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

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 $Δ H°f,298(NH2)= 44.3± 1.1 kcal mole−1$⁠.