Nonempirical parameter estimate for the statistical adiabatic theory of unimolecular fragmentation

Abstract

For the C-H bond fission in CH 3 , the reaction path, the potential profile and the frequencies of the normal modes perpendicular to the reaction path are calculated by means of ab initio quantum chemical approaches (UHF, second-order Moller-Plesset perturbation theory). On the whole, these data confirm the assumptions made in the semiempirical statistical adiabatic channel model of Quack and Troe qualitatively; in particular, the decrease of the frequencies of reactant bending modes which transform into free rotations of the product is well reproduced by an exponential switching function with a common parameter. However, this single-parameter switching function cannot be used for all vibrations changing their eigenvalues. Furthermore, within the quantum chemical approaches employed, the Morse function with a constant parameter fl is a crude approximation to the potential energy profile. On the basis of these theoretically estimated data, adiabatic rovibronic energy terms are constructed and high-pressure thermal rate constants are calculated; the latter turn out to be predominantly determined by the bond dissociation energy. For comparison, high-pressure recombination rate constants are given.

Global Analytical Potential Energy Surfaces for High‐resolution Molecular Spectroscopy and Reaction Dynamics

“…More generally, for the description of elementary reactions via statistical theories, PESs often need to be evaluated along a reaction path (Quack and Troe 1974, 1998, Kraka and Dunning 1990). In a pioneering paper Merkel and Zülicke (1987) provided an ab initio estimate of relevant potential parameters for the statistical adiabatic channel model in the bond fission reaction of CH 3 .…”

Capture and dissociation in the complex-forming CH+H2→ CH2+H, CH+H2 reactions

“…1 This reaction is important in combustion processes 2,3 and in astrochemistry, [4][5][6][7] and, consequently, there are numerous experimental and theoretical studies about it. 1,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Experimental kinetics studies provide both the capture and the abstraction rate coefficients of the title reaction for a wide range of temperatures, [8][9][10][11][12][13][14][15][16][17] including also some isotope variants. Moreover, the D atom exchange reaction, CH+D 2 -CD+HD, was studied using a crossed molecular beam technique.…”

“…On the theoretical side, attention has been devoted to the potential energy surface (PES), the kinetics and the dynamics. 1,[20][21][22][23][24][25][26][27] The most recent global PES (MHG PES), 26 which is based on high level ab initio data, indicates that the CH+H 2 -CH 2 +H reaction is barrierless, the deep CH 3 intermediate being in the minimum energy path (MEP). However, the MEP is somewhat unusual.…”

The rate of methyl radical decomposition at high temperatures and pressures

“…frigid, the rigidity factor, equals to 0.726 (see Table 11), implying a reaction with a nearly loose activated complex. In Merkel and Zulicke's study [13], the molecular data set B corresponds to the "looser" transition state. It can be seen from Figure 3 that their kdiss,, of data set B ([MI = m) are very close to eq.…”