Reinvestigation of the Branching Ratio of the CN + O2 Reaction

Global picture of isomerization and dissociation of CN2O2: new metastable isomers

A comprehensive potential energy surface of CN₂O₂, covering 15 new isomers and 30 new transition states was constructed to predict three new observable isomers 14, 22 and 29.

Parity-dependent rotational energy transfer in CN(A(2)Π, ν = 4, j F(1)ε) + N2, O2, and CO2 collisions

We report state-resolved total removal cross sections and state-to-state rotational energy transfer (RET) cross sections for collisions of CN(A(2)Π, ν = 4, j F1ε) with N2, O2, and CO2. CN(X(2)Σ(+)) was produced by 266 nm photolysis of ICN in a thermal bath (296 K) of the collider gas. A circularly polarized pulse from a dye laser prepared CN(A(2)Π, ν = 4) in a range of F1e rotational states, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5. These prepared states were monitored using the circularly polarized output of an external cavity diode laser by frequency-modulated (FM) spectroscopy on the CN(A-X)(4,2) band. The FM Doppler profiles were analyzed as a function of pump-probe delay to determine the time dependence of the population of the initially prepared states. Kinetic analysis of the resulting time dependences was used to determine total removal cross sections from the initially prepared levels. In addition, a range of j' F1e and j' F2f product states resulting from rotational energy transfer out of the j = 6.5 F1e initial state were probed, from which state-to-state RET cross sections were measured. The total removal cross sections lie in the order CO2 > N2 > O2, with evidence for substantial cross sections for electronic and/or reactive quenching of CN(A, ν = 4) to unobserved products with CO2 and O2. This is supported by the magnitude of the state-to-state RET cross sections, where a deficit of transferred population is apparent for CO2 and O2. A strong propensity for conservation of rotational parity in RET is observed for all three colliders. Spin-orbit-changing cross sections are approximately half of those of the respective conserving cross sections. These results are in marked disagreement with previous experimental observations with N2 as a collider but are in good agreement with quantum scattering calculations from the same study ( Khachatrian et al. J. Phys. Chem. A 2009 , 113 , 3922 ). Our results with CO2 as a collider are similarly in strong disagreement with a related experimental study ( Khachatrian et al. J. Phys. Chem. A 2009 , 113 , 13390 ). We therefore propose that the previous experiments substantially underestimated the spin-orbit-changing cross sections for collisions with both N2 and CO2, suggesting that even approximate quantum scattering calculations may be more successful for such molecule-molecule systems than was previously concluded.

Kinetics and mechanism of the NCCO + NO reaction

The kinetics of the NCCO + NO reaction was studied by diode infrared laser spectroscopy. The results show that the total rate coefficient of the reaction is highly pressure dependent, with k(1) (298 K) = (3.2 +/- 0.5 to 27.1 +/- 2.0) x 10(-13) cm(3) molecule(-1) s(-1) over the total pressure range 3-25 Torr. Only very small yields of CO(2) and CO products were detected, with upper limits on branching ratios estimated at Phi (CO(2) + NCN) < or = 0.01, Phi (2NCO) < or = 0.01, and Phi(CO + NCNO) < or = 0.06. The reaction mechanism was investigated by calculating the potential energy surface (PES) using ab initio methods at the DFT-B3LYP/6-311++G**//CCSD(T)/6-311++G** level. The PES shows that pathways leading to bimolecular/trimolecular product channels involve high barriers. Both experiment and theory therefore indicate that the major pathway is formation and collisional stabilization of an NCC(NO)O adduct.