A theoretical study of the dissociation of NO2

Abstract

A combination of ab initio quantum chemical and variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculations are employed in a detailed theoretical modeling of the NO2 dissociation process. Estimates of the interaction energies between NO and O in the transition state region are obtained at the multireference singles and doubles configuration interaction level employing a 6-31G* basis set. A two-dimensional variational optimization of the transition state number of states is performed employing an analytic potential energy function obtained from a fit to the present quantum chemical data. The resulting theoretical estimates of the energy resolved rate constants and product vibrational distributions are compared with the corresponding experimentally determined values. The effect of quantum mechanics on the number of states is considered via a comparison of quantum and semiclassical evaluations for an assumed center-of-mass separation distance reaction coordinate.

Photoinitiated unimolecular decomposition of NO2: Rotational dependence of the dissociation rate

“…However, above D 0 , (E) cannot be obtained spectroscopically, since absorption spectra are congested-even in the first few cm Ϫ1 above D 0 with samples cooled to a few K. 4,7,33,34 Consequently, (E) must be either calculated or obtained by extrapolation from below D 0 . 12,14,16,22 The dramatic increase of (E) observed near D 0 ͑Refs. 1, 4, 7, 32, 58, and 62͒ casts doubt on the viability of extrapolations in the immediate vicinity of D 0 .…”

“…Even state-of-the-art calculations of W ‡ (E) and (E) cannot reproduce near-threshold effects. 12,13,[59][60][61] The above limitations notwithstanding, the energy dependence of Eq. ͑12͒ can assist in the determination of how the dissociation rate constant changes with parent rotation.…”

“…Klippenstein et al also place the TS at ϳ3 Å. 12 Furthermore, NO 2 rotation facilitates tightening of the TS near threshold, since the binding energy at 3 Å is tens of cm Ϫ1 , i.e., comparable to the centrifugal barriers. Although the TS is thought to be at much larger separations right at D 0 , there is no reliable model to describe quantitatively the TS tightening, and for the present calculations the TS was set at 3 Å regardless of excitation energy and angular momentum.…”

“…[11][12][13][14][15] In the energy range of interest in the present study ͑i.e., less then 120 cm Ϫ1 above D 0 ) only the ground vibrational state of NO(X 2 ⌸ 1/2 ) and the lowest oxygen spin-orbit state ( 3 P 2 ) are accessible.…”

Unimolecular Reaction of NO2: Overlapping Resonances, Fluctuations, and the Transition State

“…We then examine dissociation at E † of several hundred to several thousand cm -1 , where the average degree of level overlap is more severe; here, the TS has significantly tightened, and its levels are best described as bending vibrations. 44,45 The model described here is based on assumptions inherent in statistical theories; i.e., the parent eigenstates are ergodic, and the formation and decay of the activated complex have uncorrelated phases. Consequently, we assume that the parent level spacings are described by a Wigner-like distribution.…”