Thermal decomposition of N2O near 900 K studied by FTIR spectrometry: Comparison of experimental and theoretical O(3P) formation kinetics

The spin-forbidden dissociation reaction of the N2O(X-1 sigma(+)) ground state has been investigated by both quantum calculations and experiments. Ab initio prediction at the CCSD(T)/CBS(TQ5)//CCSD(T)/aug-cc-pVTZ+d level of theory gave the crossing point (MSX) energy at 60.1 kcal/mol for the N2O(X-1 sigma(+)) -> N-2(X1 sigma g+) + O(P-3) transition, in good agreement with published data. The T- and P-dependent rate coefficients, k(1)(T,P), for the nonadiabatic thermal dissociation predicted by nonadiabatic Rice-Ramsperger-Kassel-Marcus (RRKM) calculations agree very well with literature data. The rate constants at the high- and low-pressure limits, k(1)(infinity) = 10(11.90) exp (-61.54 kcal mol(-1)/RT) s(-1) and k(1)(o) = 10(14.97) exp(-60.05 kcal mol(-1)/RT) cm(3) mol(-1) s(-1), for example, agree closely with the extrapolated results of Rohrig et al. at both pressure limits. The second-order rate constant (k(1)(o)) is also in excellent agreement with our result measured by FTIR spectrometry in the present study for the temperature range of 860-1023 K as well as with many existing high-temperature data obtained primarily by shock-wave heating up to 3340 K. Kinetic modeling of the NO product yields measured at long reaction times in the present work also allowed us to reliably estimate the rate constant for reaction (3), O + N2O -> N-2 + O-2, based on its strong competition with the NO formation from reaction (2) which has been better established. The modeled values of k(3) confirmed the previous finding by Davidson et al. with significantly smaller values of A-factor and activation energy than the accepted ones. A least-squares analysis of both sets of data gave k(3) = 10(12.22 +/- 0.04) exp[- (11.65 +/- 0.24 kcal mol(-1)/RT)] cm(3) mol(-1) s(-1), covering the wide temperature range of 988-3340 K.