skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O[sub 2]

Abstract

The rate coefficients of reactions that occur on potential energy surfaces without a barrier often exhibit a negative temperature dependence at low temperatures. Generally, this behavior is modeled with either the Harcourt[endash] Essen equation, k(T)=AT[sup [minus] m], or a [open quotes] negative[close quotes] activation energy, k(T)=AT[sup m] [l brace] [Delta] E/k[sub B] T[r brace]. Neither of these expressions is consistent with the Wigner threshold law. The general expression k(T)=(1+T/T[sub W]) [sup [minus]m] [summation] [sub l=0] [sup [infinity]] A[sub l] (1+T/T [sub W]) [sup [minus]l] (T/T[sub W]) [sup l] is proposed where the relative angular momentum of the reacting species is l, T[sub W] and m are independent parameters to be extracted from the data, and the amplitude of each partial wave is A [sub l]. This expression may be approximated by k(T)=A [sub 0] (1+T/T [sub W]) [sup [minus] m] [(T/T [sub W])/(1+T/T [sub W])]. For CN+O [sub 2] [r arrow] NCO+O and CO+NO the above expression reproduces the rate data, the branching ratio to the CO+NO channel, and the reactive cross section for the NCO+O channel. The rate coefficient for the NCO+O channel is given by k(cm[sup 3] [sup [minus] 1])=1.79 [times] 10 [sup [minus] 10] (+T/21.7) [sup [minus]more » 1.38] [l brace] [(T/21.7)/(1+T/21.7)] [minus] 1 [r brace] +4.62[times] 10 [sup [minus] 12] exp [(T/21.7)/(1+T/21.7)] while for CO+NO we obtain k(cm [sup 3] [sup [minus] 1])=1.79 [times] 10 [sup [minus] 10] (1+T/21.7) [sup [minus] 1.38]. An analytic form of the C[endash]O bonding potential and the electric dipole[endash]quadrupole interaction is used to show that the quantum threshold region extends up to 7 K. These results demonstrate the need of a complete quantum treatment for reactions that proceed on potential surfaces without a barrier. [copyright] [ital 1999 American Institute of Physics.]« less

Authors:
 [1]
  1. (Chemistry Division, Argonne National Laboratory, 9700 So. Cass Ave., Argonne, Illinois 60439-4831 (United States))
Publication Date:
OSTI Identifier:
6378259
Alternate Identifier(s):
OSTI ID: 6378259
Resource Type:
Journal Article
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 111:9; Journal ID: ISSN 0021-9606
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CARBON COMPOUNDS; CHEMICAL REACTION KINETICS; CYANOGEN; OXYGEN; POTENTIAL ENERGY; RADICALS; TEMPERATURE DEPENDENCE; ELEMENTS; ENERGY; KINETICS; NONMETALS; REACTION KINETICS 400201* -- Chemical & Physicochemical Properties

Citation Formats

Hessler, J.P. New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O[sub 2]. United States: N. p., 1999. Web. doi:10.1063/1.479183.
Hessler, J.P. New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O[sub 2]. United States. doi:10.1063/1.479183.
Hessler, J.P. Wed . "New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O[sub 2]". United States. doi:10.1063/1.479183.
@article{osti_6378259,
title = {New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O[sub 2]},
author = {Hessler, J.P.},
abstractNote = {The rate coefficients of reactions that occur on potential energy surfaces without a barrier often exhibit a negative temperature dependence at low temperatures. Generally, this behavior is modeled with either the Harcourt[endash] Essen equation, k(T)=AT[sup [minus] m], or a [open quotes] negative[close quotes] activation energy, k(T)=AT[sup m] [l brace] [Delta] E/k[sub B] T[r brace]. Neither of these expressions is consistent with the Wigner threshold law. The general expression k(T)=(1+T/T[sub W]) [sup [minus]m] [summation] [sub l=0] [sup [infinity]] A[sub l] (1+T/T [sub W]) [sup [minus]l] (T/T[sub W]) [sup l] is proposed where the relative angular momentum of the reacting species is l, T[sub W] and m are independent parameters to be extracted from the data, and the amplitude of each partial wave is A [sub l]. This expression may be approximated by k(T)=A [sub 0] (1+T/T [sub W]) [sup [minus] m] [(T/T [sub W])/(1+T/T [sub W])]. For CN+O [sub 2] [r arrow] NCO+O and CO+NO the above expression reproduces the rate data, the branching ratio to the CO+NO channel, and the reactive cross section for the NCO+O channel. The rate coefficient for the NCO+O channel is given by k(cm[sup 3] [sup [minus] 1])=1.79 [times] 10 [sup [minus] 10] (+T/21.7) [sup [minus] 1.38] [l brace] [(T/21.7)/(1+T/21.7)] [minus] 1 [r brace] +4.62[times] 10 [sup [minus] 12] exp [(T/21.7)/(1+T/21.7)] while for CO+NO we obtain k(cm [sup 3] [sup [minus] 1])=1.79 [times] 10 [sup [minus] 10] (1+T/21.7) [sup [minus] 1.38]. An analytic form of the C[endash]O bonding potential and the electric dipole[endash]quadrupole interaction is used to show that the quantum threshold region extends up to 7 K. These results demonstrate the need of a complete quantum treatment for reactions that proceed on potential surfaces without a barrier. [copyright] [ital 1999 American Institute of Physics.]},
doi = {10.1063/1.479183},
journal = {Journal of Chemical Physics},
issn = {0021-9606},
number = ,
volume = 111:9,
place = {United States},
year = {1999},
month = {9}
}