Reaction path shifting

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

Referring first of all to the reactions over 0.2% platinum/alumina (Table V) the major features of the product distributions may be explained by a simple reaction via an adsorbed C5 cyclic intermediate. For instance, if reaction had proceeded entirely by this path, 2-methylpentane-2-13C would have yielded 3-methylpentane labeled 100% in the 3-position (instead of 73.4%) and would have yielded n-hexane labeled 100% in the 2-position (instead of 90.2%). Similarly, 3-methylpentane-2-I3C would have yielded a 2-methylpentane labeled 50% in the methyl substituent (instead of 42.6%), and would have yielded n-hexane labeled 50% in the 1- and 3-positions (instead of 43.8 and 49% respectively). The other expectations are very easily assessed in a similar manner. On the whole, the data of Table V lead to the conclusion that some 80% or so of the reacting hydrocarbon reacts via a simple one step process via an adsorbed C5 cyclic intermediate. The departures from the distribution expected for this simple process are accounted for by the occurrence of bond shift processes. It is necessary to propose that more than one process (adsorbed C6 cyclic intermediate or bond shift) may occur within a single overall residence period on the catalyst Gault s analysis leads to the need for a maximum of three. The number of possible combinations is large, but limitations are imposed by the nature of the observed product distributions. If we designate a bond shift process by B, and passage via an adsorbed Cs cyclic intermediate by C, the required reaction paths are... 

A more detailed analysis of the results obtained over 10% platinum/ alumina (115) leads to an extended array of parallel, multistep reaction paths, and it was concluded (for 273°C) that an adsorbed species had a chance of reacting via an adsorbed C5 cyclic intermediate of about 0.3, of reacting via a bond shift of about 0.2, and a chance of desorption of about 0.5. One would expect these probabilities to be temperature dependent, but to different extents, so that the nature of the product distributions should also be temperature dependent. 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

The bathochromic shifts with increasing chain concentration are compatible with the mechanism proposed above, since an increase in the concentration of unsaturated chains will favour hydride abstraction and will therefore give allylic ions with higher degrees of conjugation, which will absorb at wavelengths greater than 450 mp. The only serious chemical (as opposed to mechanistic) uncertainty in this scheme is whether route III—> IB or III — IC, or both, or perhaps some other process, adequately represent the removal of the ions by addition of monomer. Some reaction path of this kind seems to exist since there is no evidence that either route II —> III or route II — VI is reversible. 

Equilibrium between simple salts and aqueous solutions is often relatively easily demonstrated in the laboratory when the composition of the solid is invariant, such as occurs in the KCI-H2O system. However, when an additional component which coprecipitates is added to the system, the solid composition is no longer invariant. Very long times may be required to reach equilibrium when the reaction path requires shifts in the composition of both the solution and solid. Equilibrium is not established until the solid composition is homogeneous and the chemical potentials of all components between solid and aqueous phases are equivalent. As a result, equilibrium is rarely demonstrated with a solid solution series. 

Some support for the allylic shift pictured above comes from the work of Goetz and Orchin (23) on the isomerization of allyl alcohol to propionaldehyde by DCo(CO)4. These authors found that in the deuterated aldehyde all the D was on the methyl carbon and the following reaction path was suggested ... 

Ab initio calculations at the HF/6-31G level have been used to explore energy changes, structural variation, and electron density shifts during jr-face selective addition of substituted acetylide ions to cyclohexanone and cyclohexanethione. Charge polarization of the jr-bond on approach of the nucleophile is such that the carbonyl carbon becomes considerably electron deficient for most of the reaction path (and may... 

A kinetic study of nitrous acid-catalyzed nitration of naphthalene with an excess of nitric acid in aqueous mixture of sulfuric and acetic acids (Leis et al. 1988) shows a transition from first-order to second-order kinetics with respect to naphthalene. (At this acidity, the rate of reaction through the nitronium ion is too slow to be significant the amount of nitrous acid is sufficient to make one-electron oxidation of naphthalene as the main reaction path.) The reaction that initially had the first-order in respect to naphthalene becomes the second-order reaction. The electron transfer from naphthalene to NO+ has an equilibrium (reversible) character. In excess of the substrate, the equilibrium shifts to the right. A cause of the shift is the stabilization of cation-radical by uncharged naphthalene. The stabilized cation-radical dimer (NaphH)2 is just involved in nitration ... 

In summary, these results portrayed an intermediate situation where the PT is coupled to the Sn2 geometry change and charge shifting features. It should now be apparent that a study limited to the RC and the TS and lacking a reaction path analysis would have missed most ofthe important mechanistic features discussed herein. 

According to new data for the vinylcyclopropane-cyclopentadiene rearrangement,372 particularly concerning the stereochemistry of the process, the [1,3] sig-matropic carbon shift proceeds through all four stereochemical reaction paths,... 

Two consequences arise from this model First, the coherent excitation of normal modes can result from the reactive process and must not be caused by the optical excitation. Optically inactive modes can be excited if they contribute to the reaction path. Second, the proton is passively shifted by the skeletal movements and stays all the time in its local potential well. A substitution of the proton by a deuteron should therefore not influence the dynamics. In contrast strong variations are expected, if tunneling of the proton is the central step. 

The Negative-Temperature-Coefficient Region The equilibrium constant for the reaction R + O2 ROO (R64) is strongly temperature dependent, and as the temperature increases, the equilibrium shifts in favor of R + O2. The shift in equilibrium is the primary reason for the existence of the region where the conversion decreases with an increase in temperature (i.e., where there is a negative temperature coefficient). Above about 650 K, the alkyl peroxy radical becomes less thermally stable, and alternative reaction paths for ROO begin to compete with the isomerization reaction (R65). A new product channel opens up for the R + O2 reaction... 

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

Photolysis of 1 and the J/MCM-41 complex provides evidence of cluster condensation in the pores of MCM-41. A darkening of the sample and a bathochromic maxima shift are observed for both samples. This parallels the solution chemistry of I whereby the photochemical reaction of leads to the nanocluster [Cu5o(TePh)2oTei7(PEtPh2) ]4 [6]. Although it is unlikely the same reaction path is followed in our (solid) studies, the UV data are consistent with the formation of higher nuclearity species. 

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