Knowledge Bank

The reaction of nitric acid (HONO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$) with the hydroxyl radical (OH) is of significant atmospheric interest. Kinetic studies have revealed unusual behavior including a negative temperature dependence, pressure dependence, and an overall reaction rate strongly affected by isotopic substitution.} This suggests that the reaction occurs through an intermediate, theoretically predicted to be a hydrogen-bonded OH-HONO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ complex in a six-membered ring-like configuration.} In this study, OH-HONO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ is produced in the gas phase by the association of photolytically generated OH radicals with HONO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ in the collisional region of a supersonic expansion. Infrared action spectroscopy is used to identify the fundamental OH radical stretch (\nub{1}) and OH nitric acid stretch (\nub{2}) of the complex. The rotationally structured \nub{1} band} is centered at 3216 \wn, while the extensively broadened \nub{2} band is centered at 3260 \wn, both shifted from their respective monomer. Following infrared excitation, the OH fragments resulting from vibrational predissociation are detected by laser-induced fluorescence on the OH {\it A,}$2\Sigma +$-{\it X,}$2\Pi$ (1,0) transition. The highest observed OH (v=0) product channels, {\it J,}=17/2, $\Omega$=1/2 for the OH radical stretch and {\it J,}=15/2, $\Omega$=1/2 for the OH nitric acid stretch, lead to determination of an upper limit for the binding energy of OH-HONO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$, {\it D$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$}$\le$6.2kcalmol$^{-1}$,whichisconsistentwithabinitiopredictions.$^b$