KINETICS OVER A WIDE-RANGE OF TEMPERATURE (13-744 K) - RATE CONSTANTSFOR THE REACTIONS OF CH(NU=0) WITH H-2 AND D-2 AND FOR THE REMOVAL OFCH(NU=1) BY H-2 AND D-2

Rate constants were determined for the reactions of CH((XII)-I-2,v=0) with H-2 and D-2 and for the relaxation of CH((XII)-I-2,v=1) by Hz and D-2. The method, employing pulsed laser photolysis to generate CH rad icals and laser-induced fluorescence to observe their rate of removal, was implemented between 744 and 86 K in heated and cryogenically cool ed cells and from 295; to 13 K in a Cinetique de Reaction en Ecoulemen t Supersonique Uniforme (CRESU) apparatus, The rate constants for the reaction of CH(v=0) with D-2 Were determined from 13 to 584 K and thos e for the removal of CH(v=1) by H-2 and D-2 from 23 to 584 K. These ra te constants show no dependence on total pressure and a mild negative temperature dependence, and they are clearly related closely to the ra te of capture to form a strongly bound CH3 or CHD2* energized collisi on complex. The rate constants for the reaction of CH(v=0) with H-2 we re measured from 53 to 744 K. By contrast, their values depend in a co mplex fashion on temperature and total pressure, the latter effect hav ing been studied from 4 to 400 Torr in heated and cooled cells and fro m 0.28 to 4.5 Torr al 53 K in the CRESU apparatus. It is clear that re action proceeds via a CH3 complex which, favored by high temperatures and low total pressures, can dissociate to give CH2+H, CH((XII)-I-2)H-2-->CH2+H; Delta(r)H(298)(0)=+(12.0+/-5.2) kJ mol(-1) or, especially at low temperatures and high total pressures, may be stabilized by co llisions to yield CH3 radicals CH((XII)-I-2)+H-2+M-->CH3+M; Delta(r)H( 298)(0)=-(450.(7)+/-2.0) kJ mol(-1). Careful analysis of the pressure- dependent fall-off behavior yields the following expressions for the r ate constants (in the case of the association reaction, in the limit o f low pressure): k((1a))=(3.1+/-0.7)x10(-10) exp(-(1650+/-100)/T) cm(- 3) molecule(-1)s(-1), 0)[Ar]-(5.2+/-1.5)x10(-30)(T/298)(-1.6+/-0.22)[A r] cm(3) molecule(-1)s(-1). Comparison of the rate data obtained for t he reaction of CH(v=0) with D-2 and for the removal of CH(v=1) by H-2 and D-2 is consistent with all three processes occurring through the f ormation of a CH3 (CHD2*) complex whose rate of formation is independ ent of the vibrational state of the CH. Consequently the rate constant for the reaction in the limit of high pressure can be estimated from that for the removal of CH(v=1) by H-2 yielding: nfinity)=(1.60+/-0.11 )x10(-10)(T/298)(0.08+/-0.05) cm(3) molecule(-1)s(-1). (C) 1997 Americ an Institute of Physics.