Secondary Ozonide Formation from the Ozone Oxidation of Unsaturated Self-Assembled Monolayers on Zinc Selenide Attenuated Total Reflectance Crystals

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 similar to 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 loll 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-trioxoiane, 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 similar to 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.