New empirical rate expression for reactions without a barrier: Analysis of the reaction of CN with O{sub 2}
- Chemistry Division, Argonne National Laboratory, 9700 So. Cass Ave., Argonne, Illinois 60439-4831 (United States)
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.}
- OSTI ID:
- 365977
- Journal Information:
- Journal of Chemical Physics, Vol. 111, Issue 9; Other Information: PBD: Sep 1999
- Country of Publication:
- United States
- Language:
- English
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