Isocyanate Reaction Pathways And Kinetic Rate Constants Calculated

Isocyanic acid (HNCO) plays an important role in combustion and atmospheric chemistry. Itis a potential agent for effectually eliminating nitrogen oxides in exhaust gas streams. The radicalreaction of eliminating NOX caused by HNCO at high temperature is called the RAPRENOXreaction. Using ab initio methods, three molecular reactions about Isocyanic acid were investigated inthis thesis, that is : (1) HNCO+HCOâ†'TSâ†'NCO+CH2O (2) A:HNCO+CNâ†'TSâ†'NCO+HCN B:HNCO+CNâ†'TS1â†'INTâ†'TS2â†'HNCN+CO (3) HNCO+NO2â†'TS1â†'INT1â†'TS2â†'INT2â†'TS3â†'HNNO+CO2 The theoretical studies show that the reaction (2) may occur by two different reaction channels,A and B. For the reaction systerms, by using ab initio UMP2(FULL) or UB3LYP(DFT) calculations, at6-311G(d,p) basis set level, all the stationary points on the reaction paths are optimized. Energies ofthe every stationary point on the minimum energy paths (MEP) for the reaction (1) are improved byUQCISD(T,FULL) single-point energy correction. The calculations show that the calculatedtransition state has one and only one negative eigenvalue. And we also show that the transition staterepresents an energy maximum on passing from reactants to products through IRC. Calculations show that the reaction (1) and (2) A are hydrogen abstraction reaction. The ruptureof old bonds and formation of new bond are concerted. By investigating the changes of theinteratomic distance, it is found that there exits a region "effective reaction region" defined as aregion of the new bond forming and the old bond breaking effectively. This region is characterizedby the great changes of the geometrical structure, the vibrational frequency, the electronic spindensity and so on, during the reaction process. The effective reaction regions for the reactionsmentioned above are s=-1.5～0.5(amu)1/2bohr and s=-0.8～1.5 (amu)1/2bohr. A special vibrationalmode is found which leading the reaction path defined by us as "reactive vibrational mode". Itsfrequencies change is relevant to the formation and rupture of the covalent bond. Rate constants ofthe two reactions are calculated based on traditional transition state theory (TST) and canonicalvariational transition state theory (CVT). The calculated values are in good agreement with theexperimental results. The reaction channel (2) B, which has not been discussed before, is a stepwise reaction. It goesto the products HNCN and CO via two transition states and an intermediate. The potential barrier ofthe rate controlling step is 83.42 kJ/mol, which is higher than the potential barrier of channel A(20.80 kJ/mol). So we can conclude that the hydrogen abstraction reaction of channel A is the mainreaction channel of the reaction HNCO with CN radical. The reaction (3) is a stepwise reaction. HNNO and CO2 are the ultimate products via twointermediates and three transition states. The transition state TS2 is a four- member ring structure.The first step (Reactantsâ†'TS1â†'INT1) is a rate controlling step with potential barrier 172.85kJ/mol.The rate constants calculations show that the value is very small at the low temperature (below700K), which is in agreement with the experimental results.