Chemical equilibrium of NH2−+H2⇄ H−+NH3 and the determination of D° (NH2–H)

Abstract

The proton transfer reaction NH2−+H2⇄ H−+NH3 has been investigated in a flowing afterglow system independently in both directions at 297°K. The rate constants were found to be kforward= (2.3± 0.5)× 10−11 cm3 molecule−1· sec−1 and kreverse= (9.2± 1.8)× 10−13 cm3 molecule−1· sec−1. The ratio of rate constants, kf/kr= 26−5+6 was found to be in agreement with the equilibrium constant, K= 27± 9, determined from equilibrium concentrations. The value for K corresponds to a standard free energy change, Δ G°297, of− 1.9± 0.2 kcal mole−1. A calculated value for the standard entropy change, ΔS°298, of −4.3± 0.5 cal deg−1· mole−1 leads to a value for the standard enthalpy change, Δ H°298, of−3.2± 0.3 kcal mole−1. Adoption of a recent photodetachment value for the electron affinity of NH2 leads to D°0(NH2–H)= 106.0± 1.1 kcal mole−1, D°298(NH2–H)= 107.4± 1.1 kcal mole−1, and Δ H°f,298(NH2)= 44.3± 1.1 kcal mole−1.