Unimolecular isomerization

Figure A3.4.6. Potential energy along the reaction coordinate r for an unimolecular isomerization (left) and a...

Oxirene is probably a true intermediate, but is separated from ketene by only a very low barrier. Since its instability results from unimolecular isomerization rather than from attack of other molecules, the only viable current technique for its direct observation seems to be generation and spectroscopic examination in an inert matrix at temperatures near absolute zero. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

MeSSMe and inhibited to isomerise by a significantly larger isomerization barrier of 340 kj mol Thus, both H2S=S and Me2S=S are predicted to be kinetically stable toward unimolecular isomerization and should be experimentally observable in the gas phase or after matrix isolation. HF/b-SlG calculations indicate that the allyl analogue has a similar R2S=S/RSSR energy difference but a substantially lower activation barrier (110 kJ mol ) for rearrangement [25]. 

Energy profiles for two unimolecular processes (a) the unimolecular decomposition of a bromine molecule (b) the unimolecular Isomerization of ds-2-butene. 

Although we shall restrict our discussion here to the very simple unimolecular isomerization or decomposition of a single substrate, we need to mention the effect that inhibitors have on the rate. 

Reactants AB+ + CD are considered to associate to form a weakly bonded intermediate complex, AB+ CD, the ground vibrational state of which has a barrier to the formation of the more strongly bound form, ABCD+. The reactants, of course, have access to both of these isomeric forms, although the presence of the barrier will affect the rate of unimolecular isomerization between them. Note that the minimum energy barrier may not be accessed in a particular interaction of AB+ with CD since the dynamics, i.e. initial trajectories and the detailed nature of the potential surface, control the reaction coordinate followed. Even in the absence (left hand dashed line in Figure 1) of a formal barrier (i.e. of a local potential maximum), the intermediate will resonate between the conformations having AB+ CD or ABCD+ character. These complexes only have the possibilities of unimolecular decomposition back to AB+ + CD or collisional stabilization. In the stabilization process,... 

In general, intramolecular isomerization in coordinatively unsaturated species would be expected to occur much faster than bimolecular processes. Some isomerizations, like those occurring with W(CO)4CS (47) are anticipated to be very fast, because they are associated with electronic relaxation. Assuming reasonable values for activation energies and A-factors, one predicts that, in solution, many isomerizations will have half-lives at room temperature in the range 10 7 to 10 6 seconds. The principal means of identifying transients in uv-visible flash photolysis is decay kinetics and their variation with reaction conditions. Such identification will be difficult if not impossible with unimolecular isomerization, particularly since uv-visible absorptions are not very sensitive to structural changes (see Section I,B). These restrictions do not apply to time-resolved IR measurements, which should have wide applications in this area. 

To achieve receptor desensitization and activation by ligand, multiple conformational states of the receptor are required. The binding steps represented in horizontal equilibria are rapid vertical steps reflect the slow, unimolecular isomerizations involved in desensitization (scheme 2). Rapid isomerization to the open channel state (scheme 1) should be added. To accommodate the additional complexities of the observed fast and slow steps of desensitization, additional states have to be included. 

For the gas-phase unimolecular isomerization of cyclopropane (A) to propylene (P), values of the observed first-order rate constant, kuni, at various initial pressures, P0, at 470 C in a batch reactor are as follows ... 

According to Fontana s theory, the rate-determining step in a polymerization involving a monomer-complexed cation is a unimolecular isomerization of the complex, which is kinetically of first order ... 

However, the behaviour near m = raB needs some other explanation. My proposal involves the specific solvation of the backside of the carbenium ion by the strong dipole of the solvent this displaces the monomer molecule which is located there in the absence of the solvent, so that the 7t-bond to the monomer at the front is weakened and the unimolecular isomerization-propagation becomes accelerated, despite the statistical factor which, alone, would produce a deceleration, as explained at the end of Section 3a. As the dilution proceeds from m = raB downwards, the polymerization goes through a dieidic phase, in... 

III. UNIMOLECULAR ISOMERIZATION AND FRAGMENTATION A. Selected Linear Dienes Allylic Cleavage and Isomer Distinction... 

A second example of an inverse statistical weight isotope effect is that of the secondary H/D KIE on C-C bond rupture during the gas phase unimolecular isomerization of cyclopropane to propene. Theory and experiment are compared in Fig. 14.2 for reactions 14.37 and 14.38. 

Fig. 14.2 Pressure dependence of the 2°-D isotope effect, kn/ko, on the unimolecular isomerization of cyclopropane at 755 K c-C3H6 -> C3H6 and C-C3D6 — C3D6 (The data are from Blades, A. T., Can.

Schneider, F.W. and Rabinovitch, B. S., The unimolecular isomerization of methyl-d3 isocyanide. Statistical-weight inverse secondary intermolecular kinetic isotope effects in nonequilibrium thermal systems. J. Am. Chem. Soc. 85, 2365 (1963). 

The thermal isomerization of cyclobutene to butadiene is one of the best known examples of a thermal unimolecular isomerization. Cooper... 

Case 1. Unimolecular Isomerization Consider a simple chemical reaction between species A and B at dynamic equilibrium ... 

NONEQUILIBRIUM THERMODYNAMICS A PRIMER UNIMOLECULAR CHEMICAL KINETICS MOLECULARITY ORDER OF REACTION ELEMENTARY REACTIONS Unimolecular forward/bimolecular reverse, CHEMICAL KINETICS Unimolecular isomerization,... 

Thus our unimolecular isomerization reaction actually occurs by a sequence of steps and is therefore a multiple-reaction system We need a simplified expression for the overall rate of this rate in terms of Ca alone because zl is an intermediate whose concentration is always very small just as for the free-radical intermediates and dimers in the previous examples. 

Dimerization was observed in 1 -3 and t-6, because the decay of the absorption of 1 -3 and 1-6 increased with increasing [S], respectively. The transient absorption of c-1 shifted to that of t-1 during the decay because of the bimolecular isomerization of c-1 to t-1 via a-12. Similarly, the transient absorption of c-2 shifted to that of t-2 " through bimolecular isomerization, = (2.0-4.3) x 10 sec were calculated from the dependence of kobs on [S] (Table 2). No dimerization of c-6 was observed, because kobs did not depend on [c-6]. The transient absorption of c-1 with p-MeO and methyl group (Me) on the olefinic carbon decayed with kobs according to first-order kinetics without any shifts or dependence of [c-7]. In other words, neither unimolecular isomerization nor dimerization was observed in c-7. ... 

Enhancements of Unimolecular Isomerization and Oxidation by Charge-8pin 8eparation... 

Neither the unimolecular isomerization of c-T nor the oxidation of t-8 occurred, although 7 and 8 have p-MeO. These are possibly explained in terms of a barrier to the twisting of the C=C double bond and the spin density on the olefinic carbon, respectively. The contribution of (B) is decreased by the electron-donating Me on the olefinic carbon therefore, it is suggested that the single-bond character of the C=C double bond is lower in c-T than in c-3 and that the barrier to the twisting of the C=C double bond is higher in c-1 than in c-3. An unpaired electron appears on the olefinic carbon on the side of the p-MeO in 3 because of contribution of (B), while it appears on both olefinic carbons in... 

The formation of oxidation products a-c in a range of G values (0.7-3.8) during the 7-R of S in 02-saturated DCE suggests that a-c would be produced from complicated reactions of peroxy radicals with S (Table 5). On the other hand, the regioselective formation of 3d with large G values (2.6-3.0) in oxidation of 3 with O2 is explained by spin localization on the p-olefinic carbon because of the contribution of (B) in 3. The results of products analyses are essentially identical with prediction based on k and ko for S measured with PR. It should be emphasized that the reactivities of c-t unimolecular isomerization and reaction of S with O2 can be understood in terms of charge-spin separation induced by p-MeO. 

Anglada, J. M., J. M. Bofill, S. Olivella, and A. Sole, Unimolecular Isomerizations and Oxygen Atom Loss in Formaldehyde and Acetaldehyde Carbonyl Oxides. A Theoretical Investigation, J. Am. Chem. Soc., 118, 4636-4647 (1996). 

In this problem we will consider the kinetics of this unimolecular isomerization reaction in a nitrogen bath gas at 1500 K, using several different theoretical treatments. The high-pressure Arrhenius coefficients for this reaction are A00 = 1 x 1014 s-1 and E0 = 45 kcal/mol. 

A time-dependent process, such as radiative absorption, internal conversion, intersystem crossing, unimolecular isomerization, or collision, may be treated in terms of a zero-order Hamiltonian H0 and a perturbation T. An unperturbed eigenstate of H0 evolves in time, since it is not an eigenstate to the perturbed Hamiltonian... 

Biradicals are frequently postulated to arise as intermediates in a number of chemical reactions and unimolecular isomerizations. Sometimes there are reasonable alternative concerted mechanisms in which the intermediate (or transition-state complex) is not a biradical. Such a case of much interest37,61 involves the reactions of singlet [5] and triplet [7] methylenes with olefins. We note that the permutational symmetry does not determine whether or not a reaction is concerted rather it is determined by the shapes of the intermolecular potential surfaces.78 The lowest 1Ai methylene is expected to react by a concerted mechanism, since it correlates with the ground state of the product cyclopropane higher excited singlets need not react via a concerted mechanism. 

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