Triatom-atom reaction

Reactivity of Triatomic Molecules.— The outermost, unpaired electron of the ground state of the NOj molecule lies in the i sa-Su orbital. From the arguments above, this electron is more localized on the N atom than on the 0 atoms. Reaction of NOj with a free radical is therefore likely to form a nitro-compound rather than a nitrite. Inhibition of certain free-radical reactions by NOj may plausibly be attributed to such reaction the inhibition occurs under conditions where addition of a nitro-compound has no effect, but addition of a nitrite catalyses the reaction. Similarly, initiation of chains by H-abstraction by NOj (see, e.g., McDoweU and Thomas, Trans. Faraday Soc., 1950, 46,... 

In three-atom systems, internal excitation is limited to two rotational and one vibrational degree of freedom in the reactant or product diatom. In four-atom reactions, the triatomic reactant or product (if it is non-linear) has three vibrational degrees of freedom. Thus, when a triatomic molecule is formed in a four-atom reaction it becomes possible... 

This chapter presents an overview of first principles theory for diatom-diatom and atom-triatom reactions in comparison with experiments. It is applied to the prototype H2 + OH and H + H2O reactions and their... 

Similar to the diatom-diatom reaction, the initial wavefunction is chosen as the direct product of a localized translational wavepacket for R and a specific (JMe) state for the atom-triatom system with a specific rovibrational eigenstate (z/o, Lo,Bo) f°r the triatom ABC ... 

In both the diatom-diatom and atom-triatom reactions, the energy-dependent scattering wavefunction is obtained by a Fourier transform of the propagated wavepacket ... 

The initial state-specific reaction rate constant for both diatom-diatom and atom-triatom reactions is calculated by averaging the corresponding cross-section over a Boltzmann distribution of translational energy ... 

Fig. 26. Total and final rovibrational state specific integral cross-sections for the H + H2O(00)(0) —> H2(i>i, ji) + OH(j2) reaction as a function of translational energy. The total cross-sections in crosses were calculated in the atom—triatom coordinates.

An example of a potential surface of a reacting system in Figure 22.1. This kind of plot shows the potential energy for a special (collinear or bent) arrangement of atoms during a bimolecular reaction in a triatomic system ... 

V.B). Experimental distributions obtained for the Ne+-N2 system (Fig. 54) were shown to be intermediate between those predicted by application of the Franck-Condon principle and those predicted by such a statistical model. Finally, it has been suggested by Tomcho and Haugh411 that the molecular orbitals of the transient triatomic system should be considered in predicting vibrational distributions of products from atomic-ion-diatomic-molecule reactions. Calculations based on this approach are in progress for the (Ar-N2)+ system.417... 

Calculations of potential surfaces are now in progress for a number of triatomic systems of atmospheric importance.488, 489 These have direct pertinence to various atomic-ion-diatomic-molecule reactions. 

All of the surfaces for reactions have more than three dimensions. For a tri-atomic system there are three independent coordinates (3N—6) and the potential energy function V(rlt r2, r3) is a surface in a four dimensional space. The potential function usually shown for a triatomic system ABC is a three dimensional projection of this four dimensional space, the ABC angle being held fixed. Motion restricted to such a projected surface allows no rotation of BC relative to A at large distances and no bending vibration of ABC at short distances. 

Note that in a non-linear molecule, one of the vibrational modes of the linear molecule has been replaced by a rotational coordinate. As an illustration, let us consider two examples. For the stable linear triatomic molecule CO2, there are 3 x 3 — 5 = 4 internuclear coordinates, which corresponds to the vibrational degrees of freedom, namely the symmetric and antisymmetric stretch and two (degenerate) bending modes (see Appendix E). For the three atoms in the reaction D + H — H—> D — H + H, there are 3 x 3 — 6 = 3 internuclear coordinates. These coordinates can, for example, be chosen as a D H distance, the H H distance, and the I) II H angle. 

The central concept of mode-selective chemistry is illustrated in Fig. 1, which depicts the ground and excited state potential energy surfaces of a hypothetical triatomic molecule, ABC. One might wish, for example, to break selectively the bond between atoms A and B to yield products A+BC. Alternatively, one might wish to activate that bond so that in a subsequent collision with atom D the products AD+BC are formed. To achieve either goal it is necessary to cause bond AB to vibrate, thereby inducing motion along the desired reaction coordinate. 

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