Molecular Beam Scattering from Supercooled Sulfuric Acid:  Collisions of HCl, HBr, and HNO₃ with 70 wt D₂SO₄

Abstract

Proton exchange, residence time, and gas uptake measurements are used to explore collisions and reactions of HCl, HBr, and HNO 3 with 70 wt % D 2 SO 4 at 213 K. These studies help to provide a detailed picture of HX (X = Cl, Br, NO 3 ) energy transfer to sulfuric acid and the fate of the HX molecules immediately after thermalization at the D 2 O/D 2 SO 4 surface. We find that the three molecules readily dissipate their excess kinetic energy and become trapped momentarily in the interfacial region. However, only 11 ± 3% of the thermalized HCl and 22 ± 3% of the thermalized HBr molecules undergo H → D exchange; the HCl and HBr that do not react are found to desorb from the acid within 2 × 10 -6 s. In contrast, more than 95% of the initially trapped HNO 3 molecules are converted to DNO 3 . The HX molecules that undergo exchange dissolve within the deuterated acid for characteristic times of 5 × 10 -5 s (HCl), 3 × 10 -3 s (HBr), and 1 × 10 -1 s (HNO 3 ) before they desorb thermally as DX. The scattering experiments imply that the desorption of thermalized HCl and HBr molecules is, on average, faster than their solvation and reaction in the interfacial and bulk regions of 70 wt % D 2 SO 4 . Although HNO 3 is less acidic than HCl or HBr, it appears to hydrogen bond more strongly to surface D 2 O and D 2 SO 4, enabling it to be captured by the acid in nearly every collision.