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

Abstract

Online electrospray pneumatic ionization mass spectroscopy of aq. microjets exposed to reactive gases has provided unprecedented insights on the unique chem. that takes place in the outermost water layers of water. After describing essential tech. features, I will illustrate how water's steep interfacial d. gradient affects the rates and mechanisms of reactive interfacial events. Fenton chem. in O_3 (g) or H_2O_2(g) collisions with the surface of aq. Fe^(2+) microjets proceeds ∼10^3 times faster than in bulk water satd. 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 Fe^(2+) appears to support the barrierless approach of O_3/H_2O_2, 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 O_3(g) yields Criegee intermediates at the interface, which react at similar rates with mM alkyl-C(O)OH acids and 23 M H_2O in MeCN:H O::4:1. Since both reactions proceed at very similar rates in the gas-phase, our finding is consistent with interfacial H_2O concns. that are significantly lower than in bulk water. In the oxidn. of aq. (H_2O, D_2O) benzoate-h5 and -d5 by .bul.OH(g), isotopic labeling lets us identify the phenyl-peroxyl radicals that ensue H-abstraction by .bul.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 .bul.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 .bul.OH addn. vs. Habstraction due to incomplete solvation at the low interfacial water densities.