Predicted Acid Rain Intermediate Observed

An intermediate molecule predicted to form in the atmosphere during the reaction of a hydroxyl radical with nitric acid (HONO₂) has been observed spectroscopically for the first time (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0800320105). The reaction plays a key role in controlling the atmospheric concentration of nitrogen oxides, which influences the formation of acid rain. Structural details of the OH–HONO₂ intermediate, which has a unique six-membered ringlike configuration formed by two different hydrogen bonds, were elucidated by Marsha I. Lester and coworkers at the University of Pennsylvania and Joseph S. Francisco at Purdue University. The researchers mixed hydroxyl radicals with HONO₂ in gas-phase lab experiments to make OH–HONO₂. They used infrared spectroscopy to observe its spectral fingerprint and confirmed those measurements by studying the deuterated analog (OD–DONO₂) and conducting theoretical calculations. The findings are expected to allow scientists to better model how nitrogen-based pollutants cycle through the atmosphere.