Thermochemistry Is Not a Lower Bound to the Activation Energy of Endothermic Reactions:  A Kinetic Study of the Gas-Phase Reaction of Atomic Chlorine with Ammonia

Abstract

The rate constant for Cl + NH 3 f HCl + NH 2 has been measured over 290-570 K by the time-resolved resonance fluorescence technique. Ground-state Cl atoms were generated by 193 nm excimer laser photolysis of CCl 4 and reacted under pseudo-first-order conditions with excess NH 3 . The forward rate constant was fit by the expression k 1 ) (1.08 ( 0.05) × 10 -11 exp(-11.47 ( 0.16 kJ mol, where the uncertainties in the Arrhenius parameters are (1 σ and the 95% confidence limits for k 1 are (11%. To rationalize the activation energy, which is 7.4 kJ mol -1 below the endothermicity in the middle of the 1/T range, the potential energy surface was characterized with MPWB1K/6-31++G(2df,2p) theory. The products NH 2 + HCl form a hydrogen-bonded adduct, separated from Cl + NH 3 by a transition state lower in energy than the products. The rate constant for the reverse process k -1 was derived via modified transition state theory, and the computed k -1 exhibits a negative activation energy, which in combination with the experimental equilibrium constant yields k 1 in fair accord with experiment.

Kinetic studies of the reaction of atomic chlorine with chlorobenzene

“…This prediction is consistent with the negative temperature dependence we observed for the reaction of C6H5 with HCl [10]. If k-1 exhibits Ea < 0, then Ea for k1 will be somewhat less than the endothermicity [17].…”

Analytical potential energy surface for the reaction with intermediate complexes NH3 + Cl → NH2 + HCl: Application to the kinetics study

“…Different laboratories have reported electronic structure calculations for the title reaction using very different levels (correlation energy and basis set) [24][25][26][27]. Thus, Kondo et al [25] performed ab initio calculations using the Gaussian-2 (G2) theory, finding an enthalpy of activation at 0 K, [DH = (0 K), zeropoint corrected barrier height], of 4.75 kcal mol…”

“…In 1977, using the flash photolysis resonance fluorescence technique, Westenberg and DeHass [23] determined the rate constant at room temperature, k(298 K) ¼ 1.23 Â 10 À13 cm molecule À1 s À1 . Recently, Gao et al [24], using the time-resolved resonance fluorescence technique, presented the first determination of the temperature dependence in the range 290-570 K for this endothermic reaction. Their fit to the forward rate constant was k(T) ¼ (1.08 6 0.05) Â 10 À11 exp(À2.…”

“…Later, in 2006, Gao et al [24] constructed the reaction path using the MPWB1K density functional (DFT) method [29], finding DH…”

Communication: Covalent nature of X⋯H2O (X = F, Cl, and Br) interactions

“…Interestingly, a similar 2c-3e interpretation has been proposed to explain the "strongly" bound Cl· · ·NH 3 complex, which has been repeatedly found at various levels of theory. 35,44,45 The concept of the three-electron bond, first proposed by Pauling, 46 predicts a longer bond length and a weaker bond strength than those in a typical two-electron chemical bond. Such 2c-3e bonds are prevalent in free-radical chemistry in the gas phase, solutions, solids, as well as biological systems, particularly in radical intermediates in electron transfer.…”