Ab initio study of nitrogen and position-specific oxygen kinetic isotope effects in the NO + O-3 reaction

摘要

Ab initio calculations have been carried out to investigate nitrogen (k(15)/k(14)) and position-specific oxygen (k(17)/k(16)O & k(18)/k(16)) kinetic isotope effects (KIEs) for the reaction between NO and O-3 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 parts per thousand, -1.3 parts per thousand, -44.7 parts per thousand, -14.1 parts per thousand, and -0.3 parts per thousand at 298 K for the reactions involving the (NO)-N-15-O-16, (NO)-N-14-O-18, (OOO)-O-18-O-16-O-16, (OOO)-O-16-O-18-O-16, and (OOO)-O-16-O-16-O-18 isotopologues relative to the (NO)-N-14-O-16 and 16O(3) 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 O-3 and the oxygen transferred to NO2 to be -19.6 parts per thousand and -22.8 parts per thousand, 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 NOx isotopic modeling studies aimed at understanding the processes responsible for the observed tropospheric isotopic variations of NOx as well as for tropospheric nitrate. Published by AIP Publishing.