Collisions reactive

Elementary reactions are characterized by their moiecuiarity, to be clearly distinguished from the reaction order. We distinguish uni- (or mono-), hi-, and trimoiecuiar reactions depending on the number of particles involved in the essential step of the reaction. There is some looseness in what is to be considered essential but in gas kinetics the definitions usually are clearcut through the number of particles involved in a reactive collision plus, perhaps, an additional convention as is customary in iinimolecular reactions. 

It is clear from figure A3.4.3 that the second-order law is well followed. Flowever, in particular for recombination reactions at low pressures, a transition to a third-order rate law (second order in the recombining species and first order in some collision partner) must be considered. If the non-reactive collision partner M is present in excess and its concentration [M] is time-independent, the rate law still is pseudo-second order with an effective second-order rate coefficient proportional to [Mj. 

A bimoleciilar reaction can be regarded as a reactive collision with a reaction cross section a that depends on the relative translational energy of the reactant molecules A and B (masses and m ). The specific rate constant k(E ) can thus fonnally be written in tenns of an effective reaction cross section o, multiplied by the relative centre of mass velocity... 

Quack M 1977 Detailed symmetry selection rules for reactive collisions Mol. Phys. 34 477-504... 

Although the Sclirodinger equation associated witii the A + BC reactive collision has the same fonn as for the nonreactive scattering problem that we considered previously, it cannot he. solved by the coupled-channel expansion used then, as the reagent vibrational basis functions caimot directly describe the product region (for an expansion in a finite number of tenns). So instead we need to use alternative schemes of which there are many. 

Mayne H R 1991 Classical trajectory calculations on gas-phase reactive collisions/of. Rev. Phys. Chem. 10 107-21... 

Marcus R A 1970 Extension of the WKB method to wave functions and transition probability amplitudes (S-matrix) for inelastic or reactive collisions Chem. Phys. Lett. 7 525-32... 

Figure Bl.7.10. Tliree mass spectra showing the results of reactive collisions between a projectile ion C H. NH, isomeric butenes. (Taken from Usypchiik L L, Harrison A G and Wang J 1992 Reactive...

Usypchuk L L, Flarrison A G and Wang J 1992 Reactive collisions in quadrupole cells. Part I. Reaction of [CFl3NFl2] with the isomeric butenes and pentenes Org. Mass Spectrom. 27 777-82... 

Levine R D and Wu S F 1971 Resonances in reactive collisions computational study of the H + Hj collision Chem. Rhys. Lett. 11 557... 

Neuhauser D and Baer M 1990 A new accurate (time independent) method for treating three-dimensional reactive collisions the application of optical potentials and projection operators J. Chem. Phys. 92 3419... 

The preferable theoretical tools for the description of dynamical processes in systems of a few atoms are certainly quantum mechanical calculations. There is a large arsenal of powerful, well established methods for quantum mechanical computations of processes such as photoexcitation, photodissociation, inelastic scattering and reactive collisions for systems having, in the present state-of-the-art, up to three or four atoms, typically. " Both time-dependent and time-independent numerically exact algorithms are available for many of the processes, so in cases where potential surfaces of good accuracy are available, excellent quantitative agreement with experiment is generally obtained. In addition to the full quantum-mechanical methods, sophisticated semiclassical approximations have been developed that for many cases are essentially of near-quantitative accuracy and certainly at a level sufficient for the interpretation of most experiments.These methods also are com-... 

The evidence in Figure 2 for a kinetic energy threshold for reaction of excited H2+ with He does not support the assumption of a kinetic energy transfer process for the excitation of reactant H2+ with v < 5 in reactive collisions with He. If such processes were probable, a drastic change in the maximum value of Q or k might be expected. The transfer of less than 0.5 e.v. of kinetic to internal energy would add quantum states with v = 3 and 4 to the inventory of available H2 + reactant and increase the maximum value of k by a factor of 2. 

Isotope effects which give ratios of XH +/XD + less than unity are perhaps more interesting from the standpoint of energy transfer in reactive collisions. If a collision complex between an inert gas X and HD +... 

Previous theoretical kinetic treatments of the formation of secondary, tertiary and higher order ions in the ionization chamber of a conventional mass spectrometer operating at high pressure, have used either a steady state treatment (2, 24) or an ion-beam approach (43). These theories are essentially phenomenological, and they make no clear assumptions about the nature of the reactive collision. The model outlined below is a microscopic one, making definite assumptions about the kinematics of the reactive collision. If the rate constants of the reactions are fixed, the nature of these assumptions definitely affects the amount of reaction occurring. 

Here va and va are the stoichiometric coefficients for the reaction. The formulation is easily extended to treat a set of coupled chemical reactions. Reactive MPC dynamics again consists of free streaming and collisions, which take place at discrete times x. We partition the system into cells in order to carry out the reactive multiparticle collisions. The partition of the multicomponent system into collision cells is shown schematically in Fig. 7. In each cell, independently of the other cells, reactive and nonreactive collisions occur at times x. The nonreactive collisions can be carried out as described earlier for multi-component systems. The reactive collisions occur by birth-death stochastic rules. Such rules can be constructed to conserve mass, momentum, and energy. This is especially useful for coupling reactions to fluid flow. The reactive collision model can also be applied to far-from-equilibrium situations, where certain species are held fixed by constraints. In this case conservation laws... 

This formulation assumes that the continuum diffusion equation is valid up to a distance a > a, which accounts for the presence of a boundary layer in the vicinity of the catalytic particle where the continuum description no longer applies. The rate constant ky characterizes the reactive process in the boundary layer. If it approximated by binary reactive collisions of A with the catalytic sphere, it is given by kqf = pRGc(8nkBT/m)1 2, where pR is the probability of reaction on collision. 

The simple INR concept has succeeded beautifully for many problems in atomic and nuclear physics. Unfortunately, the INR picture is seldom valid for reactive resonances, which, on the contrary, tend to be broad and overlapping. The breakdown of the INR idealization for reactive resonances was appreciated long ago in terms of the impact parameter averaging implicit in reactive collisions.38 If we imagine that an isolated reactive resonance corresponds to a vibrational state of an intermediate molecule, then the rotational energy levels built on that state have energies given by... 

These values are quite reasonable for the conversion of a molecule AB into two fragments A and B. One may alternatively, more rigorously, and less restrictively (the reactants need not be approximated by hard spheres) analyze the reactive collisions within the framework of transition state theory,18 leading to the expressions given in equations (4) and (5). 

Reactive collisions, electron nuclear dynamics (END), molecular systems, 338—342 final-state analysis, 343 -349... 

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