Atmospheric Chemistry of Hydrazoic Acid (HN₃):  UV Absorption Spectrum, HO^• Reaction Rate, and Reactions of the ^•N₃ Radical

Abstract

Processes related to the tropospheric lifetime and fate of hydrazoic acid, HN3, have been studied. The ultraviolet absorption spectrum of HN3 is shown to possess a maximum near 262 nm with a tail extending to at least 360 nm. The photolysis quantum yield for HN3 is shown to be approximately 1 at 351 nm. Using the measured spectrum and assuming unity quantum yield throughout the actinic region, a diurnally averaged photolysis lifetime near the earth's surface of 2-3 days is estimated. Using a relative rate method, the rate coefficient for reaction of HO with HN3 was found to be (3.9 +/-0.8) x 10(-12) cm3 molecule(-1) s(-1), substantially larger than the only previous measurement. The atmospheric HN3 lifetime with respect to HO oxidation is thus about 2-3 days, assuming a diurnally averaged [HO] of 10(6) molecule cm(-3). Reactions of N3, the product of the reaction of HO with HN3, were studied in an environmental chamber using an FTIR spectrometer for end-product analysis. The N3 radical reacts efficiently with NO, producing N2O with 100% yield. Reaction of N3 with NO2 appears to generate both NO and N2O, although the rate coefficient for this reaction is slower than that for reaction with NO. No evidence for reaction of N3 with CO was observed, in contrast to previous literature data. Reaction of N3 with O2 was found to be extremely slow, k < 6 x 10(-20) cm3 molecule(-1) s(-1), although this upper limit does not necessarily rule out its occurrence in the atmosphere. Finally, the rate coefficient for reaction of Cl with HN3 was measured using a relative rate method, k = (1.0+/-0.2) x 10(-12) cm3 molecule(-1) s(-1).

Accurate Study of the Two Lowest Singlet States of HN3: Stationary Structures and Energetics at the MRCI Complete Basis Set Limit

“…The hydrazoic acid (HN 3 ) is a species of growing concern in the atmosphere due to its pollutant character 1 and belongs to a class of energetic molecules. Its decomposition in the lowest electronic state…”

“…Theoretically, it was shown 22 that such photodissociation involves mainly the lowest excited singlet state (1 1 A″), with ab initio calculations being employed to explain the experimentally observed results. More recently, Fang 23 analyzed this process by performing ab initio calculations on the 1 A′, 1 A″, and 3 A″ states. He carried out optimizations of the stationary structures by constraining the system to C s symmetry from complete active-space self-consistent field calculations (CASSCF) with a reduced active space (8 valence electrons in 7 active orbitals CASSCF (8,7)), the energies being refined using the multireference MP2 approach.…”

Chemical pathways and kinetic rates of the N(4S) + N2 → N3 solid phase reaction: could the N3 radical be a temperature sensor of nitrogen ices in dense molecular clouds?

“…Also, the non-delectability of N 3 and its derivatives such as HN 3 or N 3 − would be because all those chemical species react efficiently with ions, radicals and even stable molecules, and would be a source of many chemical transformations in space. They are already used as oxidants in fast kinetic studies (Alfassi et al 1987), playing an important role in the atmospheric photochemistry (Orlando et al 2005) of hydrazoic acid (HN 3 ).…”

Thermal Decomposition of HN3

“…There are no experimental or theoretical studies of reaction 10, that of N 3 with HN 3 . N 3 is known to react rather efficiently with NO 2 (reported room temperature rate constants are 1.2 × 10 12 cm 3 mol −1 s −1 28 and 5 × 10 11 cm 3 mol −1 s −1 29 ) and even more efficiently with NO, , with attack on the nitrogen atom. These reactions, however, are not good analogies because NO and NO 2 are open-shell species, whereas HN 3 is not.…”