Experimental and theoretical investigations of the reactions

The rate coefficient of the reaction NHsX 3S−d+Ds2Sd→ k1 products s1d is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures. The NHsXd radicals are produced by quenching of NHsa Dd sobtained in the photolysis of HN3d with Xe and the D atoms are generated in a D2/He microwave discharge. The NHsXd concentration profile is measured in the presence of a large excess of D atoms. The room-temperature rate coefficient is determined to be k1= s3.9±1.5d31013 cm3 mol−1 s−1. The rate coefficient k1 is the sum of the two rate coefficients, k1a and k1b, which correspond to the reactions NHsX 3S−d+Ds2Sd→ k1a NDsX Sd+HsSd s1ad and NHsX 3S−d+Ds2Sd→ k1b NsSd+HDsX Sg d s1bd, respectively. The first reaction proceeds via the A9 ground state of NH2 whereas the second one proceeds in the A9 state. A global potential energy surface is constructed for the A9 state using the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadrupte zeta atomic basis. This potential energy surface is used in classical trajectory calculations to determine k1a. Similar trajectory calculations are performed for reaction s1bd employing a previously calculated potential for the A9 state. The calculated room-temperature rate coefficient is k1=4.1 31013 cm3 mol−1 s−1 with k1a=4.0310 13 cm3 mol−1 s−1 and k1b=9.1310 11 cm3 mol−1 s−1. The theoretically determined k1 shows a very weak positive temperature dependence in the range 250 øT /Kø1000. Despite the deep potential well, the exchange reaction on the A9 ground-state potential energy surface is not statistical. © 2005 American Institute of Physics. fDOI: 10.1063/1.1899563g