The products were dispersed mainly in the forward direction, i.e. in the initial direction of the hyperthermal oxygen beam in the center of mass frame (c.m), with an average of 16% of the available energy in translation. Here, Θ is defined as the angle at which 16O12C18O scatters relative to the direction of the reagent oxygen beam (ie, Θ = 0◦). We characterized the velocity distribution of the hyperthermal oxygen beam under two conditions: on-axis (Θ = 0◦) with a small aperture (diameter ~125 µm) to obtain the peak of the distribution, and off-axis (Θ = 3◦) with a large aperture (4 mm × 4 mm) to obtain the width of the distribution.
For the on-axis setup, an emission current of 2 mA was used in the ionizer and a potential of –18 kV at the secondary emitting electrode of the Daly ion counter. Thus, 2% of the inelastic scattering signal (m/z= 48) at each laboratory angle (Θ= 6◦– 54◦) was subtracted from the raw/z= 46 (16O12C18O+) TOF distributions to obtain scatter-only distributions reactive. . To allow easy manipulation of the P(ET) and T(θc.m.) distributions, parameterized functions were often used.
A small part of the signal can be attributed to inelastic scattering (IS) of 16O12C18O impurities in the CO2 range with 16O and 16O2, and can be subtracted. Increasing hETi in the P(ET) product shifts the fastest part of the fit to shorter flight times at all laboratory angles. The average total translational energy dissipation for the two products was 25 kcal mol−1, or 16% of the available energy; the P(ET) distribution peaked at ET.
Vibrational excitation of O2 may also be important in driving the reaction, but the vibrational temperature of O2 present in the expanded plasma is unknown.
Theoretical studies
An additional bound state was found 0.8 kcal mol−1 above the CO4 energy at the MP2/cc-pVTZ level of theory, although it does not exist at the CCSD(T)/aug-cc-pVTZ level; the species is believed to be a resonance structure of CO4 (3A00). A rotational contamination of hS2−Sz2−Szi = 0.011 in the PES region near TS3 also suggests some multireference characters. Thus, CASPT2/cc-pVTZ calculations, which represent the resonance qualitatively correctly and do not suffer from spin contamination, were performed to validate the CCSD(T) structures and energies.
CO4(3A00) was confirmed to be a resonance hybrid of two bound-state structures, although the contribution of the single reference structure to the resonance hybrid dominates the final geometry. The uncertainty in the CCSD(T)/aug-cc-pVTZ energies is unknown, although a similar study characterizing the stationary points of the closely related O(3P) + CO2 system has recently been completed (see Chapter 4) . In addition to CCSD(T)/aug-cc-pVTZ simulations, W4 calculations (near the full configuration interaction and infinite-basis-set.
CCSD(T)/aug-cc-pVTZ values (top) and CASPT2/cc-pVTZ (bottom) values are shown. boundaries; see [Karton et al., 2006]) were also carried out, because they turned out to be affordable, albeit marginally. The differences between the CCSD(T) and W4 results for the transition and limit states indicated an average unsigned error of 2.2 kcal mol−1.
Proposed mechanism
Note that the highest SOMO of CO4(3A00) and the lowest SOMO of TS3 have electron density mainly on Od, suggesting that the SOMO is a spectator during isomerization. This spin density corresponds to the radical character expected for an association of the O2 diradical with the carbon atom of CO2, because the unpaired electrons in O2 are in orthogonal π∗g orbitals: As one C–O bond is formed between the in-plane π∗orbital of O2 and the in-plane π-bu orbital of CO2 (the higher-energy πnbg orbital does not interact due to symmetry; see Figure 3.14), the out-of-plane π∗orbital on O2 is a relative spectator to the O2 radical addition. This interaction, together with the bending of the CO2, produces the SOMOs for CO4(3A00) shown in Figure 3.13.
In TS3, the lowest SOMO contains localized electron density on Od in an out-of-plane p-type orbital, suggesting that the unpaired electron on Od is still a spectator during CO4(3A00) isomerization. However, the highest SOMO contains delocalized electron density, of anti-bond (σ∗) character, in the plane of the molecule. Thisσ∗character can be interpreted as electron density taken from the lowest SOMO in CO4(3A00) while localized on Oc during isomerization.
During the isomerization of CO4(3A00), the σ Oc–Od bond cleaves homolytically (Figure 3.15): An electron moves into the ap-type orbital. A weak interaction between the π∗g orbital in O2 and the πbuorbital in CO2 is likely responsible for the small (1.5 kcal mol−1) binding energy of CO4. This pathway can account for the σ∗character of the higher-energy SOMO in TS3 (the Ob-Odbond takes an electron from the Oc-Odbond), and preserves the radical character in Od.
This mechanism is qualitatively consistent with the small reaction cross section and the preference for repulsive reactive collisions inferred from the dynamics observed in the experiment. O2 must approach CO2 at very low high-energy shock parameters to overcome the initial barrier (TS1/TS2) and reach the “shelf” of bound CO4 (3A00). Momentum along the reaction coordinate above CO4(3A00) with ~80 kcal mol-1 of excess energy should further compress the C–Oc bond with synchronous C–Oc–Od bending.
Finally, the large change in geometry from the reactants to TS3 should facilitate the transfer of some of the translational energy to the internal degrees of freedom of the products. While some lateral scattering is indeed observed in experiment, the equality of T(θc.m.) is unknown. A non-adiabatic transition to another possible surface cannot be ruled out, especially given the high internal energies observed in the products.
Conclusion
Reactive spread should become less 'rebound'-like (forward spread) as Ecollis rose above the reactive threshold; at 37 kcal mol−1 above the reactive barrier, the tight transition state, repulsive exit channel leading out of TS1/TS2, and low angular momentum should limit the extent of sideways scattering. At these high energies [80 kcal mol−1 above even the CO4(3A00) shelf state], many excited-state surfaces must exist, and radiationless transitions via conical intersections can occur. For example, the bridging oxygen atom in TS3, Od, must be equivalently bonded to Oband Oc, and chemical intuition suggests that an intersection with a singlet surface may occur by the spin-paired CO4(1A1) [Cacace et al., 2003; Elliott and Boldyrev, 2005; Jamieson et al., 2007].
Studies of the collisional energy dependence of the product angular distribution can elucidate the mechanistic origin of the low average translational energy of the products. Finally, a more complete surface, reactive trajectories, and locations of possible surface junction layers leading to ISC would be vital for a complete explanation of the reaction dynamics.
Acknowledgements
Appendices
3.A Non-reactive scattering of O 2 and CO 2
The corresponding translational energy distribution peaked at 60 kcal mol−1 with an average hETi = 62.8 kcal mol−1, or 40% of the available energy. However, this part of the distribution is uncertain because our experiment did not look at it directly. The translational energy distribution peaked at 103 kcal mol−1 with an average hETi= 100.8 kcal mol−1, or 64% of the available energy.
Transfer of vibrational energy from O2 to CO2 cannot be ruled out in our experiments because we have not characterized the vibrational temperature of the hyperthermic O2. Consequently, the similarity in inelastic scattering dynamics between O(3P) and O2 collisions with CO2 could also be coincidental. The circles in (A) and (D) are experimental data, while the lines (orange and brown) are the best-fit forward-folding simulations of the experimental data derived from c.m.
These data indicate that 16O2 is scattered primarily in the forward direction with little change in its initial direction or velocity. The circles in (A) and (D) are experimental data, while the lines are best-fit forward-convolution simulations of the experimental data obtained from the c.m. The error bars in (D) represent 2σ uncertainties in the integrated experimental TOF distributions (see Appendix 4.7).
The white arrows are the initial velocity vectors of 16O2 and 12C18O2, and the dashed white line is the maximum return velocity for.
