# Ozone Activation on TiO\({}_{2}\) Studied by IR Spectroscopy and Quantum Chemistry
## 3 Results  
### FTIR Spectroscopy Data  
When ozone is adsorbed at 77 K on titanium dioxide pretreated at 773 K, two groups of bands arise [7; 8]. One, with bands of stretching vibrations at 1109 and 1034 cm\({}^{-1}\) accompanied by a weak band of combination \(\
u_{1+3}\) mode at 2107 cm\({}^{-1}\) (close in position to those of free or dissolved ozone), gradually diminishes in intensity until almost disappearing after about 1 h. Weaker bands of another group with the maxima of fundamental vibrations at 1147 and 993-989 cm\({}^{-1}\) and a combination at 2136 cm\({}^{-1}\) are more stable and resist at 77 K.  
If we adsorb the ozone prepared from a 50:50% isotopic mixture of \({}^{16}\)O and \({}^{18}\)O, then in the region of the \(\
u_{1+3}\) combination immediately after adsorption the structure of six bands arises, forming two triplets typical of symmetrical O\({}_{3}\) molecules, as in the spectrum of ozone adsorbed on aerosolized SiO\({}_{2}\)[1]. After about 1 h, these bands significantly lose their intensity and a group of more strongly held ozone molecules becomes visible, consisting of eight maxima of almost the same intensity, as shown in Figure 1 (solid line). If ozone is prepared from an isotopic oxygen mixture containing the excess of usual \({}^{16}\)O (about 57%), the intensities of the eight constituents are not equal, as can be seen from Figure 2, where the figures at each peak show the relative intensity with respect to  
Figure 1: Experimental spectrum of a 50% \({}^{18}\)O isotopic mixture of ozone chemisorbed on TiO\({}_{2}\) (solid curve) and the calculated frequencies of the \(\
u_{1+3}\) combination mode of 8 ozone isotopomers forming a monodentate complex with the TiO\({}_{2}\) cluster (**1**), with 5- (**2**) and 4-coordinated (**3**) Ti\({}^{4+}\) ions of the Ti\({}_{20}\)O\({}_{40}\) cluster, and bidentate complexes with 5- (**4**) and 4-coordinated (**5**) Ti\({}^{4+}\) sites of the same Ti\({}_{20}\)O\({}_{40}\) cluster. The values 16 or 18 under the bands denote the positions of isotopic atoms in the ozone molecule: the lower index corresponds to the oxygen atom nearest the Ti\({}^{4+}\) cation. To provide the coincidence of the mean positions of the band of different species, the calculated values were multiplied by the following scaling factors: 0.9698, 0.8961, 0.9002, 0.9128, and 0.9107, for 1–5, respectively. the total intensity taken as 1. If the abundance of every isotopomer is purely statistical and the absorbance for \(\
u_{1+3}\) mode of all eight isotopic species is supposed to be the same (measurements for ozone isotopomers in liquid oxygen [14] show that for some vibrations it could be different for symmetrical molecules like \({}^{16}\)O\({}^{18}\)O\({}^{16}\)O and asymmetric molecules such as \({}^{16}\)O\({}^{16}\)O\({}^{18}\)O, but for the \(\
u_{1+3}\) mode the relative vibrational transition strengths are almost the same), then the relative intensities of molecules containing 0, 1, 2, or 3 \({}^{18}\)O atoms for isotopically mixed ozone with 57% \({}^{16}\)O should be 0.185, 0.140, 0.105, and 0.08, respectively. A comparison of these figures with those in Figure 2 enables us to divide the three peaks of mixed isotopomers into two groups: those with intensities of 0.17-0.13 and those with intensities of 0.10-0.09. These two groups of bands should naturally be associated with molecules containing one or two \({}^{18}\)O atoms, respectively. Thus, introduction of one \({}^{18}\)O atom shifts the band of \(\
u_{1+3}\) vibration from 2136 to 2115, 2103, or 2073 cm\({}^{-1}\), that is, by 21, 33, or 63 cm\({}^{-1}\). Apparently, the two first shifts correspond to substitution of terminal atoms, which are no more equivalent. Replacement of both by \({}^{18}\)O shifts the band to 2084 cm\({}^{-1}\) (by 52 cm\({}^{-1}\)), close to the sum of the two shifts, and we have a group of four bands, caused by the substitution of two terminal oxygens.