| Using ab initio methods, three small molecular radical reactions were systematically investigated in this thesis:1. Reaction barriers and the possible mechanism for the reactions of HNCO with radical NH2 and CH,(x=1~3)2. Reaction paths and rate constants for the reactions of CH3NO2 with radicals H, OH, CH3, CH2 [3B,] and O [3P]) respectively.3. Reaction barriers and the mechanism of reaction between CH3 and HO2For the reaction HNCO with radicals, ab initio UMP2 and UQCISD (T) calculations, at 6-311G(d, p) basis set level, were performed for the reaction between HNCO and NH2. The theoretical studies show that the reactions may occur by two different reaction channels, (A) and (B). Reaction (A) is a hydrogen abstraction process via a hydrogen-bonded complex (HBC), which is 32.38kJ/mol energetically below the reactants. The rupture of bond N-H in the reactant molecular HNCO are concerted with the formation of the bond N-H in the product molecular NH3. However, reaction (B) is a stepwise reaction with cis and trans isomers.NH2+HNCO -NH3+NCO (A) NH2+HNCO-N2H3+CO (B)The calculated UQCISD(T)//UMP2(full)6-311 G(d,p) activation energy for forward reaction (A) is 29.04kJ/mol, which is in excellent accordance with the experimental value of 29.05kJ/mol. In the temperature range 2300-2700 K, the transition theory rate constants for the reaction (A) are located in the range 1.68 X 1011-3.29X 1011 cm3 .mol-1.s-1, which is in good agreement with that the upper limit of reaction rate constants for the reaction (A) is 5.0X 1011cm3.mol-1.s-1 given by the experiment.The calculated energy barrier for the rate-control step of reaction (B) is 92.79kJ/mol, it is higher than that of the reaction (A) by 63.72kJ/mol, so we can conclude that the reaction (A) is the main reaction channel of the reaction HNCO with NH2.UMP2 method gives reasonable geometric parameters, but energies calculated usingUMP2 are poorly described. As far as the present reactions are concerned, the energy correction at UQCISD(T) level is required for improving the potential energy surface based on UMP2 geometries.After appropriate calculating methods being selected, reaction mechanisms of carbon-hydrogen radicals CHx(x=l,2,3) with isocyanic acid were drastically explored. Investigations indicate that the reactions of isocyanic acid (HNCO) with carbon-hydrogen radicals CHx(x=2,3) are similar to those of HNCO with NH2. The hydrogen abstraction reaction is taking place by a procedure that the rupture of an old bond N-H is concerted with the formation of a new bond C-H, though the rupture of bond N-H is chiefly occurring at the early reaction stage in front of the transition state and the formation at the later one in the rear of the transition state, and moreover, a weak hyper-molecular complex has been located on the side of the reactants. Besides, the barriers of reactions of HNCO with CH3(58.69kJ/mol) and CH2(44.78kJ/mol) are higher than the reaction HNCO + NH2(29.04kJ/mol).For the hydrogen abstraction reaction of HNCO with CH, the situation is quite different from the mentioned above. We found that it is a stepwise reaction process, namely,HNCO+CH-HN(CH)CO-H2CNCO-CH2(S) + NCO-H2CN + COand the intermediate product H2CNCO may continually dissociates to the molecular fragments CH2+NC0 or H2CN+CO. Calculations show that the formation of HN(CH)CO is a barrierless process and H2CNCO is formed by the intra-molecular hydrogen transfer over a reaction barrier, 99.9kJ/mol. It also shows that the barriers of decomposition of H2CNCO are very high, especially for the reaction channel of producing CH2+NC0. We can predict that the molecule H2CNCO is both stable thermodynamically and kinetically. When the reaction proceeds to form CHXNH and CO, i.e, CH,+HNCO = CH,NH + CO,theoretical calculations predict that the reaction proceeds in two-step stages. For the reaction of HNCO with CH2/CH3, the potential energy barriers of these two steps are over 90 kJ/mol, this is 30~40kJ/mol higher than those of the hydrogen abstraction reactions...
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