Nonstatistical Behavior of Reactive Scattering in the ¹⁸O+³²O₂ Isotope Exchange Reaction

Abstract

The recombination of oxygen atoms with oxygen molecules to form ozone exhibits several strange chemical characteristics, including unusually large differences in formation rate coefficients when different isotopes of oxygen participate. Purely statistical chemical reaction rate theories cannot describe these isotope effects, suggesting that reaction dynamics must play an important role. We investigated the dynamics of the 18O + 32O2 --> O3(*) --> 16O + 34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using crossed atomic and molecular beams at a collision energy of 7.3 kcal mol(-1), providing the first direct experimental evidence that the dissociation of excited ozone exhibits significant nonstatistical behavior. These results are compared with quantum statistical and quasi-classical trajectory calculations in order to gain insight into the potential energy surface and the dynamics of ozone formation.

State-to-state quantum dynamics of O + O 2 isotope exchange reactions reveals nonstatistical behavior at atmospheric conditions

“…O reaction at a significantly higher (0.32 eV) energy (11), in which the product angular distribution was found to be forward biased. The collision energies investigated here are more relevant to the ozone formation reaction in the atmosphere and in an energy region where the dynamics are presumed to be more statistical (32).…”

The non-statistical dynamics of the 18O + 32O2 isotope exchange reaction at two energies

“…Experimental details and conditions were the same as those described in Van Wyngarden et al 15 except that the collision energy (E coll ) was decreased from 7.3 kcal/mol to 5. The only significant difference in the modified ("slow") 18 O atomic beam from the "fast" 18 O atomic beam was the slower average velocity of 1887 m/s (compared to 2170 m/s).…”

“…After TOF spectra were collected at the angles noted above, the raw TOF spectra were corrected for background 34 15 At 5.7 kcal/mol, the correction ranged from 10% of the v = 0 peak height at 20 to 43% at 55, while at 7.3 kcal/mol the correction ranged from 6% of the v = 0 peak height at 10 to 48% at 60. After this background correction and rebinning the data to 3s…”

“…In the statistical limit, the lifetime of the reaction intermediate in a complex-forming reaction is sufficiently long that the energy is completely randomized among all degrees of freedom, resulting in a forward-backward symmetric angular distribution for the products. 14 Reactive scattering experiments for (R3) at a collision energy (E coll ) of 7.3 kcal/mol 15 provided the first direct experimental evidence for non-statistical behavior in the decomposition of excited ozone complexes:…”

Dynamics of the O-Atom Exchange Reaction 16O(3P) + 18O18O(3Σg–) → 16O18O(3Σg–) + 18O(3P) at Hyperthermal Energies

“…O'O( 3 Σ g − ) + O( 3 P) has attracted special interest because of its importance in atmospheric chemistry and the curious isotope effect it exhibits. 1,2 This reaction occurs in Earth's atmosphere and competes with ozone formation.…”