Cavity Ringdown Spectroscopy of cis-cisHOONO and the HOONO/HONO_2 Branching Ratio in the Reaction OH + NO_2+ M

Abstract

The termolecular association reaction OH + NO_2 + M was studied in a low-pressure discharge flow reactor, and both HONO_2 and HOONO products were detected by infrared cavity ringdown spectroscopy (IR-CRDS). The absorption spectrum of the fundamental ν1 band of the cis-cis isomer of HOONO (pernitrous or peroxynitrous acid) was observed at 3306 cm^(-1), in good agreement with matrix isolation studies and ab initio predictions. The rotational contour of this band was partially resolved at 1 cm^-1 resolution and matched the profile predicted by ab initio calculations. The integrated absorbances of the ν^1 bands of the cis-cis HOONO and HONO_2 products were measured as a function of temperature and pressure. These were converted to product branching ratios by scaling the experimentally observed absorbances with ab initio integrated cross sections for HOONO and HONO_2 computed at the CCSD(T)/cc-pVTZ level. The product branching ratio for cis-cis HOONO to HONO_2 was 0.075 ± 0.020(2σ) at room temperature in a 20 Torr mixture of He/Ar/N_2 buffer gas. The largest contribution to the uncertainty is from the ab initio ratio of the absorption cross sections, computed in the double harmonic approximation, which is estimated to be accurate to within 20%. The branching ratio decreased slightly with temperature over the range 270 to 360 K at 20 Torr. Although trans-perp HOONO was not observed, its energy was computed at the CCSD(T)/cc-pVTZ level to be E_0 = +3.4 kcal/mol relative to the cis-cis isomer. Statistical rate calculations showed that the conformers of HOONO should reach equilibrium on the time scale of this experiment. These results suggested that essentially all isomers had converted to cis-cis HOONO; thus, the reported branching ratio is a lower bound for and may represent the entire HOONO yield.

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