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

Abstract

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.