Photoreactor and bubble reactors were used to provide mechanistic insight in which singlet oxygen is directionally trapped at the air–water interface. Singlet oxygen was generated either top-down (photochemically) by delivery as a gas to an air−water interface or bottom-up (chemically) by transport to the air−water interface as a solvated species. In both cases, reactions were carried out in the presence of 7-carbon (7C), 9-carbon (9C), or 11-carbon (11C) prenylsurfactants [(CH3)2C=CH(CH2)nSO3− Na+ (n = 4, 6, 8)]. The use of the photo- and bubble-reactors for generation of singlet oxygen in alkene surfactant oxidations led to the ‘ene’ reaction of singlet oxygen and formation of a secondary and tertiary hydroperoxide at the air–water interface. Density functional theory-calculated reaction potential energy surfaces were used to help rationalize the reaction phase dependence. The reactions in the gas phase are mediated by perepoxide transition states, which evolve to well-defined stationary structures in the aqueous phase, with covalent C–O bonds and higher binding energy. The combined experimental and computational evidence points to a continuum of the perepoxide ranging from a transition state to an intermediate. The impact of the work goes beyond organic chemistry and is related to biologically relevant models of singlet oxygen at membrane or marine aerosol surfaces.

Probing the transition state-to-intermediate continuum: Mechanistic distinction between a dry vs wet perepoxide in the singlet oxygen ‘ene’ reaction at the air–water interface

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