Kinetics of n-Butoxy and 2-Pentoxy Isomerization and Detection of Primary...

Sprague, M. K., E. R. Garland, A. K. Mollner, C. Bloss, B. D. Bean, M. L. Weichman, L. A. Mertens, M. Okumura, and S. P. Sander (2012), Kinetics of n-Butoxy and 2-Pentoxy Isomerization and Detection of Primary Products by Infrared Cavity Ringdown Spectroscopy, J. Phys. Chem. A, 116, 6327-6340, doi:10.1021/jp212136r.

The primary products of n-butoxy and 2pentoxy isomerization in the presence and absence of O2 have been detected using pulsed laser photolysis-cavity ringdown spectroscopy (PLP-CRDS). Alkoxy radicals nbutoxy and 2-pentoxy were generated by photolysis of alkyl nitrite precursors (n-butyl nitrite or 2-pentyl nitrite, respectively), and the isomerization products with and without O2 were detected by infrared cavity ringdown spectroscopy 20 μs after the photolysis. We report the mid-IR OH stretch (ν1) absorption spectra for δ-HO-1-C4H8•, δ-HO-1-C4H8OO•, δHO-1-C5H10•, and δ-HO-1-C5H10OO•. The observed ν1 bands are similar in position and shape to the related alcohols (nbutanol and 2-pentanol), although the HOROO• absorption is slightly stronger than the HOR• absorption. We determined the rate of isomerization relative to reaction with O2 for the n-butoxy and 2-pentoxy radicals by measuring the relative ν1 absorbance of HOROO• as a function of [O2]. At 295 K and 670 Torr of N2 or N2/O2, we found rate constant ratios of kisom/kO2 = 1.7 (±0.1) × 1019 cm−3 for n-butoxy and kisom/kO2 = 3.4(±0.4) × 1019 cm−3 for 2-pentoxy (2σ uncertainty). Using currently known rate constants kO2, we estimate isomerization rates of kisom = 2.4 (±1.2) × 105 s−1 and kisom ≈ 3 × 105 s−1 for n-butoxy and 2pentoxy radicals, respectively, where the uncertainties are primarily due to uncertainties in kO2. Because isomerization is predicted to be in the high pressure limit at 670 Torr, these relative rates are expected to be the same at atmospheric pressure. Our results include corrections for prompt isomerization of hot nascent alkoxy radicals as well as reaction with background NO and unimolecular alkoxy decomposition. We estimate prompt isomerization yields under our conditions of 4 ± 2% and 5 ± 2% for nbutoxy and 2-pentoxy formed from photolysis of the alkyl nitrites at 351 nm. Our measured relative rate values are in good agreement with and more precise than previous end-product analysis studies conducted on the n-butoxy and 2-pentoxy systems. We show that reactions typically neglected in the analysis of alkoxy relative kinetics (decomposition, recombination with NO, and prompt isomerization) may need to be included to obtain accurate values of kisom/kO2.

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Upper Atmosphere Research Program (UARP)