Theoretical Studies Of The Mechanism Of Nh2 And Ch4

The amino radical, NH2, is the key radicel in the reaction of nitrogenous compounds. Reactions of NH2 with hydrocarbons are very important in planetary atmosphere chemistry and combustion chemistry, particularly to the studies of Jupiter and Titan.In this dissertation we use Gaussian 98 program, density functional and transition state theory to investigate dynamical calculations on the reaction of NH2 and CH4:NH2 + CH4 -?NH3 + CH3 (1)NH2 + CH4 -> CH3NH2 +H (2)CH3NH2+H^CH2NH2+H2 (3)Using density functional theory, we calculated the geometric parameters and minimum energy path by means of the intrinsic reaction coordinate method at B3LYP/6-311G(d) level. The geometries of reactants, transition states and products have been optimized and verified by frequency analysis. The relative single -point energies of the B3LYP/6-311G(d) structure have been calculated at eh QCISD(T) /6-311G(d) level. The zero- point energy (ZPE) corrections were obtained. The rate constants within the temperature range from 300-2100K are calculated using conventional transition state theory.When the atom N attacks the different atom of CH4, there create two reaction paths as follows: Reaction (1): Atom N attacks atom H; reaction (2): Atom N attacks atom C of CH4- The two reaction barriers are 16.49kcal/mol and 54.61kcal/mol respectively. By comparing the calculate barriers, it is found that reaction NH2 + CH4 椈 NH3 + CH3 is the main reaction path. The theoretical activation energies are in agreement with the experimental data. The products, CH3NH2 and H, of the reaction (2) proceed to produce the products (CH2NH2, H2 )of the reaction(3). Reaction (3) is50easy to occur because of its low barrier. NH2 + CH4 ??CHaNHi +H is the endothermic reaction and the reaction energy is 20.07 kcal/mol; reaction(3) is the exothermic reaction with the energy of 12.29kcal/mol?The calculated rate constants are in agreement with the experimental results with the temperature range from 300-2lOOK.Especially around 2100K, the theoretical results approximate better with the experimental data. No attempt on theoretical and experimental rate constants of the reaction (2) and reaction (3) documents are available up till now. Taking reaction (2) into consideration, the results by using the B3LYP and QCISD(T) level are almost the same. As for rate constants of the reaction (3), the differences between using the two levels are very small in the range of low temperature. Furthermore, when we considering the quantum mechanical tunneling effects, the results show that the influences on the theoretical rate constants are obvious only at low temperature range.