The ozone oxidation of terminal unsaturated 7-octenyltrichlorosilane self-assembled monolayers (C8= SAMs) on ZnSe and on SiOx-coated ZnSe was followed as a function of time using Fourier transform infrared spectroscopy (FTIR). Zinc selenide substrates have a major advantage over silicon crystals used in previous studies in that they transmit to approximately 650 cm-1, well beyond the cutoff of -1500 cm-1 for silicon ATR crystals, and thus allow detection of additional product species. When uncoated ZnSe or SiOx-coated ZnSe ATR crystals with a C8= SAM are exposed to gaseous ozone at concentrations from 1013 to 1015 molecules cm-3 at 1 atm pressure in He at 296 K, peaks due to C=CH decrease and a strong product peak at 1110 cm-1 as well as a weaker peak at 1385 cm-1 increase, both of which are attributed to the formation of a stable secondary ozonide (1,2,4-trioxolane, SOZ). Peaks at 2860 and 2929 cm-1 are also formed, which we tentatively assign to the C-H stretch of the OC-H group in the SOZ. The magnitude of the 1110 cm-1 band relative to those due to carbonyl groups at ~1722 cm-1 suggests that SOZ may, in fact, be the major reaction product. The SOZ has a sufficiently long lifetime when formed in the condensed phase that it is stabilized on the surface, indicating secondary ozonides may be formed during organic oxidations on surfaces in the atmosphere.
McIntire, T. M., O. Ryder, B. J. Finlayson-Pitts
J. Phys. Chem. C