title: 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 + NH3 --> HCl + NH2 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 CCl4 and reacted under pseudo-first-order conditions with excess NH3. The forward rate constant was fit by the expression k1 = (1.08 +/- 0.05) x 10(-11) exp(-11.47 +/- 0.16 kJ mol(-1)/RT) cm3 molecule(-1) s(-1), where the uncertainties in the Arrhenius parameters are +/-1 sigma and the 95% confidence limits for k1 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 NH2 + HCl form a hydrogen-bonded adduct, separated from Cl + NH3 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 k1 in fair accord with experiment.
authors.affiliation: Department of Chemistry, University of North Texas, P. O. Box 305070, Denton, Texas 76203-5070, USA.
authors.forename: Yide
authors.initials: Y
authors.lastname: Gao
authors.forename: I M
authors.initials: IM
authors.lastname: Alecu
authors.forename: P-C
authors.initials: PC
authors.lastname: Hsieh
authors.forename: Brad P
authors.initials: BP
authors.lastname: Morgan
authors.forename: Paul
authors.initials: P
authors.lastname: Marshall
authors.forename: Lev N
authors.initials: LN
authors.lastname: Krasnoperov