Fragmentation, unimolecular

Like fragmentations, unimolecular rearrangements are always uphill at the beginning of the process, because a bond breaks. Unlike fragmentations, rearrangements are downhill at the end as a new bond forms. An example of this kind of energy profile for the Isomerization reaction of ds-2-butene appears in Figure 15-14Z). 

In the FFR of the sector mass spectrometer, the unimolecular decomposition fragments, and B, of tire mass selected metastable ion AB will, by the conservation of energy and momentum, have lower translational kinetic energy, T, than their precursor ... 

Mass spectrometric studies yield principally three types of information useful to the radiation chemist the major primary ions one should be concerned with, their reactions with neutral molecules, and thermodynamic information which allows one to eliminate certain reactions on the basis of endothermicity. In addition, attempts at theoretical interpretations of mass spectral fragmentation patterns permit estimates of unimolecular dissociation constants for excited parent ions. 

In a unimolecular reaction, a singie moiecule fragments into two pieces or rearranges to a new isomer. A simple example of a unimolecular reaction is the decomposition of N2 O4. An N2 O4 molecule continually vibrates, and these vibrations cause its N—N bond to stretch. If the molecule has sufficient energy, the bond breaks and the moiecuie separates into two moiecuies of NO2, much like a spring that breaks if it is stretched too far. 

A unimolecular elementary reaction is a fragmentation or rearrangement of one chemical species, so its rate law contains the concentration of only that species ... 

In a unimolecular reaction, a molecule fragments into two pieces or rearranges to a different isomer, hi either case, a chemical bond breaks. For example, in the fragmentation of bromine molecules, breaking a ffbond gives a pair of bromine atoms Bf2 2 Br Recall that this unimolecular process is the first step of the reaction between molecular hydrogen and molecular bromine to give HBr. 

The turbine type of helicity in canals is a toroidal turbine, and can occur with proper rotation axes and with 2V 42, 63 screw axes, and depends on the object rotated. It is necessary that a distinctive principal plane or axis of the host molecule or molecular fragment be canted in the cylindrical surface of the canal so that it is neither parallel to or perpendicular to the axis of the canal. No unequivocal instance of this type of helical canal has been reported. The cyclodextrin unimolecular hosts 3 4) might be... 

The quasi-equilibrium theory (QET) is the most widely used theoretical framework for the discussion of the fragmentation pattern of the parent ion in a uni-molecular process. Although other unimolecular theories (see Levine, 1966) have been subsequently proposed, the QET has traditionally been applied for... 

In a similar vein, various electron acceptors yielding anion radicals that undergo rapid unimolecular decomposition also facilitate the efficacy of Scheme 1 by effectively obviating the back-electron transfer. For example, the nitration of enol silyl ether with tetranitromethane (TNM) occurs rapidly (despite an unfavorable redox equilibrium)78 owing to the fast mesolytic fragmentation of the TNM anion radical79 (Scheme 15). 

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Subsequently, the rapid unimolecular (mesolytic) fragmentation of the resulting pinacol cation radical followed by proton (or trimethylsilyl cation) transfer to quinone anion radical (within the solvent cage) yields the retropinacol products in equations (55) and (56) (equation 58). 

Charge-transfer activation of the EDA complex leads to the ion-radical pair, in which the bicumene cation radical undergoes a unimolecular fragmentation (equation 64). 

The frequency with which the transition state is transformed into products, iT, can be thought of as a typical unimolecular rate constant no barrier is associated with this step. Various points of view have been used to calculate this frequency, and all rely on the assumption that the internal motions of the transition state are governed by thermally equilibrated motions. Thus, the motion along the reaction coordinate is treated as thermal translational motion between the product fragments (or as a vibrational motion along an unstable potential). Statistical theories (such as those used to derive the Maxwell-Boltzmann distribution of velocities) lead to the expression ... 

Product Ion An ion formed as the product of a reaction involving a particular precursor ion. The reaction can be unimolecular dissociation to form fragment ions, an ion/ molecule reaction, or simply involve a change in the number of charges. The terms fragment ion and daughter ion are deprecated (but still very much in use). 

One method to study energy-selected ions is threshold ionization, in which ions with precisely defined energy contents are produced. These ions can then be used to study unimolecular fragmentation, ion-molecule reactions, van der Waals clusters, and hydrogen-bonded clusters [62]. 

The thermal and photochemical activations of EDA complexes by electron transfer are both enhanced when the radical ions D+- or A--(either paired or free) undergo a facile first-order (unimolecular) transformation such as fragmentation, rearrangement, bond-formation, etc., which pulls the redox equilibrium and thus renders the competition from the energy-wasting back electron transfer less effective (compare Scheme 5). Critical to the quantitative evaluation of the reaction dynamics is the understanding that the typical [D+% A--] intermediates, as described in... 

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