Kathleen M. Fiehrer and Gilbert M. Nathanson*

Department of Chemistry, University of Wisconsin 1101 University Ave., Madison, Wisconsin 53706-1322

Received August 19, 1996

Concentrated sulfuric acid readily protonates organic moleules with oxygen-containing functional groups. This protonation is often strongly exothermic and shifts equilibrium between gaseous and dissolved species toward the solvated state.To dissolve in the acid, an approaching gas molecule must first collide with the acid's surface. The molecule may then scatter inelastically from the surface or bind momentarily to interfacial H2S04 before evaporating or remaining behind.The range of approach energies and angles toward the surface broad at thermal equilibrium, spanning a Boltzmann distribution of incident energies (Einc)and a cosine distribution of incident angles (THETAinc).

We explore in this letter how gas uptake in 98.8 wt % H2S04 depends on Ein, and THETAinc for organic species of

very different basicities. The molecular beam experiments demonstrate that sulfuric acid efficiently captures impinging organic molecules over a wide range of impact energies and approach directions. However, the net uptake

increases sharply with basicity, indicating that strongly basic molecules undergo longtime solvation while weakly basic species return more quickly to the gas phase.