9098 J. Phys. Chem. B 1997,101, 9098-9106

Jane K. Klassen,t Kathleen M. Fiehrer,* and Gilbert M. Nathanson*

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

Received: July 17, 1997

Molecular beam experiments demonstrate the diverse ways in which propane, propene, acetaldehyde, formic acid, dimethyl ether, and ethanol scatter from and react with 98.8 wt % H2SO4. The events following contact between gas and liquid include direct inelastic scattering from the acid's surface, trapping in the interfacial region followed by immediate desorption or desorption delayed by reversible protonation, and trapping followed by nearly irreversible protonation or further reaction. Molecules that scatter directly from the surface at incident and exit angles of 45 transfer from 70 to 80% of their impact energy of 90-110 kJ/mol. The fate of a molecule trapped at the interface depends on solute basicity and reactivity: the more basic molecules preferentially dissolve and undergo protonation rather than immediately desorb. Propane and propene desorb promptly, acetaldehyde and formic acid dissolve reversibly, and dimethyl ether and ethanol enter the acid for long times with little or no desorption following trapping. Substitution of 1-butanol for ethanol increases the size of the hydrophobic group but is insufficient to protect the OH group from protonation following thermal accommodation at the interface.