No Barrier for the Gas-Phase C2H + NH3 Reaction

Abstract

The absolute rate constant of the reaction C2H + NH3 → products has been experimentally determined over the temperature range 295−765 K at a pressure of 10 Torr (He) and over the pressure range 5−30 Torr at 295 K. The concentration of C2H radicals was monitored in real time using the CH chemiluminescence method following their production by pulsed laser photolysis of C2H2 at 193 nm in a slow-flow reaction vessel. The pressure-independent rate constant is large and exhibits a pronounced negative temperature dependence: k1(T) = (1.2 ± 0.2) × 10-11 exp[(370 ± 40) K/T] cm3 s-1, both unusual for reaction of a radical with a saturated molecule. This may be rationalized by strong dipole−dipole and dipole − quadrupole interactions, as have been put forward for the isoelectronic CN + NH3 reaction. The C2H + NH3 reaction is somewhat faster than CN + NH3, even though ab initio calculations predict the CCH dipole moment to be smaller than that for CN. However the opposite sign of the dipole moment of C2H implies an initial HCC- - -HNH2 alignment, favorable for subsequent H-abstraction. The (extrapolated) high rate constant both at combustion temperatures and at interstellar temperatures should make this reaction very important in those environments.