Spin-free activation energy

Table III. Estimated Spin-free Activation Energies for Selected CobaIt(ni) Species...

Cobalt (III) complexes appear to have B and C values of about 500 and 3800 cm. , respectively. These typical values give a crossover point of 16,500 cm. , which has been used in Table III to estimate the spin-free activation energies of the other species, the spin-free activation energies being the amount by which 2A exceeds 5 B + 8 C. That the position of the spin-free crossover is not too low is indicated by the fact that the hypothetical spin-paired [Co Fe]" ion has a A value (Table IV) about equal to the crossover point estimated from the Racah parameters, B and C. The spin-paired activation energies, estimated at 0.4A, are also given in Table III for comparison purposes. 

The 500 MHz H-NMR of the primary organozinc iodides 44a and 44b have been reported . The methylenic protons a to the zinc atom occur as the AB part of an ABC spin system, indicating slow inversion rates. Applying equation 34 (see Appendix, Section IV.A) to the given chemical shifts and coupling constants, a lower limit for the free activation energy can be established as AG > 15 kcalmol" in DMF-rfv or THF-rfg at 25 °C. No further attempts to approach closer to the coalescence temperature were undertaken (equation 26). 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

The effect of temperature on the association of vanadium compounds in asphaltenes was investigated by Tynan and Yen (1969). Using electron spin resonance (ESR), they observed both anisotropic and isotropic hyperfine structures of vanadium, interpreted as bound or associated and free vanadium, from asphaltenes precipitated for a Venezuelan petroleum and reintroduced to various solvents. Higher temperatures and more polar solvents resulted in a transition from bound to free vanadium, as shown in Fig. 12. At 282°C, only 1% of the anisotropic spectrum was observed. An activation energy of 14.3 kcal/mole was observed for the transition. 

One way of circumventing this activation energy barrier involves a free radical pathway in which a singlet molecule reacts with 302 to form two doublets (free radicals) in a spin-allowed process (Fig. 4.1, Reaction (1)). This process is, however, highly endothermic (up to 50 kcal mol-1) and is observed at moderate temperatures only with very reactive molecules that afford resonance stabilized radicals, e.g. reduced flavins (Fig. 4.1, Reaction (2)). It is no coincidence, therefore,... 

The major contribution to the EPR signal at room temperature is from Curie-like spins, which are associated with localized free radicals (trapped neutral solitons). The high activation energy, AEj, and broad line width of PMQl shows the effect of short conjugated sequences on trapped spins. 

From the temperature dependence of the signal intensity (amplitude) the activation energy for this reaction was derived as 140 20 kj mol (SS bonds) or 70 kJ mor (spins). From the line-width and its temperature dependence the hfetime of the free radicals was estimated which is obviously determined by the following rapid radical displacement reaction ... 

The electrical conductivity a of liquid sulfur increases with temperature except near the viscosity maximum of ca. 170 °C where a minimum of the conductivity is observed. Above 200 °C the plot of log a vs 1/T was found by several authors to be linear but the slopes of these linear relationships as well as the absolute conductivities vary considerably [118-122]. On the assumption that the conductivity at these temperatures is intrinsic, values of about 1.6 eV were derived for the activation energy at high temperatures (up to 900 °C) [121, 122], an energy which is much higher than the activation energy for the formation of free spins by homolytic bond dissociation (see above). 

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