Simple fission reactions

The treatment of these simple associations directly follows that of the simple fission reactions discussed previously. For example, these reactions proceed via the formation of a loose transition state and without an activation energy barrier. The rates and rate parameters of simple associations can be determined either directly, by the application of bimolecular TST, or from their reverse, simple unimolecular fission reactions, through the use of the principle of microscopic reversibility. 

This procedure is an example of a simple fission reaction of AMieter-ocyclic compounds by thiophosgene and base2 wherein the dihydro intermediate 1 undergoes ring fission to yield the Z-isothiocyanate 2 which isomerizes in situ to the -isomer 3. The reaction may be applied to... 

Simple fissions are responsible for the initiation of radical chain reactions, and some examples are... 

The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

Brouwer, L., Cobos, C.J., Troe, J., Dubai, H.-R., and Crim, F.F. (1987). Specific rate constants k(E, J) and product state distributions in simple bond fission reactions. II. Application to HOOH —> OH+OH, J. Chem. Phys. 86, 6171-6182. 

Reisler, H. and Wittig, C. (1992). State-resolved simple bond-fission reactions Experiment and theory, in Advances in Chemical Kinetics and Dynamics, ed. J.A. Barker (JAI Press, Greenwich). 

The energy liberated in fission is, of course, to some degree dependent upon the target used and the product nuclei which are formed. In a typical fission of U285, about 200 Mev of energy is released, almost 20 times the energy of the most energetic simple nuclear reactions. 

All the foregoing information allows one to state that the formation of C-dihydrotoxiferine I from hemidihydrotoxiferine I (LXVIII) in acetic acid involves the condensation of two molecules of LXVIII, with the loss of two molecules of water and with the disappearance of the aldehyde and >Na—H functions. Evidence for such a condensation is provided by the fact that a mixture of equivalent amounts of norhemidihydrotoxiferine I and hemidihydrotoxiferine I in acetic acid gives a reaction product containing nordihydrotoxiferine I, its mono-Ab-metho salt, and C-dihydrotoxiferine I (86). The formation of toxiferine I from hemitoxiferine I (LXVII) must be strictly analogous, and the only structures which can be written to accommodate the above evidence are LXXI (R = H) or LXXII (R = H) for C-dihydrotoxiferine I and LXXI (R = OH) or LXXII (R = OH) for toxiferine I. Structure LXXIII can be written as a formal representation of the intermediate in both condensation and fission reactions. Of these alternatives, the structures LXXI were at first preferred (86, 31), mainly because of the striking similarity between the UV-spectra of the two alkaloids and those of simple a-methyleneindolines. The above chemical evidence allows of no distinction between formulas LXXI and LXXII. 

Chain reactions are recursive reaction cycles that regenerate their intermediates. Such cycles occur in combustion, atmospheric chemistry, pyrolysis. photolysis, polymerization, nuclear fusion and fission, and catalysis. Typical steps in these systems include initiation, propagation, and termination. often accompanied by chain branching and various side reactions. Examples 2.2 to 2.5 describe simple chain reaction schemes. 

There is no evidence in any of the gas phase systems for initial multiple bond rupture (i.e., fragmentation reactions). Because of the low reaction temperatures, the alkoxy radical intermediates of the bond fission reactions (or radicals resulting from alkoxy radicals) are mainly involved in radical-radical termination processes ( 0) rather than participating in hydrogen abstraction from the parent peroxide E oi 6-8). Thus it has been commonly believed that the peroxide decompositions were classic examples of free radical non-chain processes. Identification of the rate coefficients and the overall decomposition Arrhenius parameters with the initial peroxide bond fission kinetics were therefore made. However, recent studies indicate that free radical sensitized decompositions of some peroxides do occur, and that the low Arrhenius parameters obtained in many of the early studies (rates measured by simple manometric techniques) were undoubtedly a result of competitive chain processes. The possible importance of free radical reactions in peroxide decompositions is illustrated below with specific regard to the dimethyl peroxide decomposition. 

Resonance stiffening in the transition state seems to lower activation entropies by about 3 eu per resonance interactions. Therefore the following ranges of A-factors are reasonable in simple bond fission reactions. 

A useful variant of the reflected shock technique has been to add a chemical substance in trace amounts whose unimolecular rate constant is well known. The products of this trace reaction are then used as an internal standard for measuring the mean temperature in the system (66, 67, 68, 69). A number of rate constants for four-center and six-center elimination reactions measured in this way have been found to be in excellent agreement with rate constants obtained more conventionally at lower temperatures. However, almost all of the rate constants for simple fission of branched hydrocarbons obtained in this way have yielded anomalies (48) which have not yet been resolved. 

Fragment Reactions. The nonaromatic fragments formed from aromatic ring breakdown can undergo a variety of reactions (a) molecular reactions such as simple fission (pyrolysis) or hydrogenation-dehydro-... 

In applications to a wide range of experimental results, one needs an efficient theoretical tool allowing for quick comparison of experiment and theory, perhaps followed by adjustment of theoretical parameters to experiment. With this goal in mind, the early formulation of the SACM included a simple empirical representation of the main features of the electronic potential by a very few adjustable parameters. Furthermore, the complicated calculation of adiabatic channels by a solution of the multidimensional clamped -q- rovibrational Schrbdinger equation, which is an exceedingly demanding task even today for larger than triatomic systems, was completely circumvented by a simple channel interpolation procedure. We shall present here a very brief description of this empirical approach for simple bond fission reactions. 

The second most important feature of the potential for simple bond fission reactions is the angular potential for those coordinates which correspond to free relative rotations for the separated fragments, changing into hindered rotations at shorter distances and finally vibrations in the bound molecule. Such potentials can be parametrized by general expressions of the form given in equations (108) and (109), where is the reference value of the angle (p ... 

For a simple bond fission reaction, H is given to a first approximation by (Troe, 1977b)... 

In this pressure range the actual populations of excited molecular states are close to the equilibrium populations given by f. The rate coefficient /c2 is therefore the thermal equilibrium average of the specific rate coefficient k(E) of the unimolecular reaction. The rate coefficient for the reverse recombination of a simple bond fission reaction... 

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