Ab initio study of nitrogen and position-specific oxygen kinetic isotope effects in the NO + O₃ reaction

Ab initio calculations have been carried out to investigate nitrogen (k¹⁵/k¹⁴) and position-specific oxygen (k¹⁷/k¹⁶O & k¹⁸/k¹⁶) kinetic isotope effects (KIEs) for the reaction between NO and O₃ using CCSD(T)/6-31G(d) and CCSD(T)/6-311G(d) derived frequencies in the complete Bigeleisen equations. Isotopic enrichment factors are calculated to be -6.7‰, -1.3‰, -44.7‰, -14.1‰, and -0.3‰ at 298 K for the reactions involving the ¹⁵N¹⁶O, ¹⁴N¹⁸O, ¹⁸O¹⁶O¹⁶O, ¹⁶O¹⁸O¹⁶O, and ¹⁶O¹⁶O¹⁸O isotopologues relative to the 14N¹⁶O and 16O₃ isotopologues, respectively (CCSD(T)/6-311G(d)). Using our oxygen position-specific KIEs, a kinetic model was constructed using Kintecus, which estimates the overall isotopic enrichment factors associated with unreacted O₃ and the oxygen transferred to NO₂ to be -19.6‰  and -22.8‰, respectively, (CCSD(T)/6-311G(d)) which tends to be in agreement with previously reported experimental data. While this result may be fortuitous, this agreement suggests that our model is capturing the most important features of the underlying physics of the KIE associated with this reaction (i.e., shifts in zero-point energies). The calculated KIEs will useful in future NO_x isotopic modeling studies aimed at understanding the processes responsible for the observed tropospheric isotopic variations of NO_x as well as for tropospheric nitrate.