Theoretical Calculation of ClONO2 and BrONO2 Bond Dissociation Energies

Dissociation Mechanism of a Stable Intermediate: Perfluorohydroxylamine

The mechanism of dissociation of F2NOF has been studied using density-functional (B3LYP, BB1K, and MPWB1K) and wave function methods (CCSD). Variational transition state theory was used to calculate the rate constants for cis-F2NOF --> FNO + F2 (concerted), cis-F2NOF --> F2NO + F, and cis-F2NOF --> trans-F2NOF --> F3NO. Rate constants were also calculated for the dissociation of F2NOF by using transition state theory. The enthalpies of the transitions states (CCSD(T)/cc-pVQZ//B3LYP/6-311+G(d)) were very close to the enthalpy of separated F2NO + F radicals which implies temperature-dependent competition between concerted rearrangement and fragmentation-recombination. The picture is further complicated by the fact that F2NO undergoes fragmentation into FNO + F with a very low barrier. Thus, formation of F3NO, the global minimum on the potential energy surface, can only occur by a concerted process (not from F2NO + F). The data were fit to a temperature-dependent rate in the range 200-1000 K in the form k2 = 8.14 x 10(13) exp(-7860/T) s(-1), k(1) = 6.37 x 10(13) exp(-7855/T) s(-1), and k(10) = 1.42 x 10(12) exp(-7420/T) for cis-F2NOF --> FNO + F2 (concerted), cis-F2NOF --> F2NO + F, and cis-F2NOF --> F3NO, respectively. The calculated lifetime of cis-F2NOF at 298K is 2.5 x 10(-3) s via k1.

Ab initio Studies of ClOx Reactions: Prediction of the Rate Constants of ClO+NO2 for the Forward and Reverse Processes

The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of -25.6, -0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol-1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol-1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200-600 K can be represented by: (N2)=3.19 x 10-17 T-5.54 exp(-384 K/T) cm6 molecule-2 s-1 and =3.33 x 10-7 T-1.48 exp(-18 K/T) cm3 molecule-1 s-1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200-600 K can be expressed, respectively, by =6.08 x 1013 T-6.54 exp(-13813 K/T) cm3 molecule-1 s-1 and =4.59 x 1023 T-2.43 exp(-13437 K/T) s-1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19 x 10-9 T-0.60 exp(58 K/T) cm3 molecule-1 s-1..