KINETICS OF THE NH2+NO REACTION - EFFECTS OF TEMPERATURE ON THE TOTAL RATE-CONSTANT AND THE OH/H2O BRANCHING RATIO

The rate constant for the reaction of NH2 with NO has been measured between 297 and 673 K using the cavity-ring-down technique to monitor the disappearance of the NH2 radical. The measured bimolecular rate constant can be effectively represented by the expression k(II) = (2.2 +/- 0.7) X 10(-12) exp[525 +/- 80)/T] cm(3)/s, which agrees reasonably well with the results of several other recent measurements employing various diagnostic methods. A multichannel RRKM calculation has been carried out to account for the observed negative temperature dependence and the product branching ratio, OH/H2O, based on Walch's recent potential energy surface data for various transition states and stable intermediates leading to the formation of the OH and H2O products. The predicted temperature dependencies agree reasonably well with experimental observations. We have also performed kinetic modeling using a set of of reactions involving H, NH3, NH2, NO, and their anticipated products. The result of the modeling aided by sensitivity analysis suggests that the unknown ''third channel'' responsible for the decline of the ([OH] + [H2O])/[NH2](0) ratio at high temperatures (ref 23) may result from secondary reactions which produce neither OH nor H2O. These reactions include NH2 + H --> NH + H-2 and NH2 + NH2 --> NH + NH3.