Online electrospray pneumatic ionization mass spectroscopy of aqueous microjets exposed to reactive gases has provided unprecedented insights on the unique chemistry that takes place in the outermost water layers of water. After describing essential technical features, I will illustrate how water’s steep interfacial density gradient affects the rates and mechanisms of reactive interfacial events. Fenton chemistry in O3(g) or H2O2(g) collisions with the surface of aqueous Fe2+ microjets proceeds ~103 times faster than in bulk water saturated with these gases, and generates mono- and poly-iron, high-valent, high-spin Fe(II/III)n●Fe(IV)=O metal-oxos in high yields. The incomplete hydration shell of interfacial Fe2+ appears to support the barrierless approach of O3/H2O2, and, hence, facilitate 2-e- O-atom over 1-e- transfers. The unambiguous mass-specific identification of metal-oxos let us characterize the higher reactivity of interfacial poly-iron species as O-atom donors. The ozonolysis of sesquiterpene solutes by O3(g) yields Criegee intermediates at the interface, which react at similar rates with mM alkyl-C(O)OH acids and 23 M H2O in MeCN:H2O::4:1. Since both reactions proceed at very similar rates in the gas-phase, our finding is consistent with interfacial H2O concentrations that are significantly lower than in bulk water. In the oxidation of aqueous (H2O, D2O) benzoate-h5 and -d5 by ●OH(g), isotopic labeling lets us identify the phenyl-peroxyl radicals that ensue H-abstraction by ●OH and establish a lower bound (~26%) for this process in interfacial water. This finding stands in marked contrast with the negligible extent of H-abstraction by ●OH both in the gas-phase and bulk water. The enhancement of H-atom abstraction in interfacial water is consistent with the relative destabilization of the more polar transition state for ●OH addition vs. H-abstraction due to incomplete solvation at the low interfacial water densities.

Chemistry on a slide: Hydration gradient effects on rates and mechanisms at the air-water interface

254th National Meeting (2017)

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