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

Online electrospray pneumatic ionization mass spectroscopy of aq. microjets exposed to reactive gases has \nprovided unprecedented insights on the unique chem. that takes place in the outermost water layers of water. \nAfter describing essential tech. features, I will illustrate how water's steep interfacial d. gradient affects the \nrates and mechanisms of reactive interfacial events. Fenton chem. in O_3 (g) or H_2O_2(g) collisions with the \nsurface of aq. Fe^(2+) microjets proceeds ∼10^3 times faster than in bulk water satd. with these gases, and \ngenerates mono- and poly-iron, high-valent, high-spin Fe(II/III)_n ·Fe(IV)=O metal-oxos in high yields. The \nincomplete hydration shell of interfacial Fe^(2+) appears to support the barrierless approach of O_3/H_2O_2, and, \nhence, facilitate 2-e- O-atom over 1-e- transfers. The unambiguous mass-specific identification of metal-oxos let \nus characterize the higher reactivity of interfacial poly-iron species as O-atom donors. The ozonolysis of \nsesquiterpene solutes by O_3(g) yields Criegee intermediates at the interface, which react at similar rates with \nmM 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 \nthe gas-phase, our finding is consistent with interfacial H_2O concns. that are significantly lower than in bulk \nwater. In the oxidn. of aq. (H_2O, D_2O) benzoate-h5 and -d5 by .bul.OH(g), isotopic labeling lets us identify the \nphenyl-peroxyl radicals that ensue H-abstraction by .bul.OH and establish a lower bound (∼26%) for this \nprocess in interfacial water. This finding stands in marked contrast with the negligible extent of H-abstraction \nby .bul.OH both in the gas-phase and bulk water. The enhancement of H-atom abstraction in interfacial water \nis consistent with the relative destabilization of the more polar transition state for .bul.OH addn. vs. Habstraction \ndue to incomplete solvation at the low interfacial water densities.