Activation energies relative

The prototype hole-catalyzed Diels-Alder reaction between the butadiene radical cation and ethylene has also been studied by Bauld [53]. He finds strongly exothermic formation of a l-hexene-3,6-diyl radical cation intermediate without activation energy followed by a weakly activated (activation energy 2.3 kcal mol ) closure of the second C-C bond to form the cyclohexene radical cation, The reaction shows no overall activation energy relative to the... 

A second set of comparisons assesses the consequences of use of approximate reactant and transition-state geometries for relative activation energy calculations, that is, activation energies for a series of closely related reactions relative to the activation energy of one member of the series. Two different examples have been provided, both of which involve Diels-Alder chemistry. The first involves cycloadditions of cyclopentadiene and a series of electron-deficient dienophiles. Experimental activation energies (relative to Diels-Alder... 

Fig. 33. Calculated activation energies relative to Cr(II) (top portion) compared to -log k, (bottom portion) where k are the experimental exchange-rate constants. No suitable D pathway could be located for Zn which is therefore omitted from the graph.

Various reaction mechanisms are known for ene reactions. Both single-step synchronous reaction and stepwise processes involving diradicals or zwitterionic transition states have been discussed. One of the three bonds broken in the course of the reaction is a a bond, which dictates a high activation energy relative to a Diels-Alder reaction (see Chapter 2). For this reason if the reaction is conducted thermally, temperatures above 100 C are required. However, the reactivity of the enophile can be increased by addition of a Lewis acid, permitting milder reaction conditions. The Lewis acid coordi-... 

It is seen again that the activation energy typical of the pure metal is lowered by contact of the metal with an oxidic support. Even more important, p-type doping with bivalent oxides lowers, n-type doping with tetravalent oxides increases the activation energy relative to that observed with undoped alumina. This is consistent with the concept... 

Heterocyclic thiazyl radicals hold considerable potential in the design of both conductive and magnetic materials. In the pursuit of improved conductivity, a series of resonance-stabilized radicals based on diselenadiazoles, sulfaselenazoles, and diselenazoles were obtained (185-188) (Fig. 17) [298-303], Structural analyses of 187 and 188 (R1 = Me, R2 = H) confirm that lattice and n-delocalization energies are sufficient to prevent solid state dimerization of the radicals. Incorporation of selenium leads to a dramatic increase in conductivity and reduction in thermal activation energy relative to sulfur-based radicals [300],... 

Activation energy relative to s-trans-1,3-butadiene + ethylene. Most calculations include zero-point vibrational energy. 

Before we move on, it is worthwhile to clarify the implications of the words allowed and forbidden. An allowed reaction is simply one with a low activation energy relative to some other pathway, while a forbidden reaction is a process for which there is a significant activation energy. 

Hoz and co-workers used the Markus equation and theory at the HF/6-31G level to determine that the strain relief contribution to ring opening of thiirane by MeS is 8.5kcalmol <2001JOC915>. There is an additional contribution to the lowering of the activation energy, relative to that for the unstrained model (MeS -I- MeSMe) of 13.3 kcal mol . The additional factor was not identified. 

At 75° (Ref. Tj). Relative to 3/3-ch.oiestanyl tosylate = i. c Difference in activation energy relative to 3/ -cholestanyl tosylate. 

A hierarchical series of ab initio methods was applied to obtain benchmark potential energy surface for the oxidative addition of methane to the Pd atom. The best estimate of kinetic and thermodynamic parameters is -8.1 kcal mol for the formation of the reactant complex, 5.8 kcal mol for the activation energy relative to the separate reactants, and 0.8 kcal mol for the reaction energy. The values agree well with those obtained by Siegbahn et al. with the PCI-80 method. Calculations were based on a model reaction involving the f --C,H transition state. 

Plotting such data in terms of the logarithm of the specific reaction rate vs the reciprocal of the absolute temperature (i.e., an Arrhenius plot) is helpful in assessing which step is rate controlling. A reduction in the activation energy relative to that of the native enzyme, indicates that kinetics are not rate limiting. 

Table XI. Complexation energy of the ion-tnoiecules complex (AEcomp), activation energy (AE ), and overall activation energy relative to reactants (AE ) for the Walen inversion. All the values are in kcal/mol.

The bond lengths in the optimized transition structures at the three levels of theory are tabulated (Table 4.1). B3LYP/6-31G gave the best agreement with activation energy relative to the experimentally observed activation every of the rearrangement of 2-OTMS allyl vinyl ether. The calculated relative activation energies for the 2-OH versus the parent allyl vinyl ether were most accurate at the RHF/ 6-31G level. 

Scheme 11.41 Predicted structural and energetic data for the Claisen rearrangement of 2-substitued allyl vinyl ethers according to Aviyente and Houk (2001) [57], Gas-phase calculations were performed on the B3LYP/5-31G level of theory./< = dipole moment, AS = charge separation in the transition state, A = reaction energy, AE" = activation energy, = relative intrinsic contribution to the activation energy ...

The stability of the cis- and trans-alkenes produced in the E2 reaction reflects the stabilities of the transition states leading to them. Steric crowing in the transition state leading to the cis isomer raises the activation energy relative to the transition state for forming the trans isomer. Therefore, the trans isomer forms faster and is the major product. 

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