Cavity Ringdown Spectroscopy of cis-cis HOONO and the HOONO/HONO₂ Branching Ratio in the Reaction OH + NO₂ + 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 ν 1 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 transperp 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 exeriment. 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.

Rotational spectrum of cis–cis HOONO

“…The parameters determined in the least-squares fit are given in Table I. The A, B, and C rotational constants were determined from the fitted Hamiltonian parameters. These values are given in Table II along with the rotational constants determined from the optimized CCSD͑T͒/cc-pVTZ structure reported by Bean et al 16 The inertial defect ⌬ϭI cc ϪI aa ϪI bb is 0.076 amu Å 2 , consistent with the small, positive inertial defects that have been observed for planar ring molecules such as furan and pyrrole. 30 This evidence indicates that ciscis HOONO is a planar molecule with small out-of-plane deformations caused by zero-point vibrational motion.…”

“…This work was followed in rapid succession by detection of the cis-cis 1 fundamental by Bean et al using cavity ringdown spectroscopy 16 and a high-resolution study of the trans-perp 2 1 band in a supersonically cooled molecular beam by Pollack et al 17 18 The vibrational frequencies and intensities calculated in this study enabled Lee and co-workers 11,13 to distinguish between the trans-perp and cis-cis conformers in their matrix IR spectra. McGrath and Rowland subsequently used G2, G2͑MP2͒, QCISD(T)/6-311G(d,p), and MP2 calculations with basis sets up to 6-311ϩG͑3df,2p͒ to improve characterization of the relative energetics of the three stable conformers.…”

“…19 Bean et al found that the cis-perp conformer was not a true stationary point at the QCISD͑T͒/ cc-pVTZ or CCSD͑T͒/cc-pVTZ levels of theory. 16 Bean et al also reported rotational constants and vibrational fre-quencies for cis-cis and trans-perp HOONO for QCISD and CCSD͑T͒ calculations. At the CCSD͑T͒/cc-pVTZ level of theory, the cis-cis conformer was found to be 3.4 kcal/ mol more stable than the trans-perp conformer ͑including zero-point correction͒ ͑Ref.…”

“…1. Simulations of the cis-cis HOONO rotational spectrum, based on ab initio rotational constants and dipole moments, 16 predicted the presence of characteristic asymmetry doublets of both a-and b-type transitions ͑i.e., quartets͒ for low-K a R-type transitions near 400 GHz. Survey scans in the 399-454 GHz region revealed quartet patterns consistent with the cis-cis HOONO simulations.…”

“…A combined submillimeter and infrared experiment could also be used to quantify the band strengths of the infrared transitions. 14,16,17 This research was performed at the Jet Propulsion Laboratory ͑JPL͒, California Institute of Technology, under contract with the National Aeronautics and Space Administra- …”