexes, where the interaction with five-coordinated is stronger. Here we must consider that, as seen from Figure 6, the distance of the terminal oxygen atom from the titanium is even greater, at about 3.4 A. At approximately the same distance from the oxygen there is one more Ti\({}^{4+}\) ion and ozone complex can be considered as asymmetric \"bridged\" one. The formation of two bonds with the same cation here means an increase of its coordination number up to seven, that is completely not typical of titanium. Therefore, we suppose that the reason for the greater adsorption energy of such a complex with a weaker five-coordinated site is an additional attraction to the second cation. This means that interaction of ozone with a surface of oxide is not restricted to the bond formation with a single cation, but includes the influence of neighboring atoms. The difference between the fundamental frequencies \(\
u_{1}-\
u_{3}\) was considered as a criterion for the choice between the predicted ozone complexes on the titania surface [10]. These values for ozone in complexes **I**-**IV** are given in the right column of Table 1, and those calculated for the \({}^{18}\)O substituted ozone can be seen in Table 3. For a free ozone molecule this difference is 61.0 cm\({}^{-1}\)[14]. The calculated value is 66.5 cm\({}^{-1}\), and it does not change greatly with the rather strong interaction with the TiO\({}_{2}\) cluster (64.2 cm\({}^{-1}\)). The proximity of these values signals the reliability of our calculation, and the value itself can be considered as a measure of the resonance interaction between the vibrations of the two O-O bonds. An increase in the \(\
u_{1}-\
u_{3}\) difference for chemisorbed ozone of up to 152 cm\({}^{-1}\) in the experimental spectra or even more in our model calculations testifies to the redistribution of the fourth constants of the two bonds, apparently overestimated in the calculation. The calculated values of the \(\
u_{1}-\
u_{3}\) difference for the substituted ozone molecules are consistent with the suggested assignment. In fact, substitution of the first O atom nearest titanium by \({}^{18}\)O lowers the frequency of \(\
u_{3}\) vibration and does not affect the \(\
u_{1}\), leading to an increase in the \(\
u_{1}-\
u_{3}\) distance.