Polyatomic reactions

The reaction of an atom with a diatomic molecule is the prototype of a chemical reaction. As the dynamics of a number of atom-diatom reactions are being understood in detail, attention is now being turned to the study of the dynamics of reactions involving larger molecules. The reaction of Cl atoms with small aliphatic hydrocarbons is an example of the type of polyatomic reactions which are now being studied [M, 72, 73]. 

Zhang J Z H 1999 The semirigid vibrating rotor target model for quantum polyatomic reaction dynamics J. 

We will now discuss some very recent applications of the soft El ionization method for product detection in CMB experiments. We will first deal with two polyatomic reactions of ground state oxygen atoms with unsaturated hydrocarbons (acetylene and ethylene) these reactions are characterized by multiple reaction pathways and are of great relevance, besides being from a fundamental point of view, in combustion and atmospheric chemistry. 

In the adiabatic bend approximation (ABA) for the same reaction,18 the three radial coordinates are explicitly treated while an adiabatic approximation was used for the three angles. These reduced dimensional studies are dynamically approximate in nature, but nevertheless can provide important information characterizing polyatomic reactions, and they have been reviewed extensively by Clary,19 and Bowman and Schatz.20 However, quantitative determination of reaction probabilities, cross-sections and thermal reaction rates, and their relation to the internal states of the reactants would require explicit treatment of five or the full six degrees-of-freedom in these four-atom reactions, which TI methods could not handle. Other approximate quantum approaches such as the negative imaginary potential method16,21 and mixed classical and quantum time-dependent method have also been used.22... 

Truhlar, D. G. and Steckler, R. Potential energy surfaces for polyatomic reaction dynamics, Chem. Rev., 87(1987), 217-236... 

Occasionally, the rates of bimolecular reactions are observed to exhibit negative temperature dependencies, i.e., their rates decrease with increasing temperature. This counterintuitive situation can be explained via the transition state theory for reactions with no activation energy harriers that is, preexponential terms can exhibit negative temperature dependencies for polyatomic reactions as a consequence of partition function considerations (see, for example, Table 5.2 in Moore and Pearson, 1981). However, another plausible explanation involves the formation of a bound intermediate complex (Fontijn and Zellner, 1983 Mozurkewich and Benson, 1984). To... 

N. Balucani, G. Capozza, F. Leonori, E. Segoloni, P. Casavecchia, Crossed molecular beam reactive scattering From simple triatomic to multichannel polyatomic reactions, Int. Rev. Phys. Chem. 25 (2006) 109. 

Wang, M.L. and Zhang, J.Z.H. (2002) Generahzed semirigid vibrating rotor target model for atom-polyatom reaction Inclusion of umbrella mode for the H + CH4 reaction, J. Chem. Phys. 

Accurate quantiun dynamics calculations for polyatomic reactions... 

In this chapter, we have shown how the recent advances in the crossed molecular beam technique allow us to study complex polyatomic reactions of relevance in astrochemistry. The focus was on the CN radical reactions with simple alkynes, but the same approach has been also applied to the study of other CN radical reactions with unsaturated small organic molecules, such as ethylene, benzene, and allene, which are of relevance in astrochemistry as well [77,81,84]. 

The higher dimensionality of polyatomic reactions makes them more of a challenge to treat theoretically. Variational transition state theory with multidimensional tunneling has been developed to allow calculations for a wide variety of polyatomic systems. In this section we consider issues that arise when treating polyatomic systems. The Cl -1- CH4 reaction provides a good system for this pur-... 

Several approaches to obtaining both qualitative and quantitative information on polyatomic reaction dynamics via reaction path potentials have been discussed by Miller [13], Truhlar and Gordon [14], Kraka and Dunning [1], Collins [15] and Hammes-SchifFer [16]. 

Ideally, one wishes to develop accurate computational methods to study polyatomic reactions without resorting to drastic dimensionality reductions. In this regard, progress has recently been reported for rigorous quantum dynamics calculations for the four-atom reaction H2 + OH by Zhang and Zhang (77-80), Manthe et al. (81,82), and Neuhauser... 

While it is tempting to apply the concepts developed for atom plus diatomic reactions to polyatomic reactions in general, it becomes rapidly apparent that the situation is much more complex. One of the first reactions studied was the reaction of O3 with NO, reaction (6). Two channels are available, one producing electronically excited NOj [exothermic, reaction... 

The relative importance of vibrational and translational energy in promoting chemical reactions is of both theoretical and practical interest. In reactions of diatomic molecules with atoms it has been substantiated both experimentally and theoretically that for endothermic reactions vibrational energy is more important, while for exothermic reactions the opposite is true. For polyatomic molecules, however, there is insufficient experimental and theoretical evidence to draw conclusions. The major work on laser-excited polyatomic reactions has involved the vibrational excitation of ozone in its exothermic reaction with nitric oxide. Although the vibrational energy increased the reaction rate, comparison with statistical models and the temperature dependence of the thermal reaction indicate about equal importance for vibrational and translational energy. On the other hand, a molecular beam study of the temperature dependence of the reaction of potassium with sulfur hexafluoride" has shown a definite preference for vibrational energy of the SF. ... 

Finally, time dependent methods are yielding interesting views on polyatomic reaction dynamics, although not actually leading to the full state-to-state information. Most promising for the future will be the blending of these methods with the hyperspherical approach under focus in the present paper. 

While Figs. 1 and 2 apply strictly to a collinear A + BC reaction, we know that the picture of a potential energy valley confined (more or less) by repulsive walls also holds for the PES of polyatomic reactions in three dimensions. 

However, the real power of classical trajectory studies comes from the ease with which polyatomic reactions involving many degrees of freedom can be studied using Cartesian coordinates. If one can write the PES in a set of internal coordinates, R, Hamilton s equations of motion are given by... 

---