Experimental and theoretical investigation of the reaction NH(X3Sigma-) + H(2S)-->N(4S) + H2(X1)Sigmag +)

The rate coefficient of the reaction NH(X (3)Sigma(-)) + H((2)S)-->(k(1a) )N((4)S) + H(2)(X (1)Sigma(g) (+)) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures (2 mbar< or =p< or =10 mbar). The NH(X) radicals are produced via the quenching of NH(a(1)Delta) (obtained by photolyzing HN(3)) with Xe whereas the H atoms are generated in a H(2)He microwave discharge. The NH(X) concentration profile is measured under pseudo-first-order condition, i.e., in the presence of a large excess of H atoms. The room temperature rate coefficient is determined to be k(1a) = (1.9 +/- 0.5) x 10(12) cm(3) mol(-1) s(-1). It is found to be independent of the pressure in the range considered in the present experiment. A global potential energy surface for the (4)A(") state is calculated with the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadruple zeta atomic basis. The title reaction is investigated by classical trajectory calculations on this surface. The theoretical room temperature rate coefficient is k(1a) = 0.92 x 10(12)cm(3) mol(-1) s(-1). Using the thermodynamical data for the atoms and molecules involved, the rate coefficient for the reverse reaction, k(-1a), is also calculated. At high temperatures it agrees well with the measured k(-1a).