Diatomic molecules atomic reactions with

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. ... 

In measuring this rate constant, one must take into account the self reactions of OH and H which occur concurrently with their cross reaction (R7 in Table 1). Data obtained by Buxton and Elliot [14] show that k(H + OH) has the same temperature dependence as A (OH + OH), with P= and Eact = 0 kJ mol , so that kreaci has au influence on kob + OH). This result is quahtatively consistent with transition-state theory [21] which, for the gas phase, predicts that the reaction probability for an atom and a diatomic molecule is 10-100 times smaller than for two atoms. A comparison of /c(H + OH), k(H + H) and k(OH +OH) is shown in Figure 5. The similarity in the values of k(H + H) and /c(OH +OH) at 25 °C is due mainly to the different spin factors, p, for the two reactions whidi counterbalance the difference between Dh and Dqh-... 

When an electric discharge is passed through a cold diatomic gas at low pressure it is partially dissociated into atoms in this way reasonable concentrations of O, H, D, N, halogen or other atoms can be produced in a chemically inert diluent. The recombination of these atoms, and their reaction with other molecules can be observed as the gas flows down a long tube. Many of the reactions produce molecules in excited electronic states the resulting chemiluminescence can be used to measure the concentration of atomic species as a function of distance, and hence time, down the tube. Dr Clyne describes this important technique, which has produced direct measurements of the rates of many exothermic reactions of atoms and free radicals at room temperature and below. The reverse of the recombination steps are, of course, the dissociation reactions whose kinetics at high temperatures were described in the first chapter if the ratio of forward and reverse rate constants is equal to the equilibrium constant, the temperature dependence of these rates can be deduced over very wide ranges of temperature. 

Now consider a reaction between an atom and a diatomic molecule, A + BC — AB + C, where a stable triatomic molecule ABC does not exist. The PES can be described with the same coordinates used for the stable triatomic, but it will have a different shape because the minimum... 

Consider first the weak charge redistribution such as for the reaction of a hydrogen atom with a hydrogen molecule. Here, Ta,bc ab,c he constructed as the product of the electronic wave function of a free atom and a diatomic molecule, the latter being expressed by the Heitler-London approximation [133, 162]. Then V corresponds to the interaction between two valence structures, one for spin coupling of electrons localized on protons B and C, and the other for those localized on A and B. Calculation yields the London expression for the interaction energy of free atoms in the S state [280]. 

The experimental results which we will describe are primarily those obtained in this laboratory but a few experimental data exist which have been collected elsewhere. Our experimental program in fluorine atom chemistry has been motivated primarily by two facts which have also been important to studies performed by other methods (1) atomic fluorine abstraction of hydrogen atoms from appropriate molecules has been demonstrated to be an important class of reactions for chemical lasers W. In particular, the reactions of F + H2 HF + H and F + D DF + D have been investigated in great detail by various theoretical and experimental approaches (3-11) the latter reaction provides us with an example from the general class of reactions of fluorine atoms with diatomic molecules. (2) Substitution reactions of fluorine atoms with unsaturated hydrocarbons Involving the formation of C-F bonds frequently are observed to proceed through a "complex" which... 

The next simplest reaction of F is the reaction with molecular hydrogen and its stable isotopic variants. A typical result is giv n,in Fig. where the cross sections of the reaction of F ( P) and F D) are plotted vs. center of mass kinetic energy assuming that the reaction is proceeding by a direct mechanism involving one atom of the diatomic molecule. 

Next, consider a reaction between an atom and a diatomic molecule, A + BC. Reactions can differ also in their energetic requirements, but to focus attention on the steric requirements with the energetic effects being equal we take the barrier height Eq to be the same as in the previous reaction. To have minimal steric requirements let us take the transition state, ABC, to be bent. This choice allows A to approach BC within a cone. Because BC has an internal structure, the partition function for the reactants becomes Q = Q Q QvQ - The transition state is a bent triatomic. It has three vibrations, one of which is the reaction coordinate. (As we saw in Section 5.1, this is the asymmetric stretch vibration.) The bent transition state has three planes of rotation, = Q QIQ - Accordingly, for reasons that will become immediately apparent, we write k T) as... 

Advances in quantum chemistry and scattering theory have enabled essentially exact calculations to be performed on very simple reactions of atoms with diatomic molecules such as ... 

Quantum scattering calculations on the reactions of atoms with diatomic molecules for linear geometry have played a very useful role (for example, clearly demonstrating the importance of vibrational enhancement and resonances in such reactions, which still exist in three-dimensional studies ). Many three-atom reactions have minimum energy paths in the potential energy surface that are close to linear geometries, so... 

There continues to be considerable interest in the dynamics of reactions of atoms with diatomic molecules, and classical trajectory calculations continue to play an important role. These reactions are important since there is the possibility that we can realize a nearly complete understanding of them, and it is feasible to do extensive calculations of the potential energies by ab initio methods thus allowing for purely theoretical predictions to be made. Furthermore, it is possible to have rigorous quantum mechanical scattering results with which to... 

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]. 

Fast transient studies are largely focused on elementary kinetic processes in atoms and molecules, i.e., on unimolecular and bimolecular reactions with first and second order kinetics, respectively (although confonnational heterogeneity in macromolecules may lead to the observation of more complicated unimolecular kinetics). Examples of fast thennally activated unimolecular processes include dissociation reactions in molecules as simple as diatomics, and isomerization and tautomerization reactions in polyatomic molecules. A very rough estimate of the minimum time scale required for an elementary unimolecular reaction may be obtained from the Arrhenius expression for the reaction rate constant, k = A. The quantity /cg T//i from transition state theory provides... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

Other reactions have extremely small equilibrium constants. For example, elemental fluorine, a diatomic molecule under standard conditions, is nevertheless at equilibrium with fluorine atoms ... 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

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