Kinetics of the Removal of OH(v = 1) and OD(v = 1) by HNO3 and DNO3 from 253 to 383 K

We have measured the rate coefficients for the removal of OH( v = 1) and OD( v = 1) by HNO 3 and DNO 3 as a function of temperature from 253 to 383 K. OH( v = 1) and OD( v = 1) were produced by photolysis of HNO 3 /DNO 3 at 248 nm; laser-induced fluorescence was used to monitor the kinetics of the vibrationally excited radicals. The measured rate coefficients at 295 K range from 2.5 × 10 -11 cm 3 molecule -1 s -1 for the removal of OH( v = 1) by HNO 3 to 6 × 10 -12 cm 3 molecule -1 s -1 for the removal of OH( v = 1) by DNO 3; the rate coefficients for the like-isotope processes [removal of OH( v = 1) by HNO 3 and removal of OD( v = 1) by DNO 3 ] are 2−4 times higher than the rate coefficients for the unlike-isotope processes. All four rate coefficients show negative temperature dependences that are too strong to be attributable only to long-range interactions between the reactants. Expressed as negative activation energies, the temperature dependences yield values of E a / R from −520 to −750 K. We suggest that the removal of the vibrationally excited radicals occurs via formation of the hydrogen-bonded, cyclic OH·HNO 3 reaction complex (or the appropriate isotopomer of the complex) invoked to explain the unusual kinetics of the reaction of ground-state OH with nitric acid. We postulate that dissociation of the reaction complex to regenerate nitric acid and vibrationally excited OH or OD competes with intramolecular vibrational redistribution of the OH/OD vibrational excitation energy within the reaction complex, leading to the observed negative temperature dependence. We attribute the higher rate coefficients of the like-isotope processes (relative to the unlike-isotope processes) to faster, resonant, intramolecular vibrational energy redistribution within the reaction complexes containing the same isotopes. Additionally, we estimate the yields of OH( v = 1) to be ∼1% and OH( v = 2) to be ∼0.4% of that of OH( v = 0) from the photolysis of HNO 3 at 248 nm.