A crossed beam study of 18O(3P)+NO2 and 18O(1D)+NO2: Isotope exchange and O2+NO formation channels

The products and dynamics of the reactions O-18(P-3)+NO2 and O-18(D-1)+NO2 have been investigated using crossed beams and provide new constraints on the structures and lifetimes of the reactive nitrogen trioxide intermediates formed in collisions of O(P-3) and O(D-1) with NO2. For each reaction, two product channels are observed - isotope exchange and O-2+NO formation. From the measured product signal intensities at collision energies of similar to 6 to 9.5 kcal/mol, the branching ratio for O-2+NO formation vs. isotope exchange for the O(P-3)+NO2 reaction is 52(+6/-2)% to 48(+2/-6)%, while that for O(D-1)+NO2 is 97(+2/-12)% to 3(+12/-2)%. The branching ratio for the O(P-3)+NO2 reaction derived here is similar to the ratio measured in previous kinetics studies, while this is the first study in which the products of the O(D-1)+NO2 reaction have been determined experimentally. Product energy and angular distributions are derived for the O(P-3)+NO2 isotope exchange and the O(D-1)+NO2 -> O-2+NO reactions. The results demonstrate that the O(P-3)+NO2 isotope exchange reaction proceeds by an NO3* complex that is long-lived with respect to its rotational period and suggest that statistical incorporation of the reactant O-18 into the product NO2 (apart from zero point energy isotope effects) likely occurs. In contrast, the O-18(D-1)+NO2 -> O-2+NO reaction proceeds by a direct "stripping" mechanism via a short-lived O-18-O-NO* complex that results in the occurrence of O-18 in the product O-2 but not in the product NO. Similarly, O-18 is detected in O-2 but not NO for the O(P-3)+NO2 -> O-2+NO reaction. Thus, even though the product energy and angular distributions for O(P-3)+NO2 -> O-2+NO derived from the experimental data are uncertain, these results for isotope labeling under single collision conditions support previous kinetics studies that concluded that this reaction proceeds by an asymmetric O-18-O-NO* intermediate and not by a long-lived symmetric NO3* complex, as earlier bulk isotope labeling experiments had concluded. Applicability of these results to atmospheric chemistry is also discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4736567]