Atmospheric oxidation of dimethylsiloxanes, a source of Si=O double bonds?

Volatile methylsiloxanes are pollutants in the atmosphere and are oxidized therein by hydroxyl radicals to siloxanols and other product classes. Under (simulated) atmospheric conditions, some stable oxidation products were recently obtained that were assigned by their authors to structures comprising Si=O double bonds, based on molecular formulas only that were derived from mass spectrometry. Here we compare the stability and even isolability of such compounds to what is known about Si=O double bonds and the few known compounds comprising them. The latter are highly reactive, thermodynamically and kinetically labile, and cannot be isolated except when stabilized by strong steric or electronic substituent effects. Si=O comprising compounds without such effects are at most observable at a very low temperature (12 K) in an argon matrix. In siloxane atmospheric oxidation products that are completely stable at ambient temperature and above, no stabilizing substituents are present. Therefore, structures featuring Si=O double bonds for these products are extremely unlikely. As for alternatives, we consider structures that instead of a Si=O double bond contain an additional ring, e.g., a Si-O-Si bridge, and a possible mode of such bridge formation. Our proposed structures, in contrast to those with Si=O double bonds, are compatible with all experience; in particular, they explain the stability of these compounds.