Thermochemistry is not a lower bound to the activation energy of endothermic reactions: A kinetic study of the gas-phase reaction of atomi-chlorine with ammonia

Document Type

Article

Publication Date

6-1-2006

Abstract

The rate constant for Cl + NH 3 → 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 -1/RT) cm 3 molecule -1 s -1, 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 + HC1 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. © 2006 American Chemical Society.

Identifier

33745457613 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp056406l

ISSN

10895639

First Page

6844

Last Page

6850

Issue

21

Volume

110

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