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H magnetic-angle spinning (MAS) NMR measurements have been performed on a number of crystalline titanias, and on amorphous silica-supported titania and titania-silica, with the aim of measuring the characteristic proton chemical shifts of hydroxy groups bound to titanias of different crystalline form. In the case of anatase, signals observed at 6 = 2.3 and 6.7 ppm correspond to terminal and bridging hydroxy groups, the results of deuterium exchange experiments (using D,O) and IR data supporting these assignments. For rutile, signals observed at 6 = 2.2 and 5.3 ppm are similarly assigned. Hydroxy groups bound to amorphous titania supported on silica (containing tetrahedrally coordinated Ti") are found to possess a characteristic chemical shift of 6 = 3.3 ppm. Deconvolution of 'H NMR spectra of titania-silica (containing 8 wt.% Ti) indicate the presence of a signal at 6 3.3 ppm, which is similarly assigned to hydroxy groups bound to tetrahedrally coordinated Ti", together with signals assigned to anatase and silanol groups. These observations are consistent with literature reports indicating the presence of two main titania phases in titania-silicas : an amorphous phase containing isolated Ti sites tetrahedrally coordinated by Si-0 and OH groups, and segregated nanodomains of TiO, (anatase or rutile).
The n-π* transitions (absorption, circular dichroism, luminescence, linear and circular polarization of luminescence) of trans-β-hydrindanone (I) and trans-β-thiohydrindanone (II) are discussed. The C2 symmetry of I and II is taken into account with the aid of expressions that are derived for nonrigid molecules having a twofold axis of symmetry. This allows the absorption of I to be split into a part due to transitions polarized along the twofold axis (z axis) and a part due to x, y-polarized vibration-induced transitions. Accurate values for the electric and magnetic transition moments then are obtained. The vibrational structure of absorption and fluorescence and the latter's low degree of circular polarization are related to nonplanarity of the carbonyl group in the (n → π*)1 state. A larger nonplanarity probably is present in the triplet state of I. Circular polarization in the fluorescence was also observed when racemic I was excited with circularly polarized light. For the absorption of II, an upper limit is found for the electric transition moment and a lower limit for the magnetic moment. These give no indication that 3d orbits play an important part in the optical activity of thioketones. The luminescence at − 196°C and the longest wavelength part of the visible absorption band do not show circular polarization. They probably are singlet–triplet transitions. There is some evidence that the thiocarbonyl group is planar in the triplet and in the excited singlet state.
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