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 SiO x -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 SiO x -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.

Heterogeneous oxidation of a phosphocholine on synthetic sea salt by ozone at room temperature

“…This allows small changes in the presence of a large ''background'' to be followed quantitatively, as has been done for example, for the oxidation of self-assembled monolayers. [53][54][55][56] ) decrease. Ozone reacts with the double bond in POPC via the Criegee mechanism (Scheme 1).…”

Reactions at surfaces in the atmosphere: integration of experiments and theory as necessary (but not necessarily sufficient) for predicting the physical chemistry of aerosols

“…249 More recently, the ozonolysis of the 8-carbon alkene SAM has been revisited using ZnSe and SiO x -coated ZnSe ATR crystals. 250 SAMs on ZnSe are not nearly as well characterized as those on silicon, [251][252][253] but have the advantage that ZnSe transmits down to B650 cm À1 , well beyond the B1500 cm À1 cut-off for the silicon ATR crystal. This allows one to probe for additional products that absorb below 1500 cm À1 .…”

Reaction of gas phase OH with unsaturated self-assembled monolayers and relevance to atmospheric organic oxidations

“…The coating was repeated using 10 min exposure time, followed by boiling three times in dichloromethane after which the crystal was wiped using dichloromethane to remove physisorbed residues. 27,31 After coating, the ATR crystal was placed in the reaction cell under a flow of dry nitrogen overnight.…”

“…This is in contrast to previous studies of the ozonolysis of alkene SAMs in which complete reaction of the monolayer was manifested with no further changes in the vinyl peak intensities after B10 min of reaction with B10 14 O 3 cm À3 . 27,31 The fraction of CQC loss due to the OH reaction in the present studies was determined using the absorbance relative to that of a fully oxidized monolayer obtained by further exposing the OH oxidized film to ozone. Fig.…”

“…29 Reaction mechanisms can also be different. For example, ozonolysis of alkenes on solid substrates forms secondary ozonides (SOZ) 30,31 because the Criegee intermediate and aldehyde or ketone precursors are stabilized and held in sufficiently close proximity to combine, whereas the SOZ yields are negligible under most conditions in the gas phase. Thus, understanding the kinetics and mechanisms of interaction of atmospheric oxidants with condensed phase organics and how they differ from gas phase chemistry is of fundamental chemical interest.…”