NCO quantitative measurement in premixed low pressure flames by combining LIF and CRDS techniques.

NCO is a short-lived species involved in NO(x) formation. It has never been quantitatively measured in flame conditions. In the present study, laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS) were combined to measure NCO radical concentrations in premixed low-pressure flames (p = 5.3 kPa). NCO LIF excitation spectrum and absorption spectrum (using CRDS) measured in a stoichiometric CH(4)/O(2)/N(2)O/N(2) flame were found in good agreement with a simulated spectrum using PGOPHER program that was used to calculate the high-temperature absorption cross section of NCO in the A(2)Σ(+)-X(2)Π transition around 440.479 nm. The relative NCO-LIF profiles were measured in stoichiometric CH(4)/O(2)/N(2)O/N(2) flames where the ratio N(2)O/O(2) was progressively decreased from 0.50 to 0.01 and in rich CH(4)/O(2)/N(2) premixed flames. Then, the LIF profiles were converted into NCO mole fraction profiles from the absorption measurements using CRDS in a N(2)O-doped flame.

A CASPT2 theoretical study of the kinetics of the 2-, 3-, and 4-methylbenzylperoxy radical isomerization.

The rate constants of the 2-, 3-, and 4-methylbenzylperoxy isomerization reactions have been computed using the elaborated CASPT2 method. Geometry optimizations and vibrational frequency calculations are performed with two methods (B3LYP and MPW1K) combined with the cc-pVDZ and 6-31+G(d,p) basis sets, respectively. Single-point energy calculations are performed at the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory as recommended by Canneaux et al. (J. Phys. Chem. A 2008, 112, 6045). Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. They are given by the following relations for the 2-, 3-, and 4-methylbenzylperoxy (MBP) (1,3s) isomerizations and for the 2-methylbenzylperoxy (1,6p) isomerization, respectively: k(2-MBP(1,3s))(600-2000 K) (in s(-1)) = (3.33 x 10(10))T(0.79) exp((-142.6 in kJ mol(-1))/RT); k(3-MBP(1,3s))(600-2000 K) (in s(-1)) = (0.74 x 10(10))T(0.79) exp((-130.7 in kJ mol(-1))/RT); k(4-MBP(1,3s))(600-2000 K) (in s(-1)) = (1.12 x 10(10))T(0.79) exp((-133.6 in kJ mol(-1))/RT); k(2-MBP(1,6p))(600-2000 K) (in s(-1)) = (5.10 x 10(8))T(0.85) exp((-87.1 in kJ mol(-1))/RT). These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. In the case of the 2-methylbenzylperoxy radical, the (1,6p) H-atom transfer reaction is consistently the most important channel over the studied temperature range, and the (1,3s) H-atom transfer reaction is not energetically favored.

A theoretical study of the kinetics of the benzylperoxy radical isomerization.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.