Ethyl radical dissociation

Flanagan and Rabinovitch were able to establish another point of general interest. They deduced from the relative rates of exchange and isomerization of isotope effect in the rupture of the carbon- hydrogen bond when adsorbed ethyl radicals dissociate to form adsorbed ethylene molecules. The ratio of the rupture probabilities of C—H and C—D decreased from 15.9 at —78° to 1.4 at 429°. More evidence of this kind would obviously be valuable because it suggests that some revision may be necessary of the theory for calculating initial distributions of... 

Trajectory calculations have been used to study the intrinsic RRKM and apparent non-RRKM dynamics of ethyl radical dissociation, i.e. C2H5 — H - - C2H4 [61,62]. When C2H5 is excited randomly, with a microcanonical distribution of states, it dissociates with the exponential P t) of RRKM theory [61], i.e. it is an intrinsic RRKM molecule. However, apparent non-RRKM behavior is present in a trajectory simulation of C2H5... 

Another unimolecular reaction which has been studied by classical trajectories is ethyl radical dissociation ... 

From the results of classical trajectory calculations intrinsic non-RRKM behavior has been predicted for ethane dissociation, ethyl radical dissociation,and methyl isocyanide isomerization. These predictions are supported by classical trajectory calculations for model H-C-C -> H + C=C dissociation. To generalize, classical trajectory calculations have predicted intrinsic non-RRKM behavior for molecules with isolated high frequency modes [e.g, CH3NC, clusters like Li (H20)j, and van der Waals molecules], molecules like acetylene with linear geometries for which bending and stretching motions are nearly separable, and molecules with tight activated complexes. 

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

Cleavage of the carbon-carbon bond in ethane yields two methyl radicals whereas propane yields an ethyl radical and one methyl radical Ethyl radical is more stable than methyl and so less energy is required to break the carbon-carbon bond in propane than in ethane The measured carbon-carbon bond dissociation energy in ethane is 368 kJ/mol (88 kcal/mol) and that in propane is 355 kJ/mol (85 kcal/mol)... 

Irradiation of the molecular radical anion of DESO, which has a yellow color, with light of X = 350-400 nm partially restores the red color and the ESR spectrum of the radical-anion pair. Similarly to the case of DMSO-d6 a comparison of the energetics of the photodissociation of the radical anion and dissociative capture of an electron by a DESO molecule permits an estimation of the energy of the hot electrons which form the radical-anion pair of DESO. This energy is equal to 2eV, similarly to DMSO-d6. The spin density on the ethyl radical in the radical-anion pair of DESO can be estimated from the decrease in hfs in comparison with the free radical to be 0.81, smaller than DMSO-d6. 

Fig. 16. The dissociation rate of ethyl radical as a function of excitation energy. The open circles correspond to data obtained with ethyl iodide used as the precursor, while the closed diamonds correspond to data obtained with n-propylnitrite used as precursor. (From Gilbert et al.1 2 7)...

The prompt dissociation of the fast H atom in the pathway, for which character change and collapse of the 3s Rydberg orbital on the ethyl radical to the Is orbital of the H product are required,121-123 could be assisted by the conical intersection. Also, relaxation and internal conversion from the 3s state to the ground state ethyl can be facilitated by this conical intersection, in addition to other possible vibronic couplings in the A symmetry.39... 

The molecular ion (M ) of 16 was reported in <1998GHE297> and was shown to be unstable and dissociates in a number of directions. The most stable ion has mjz 262, and high-resolution MS showed that this ion is formed by removal of the isopropyl group and ethyl radicals from M. To determine the structure of this ion, the collision-activation spectrum was recorded and was found to be compound 27 <1997GHE1306>. 

Similar considerations apply to processes occurring on the surface of the catalyst i.e., the rate of dissociation of ethyl radicals to ethylene molecules will be equal to the rate of the reverse reaction. An alternative method of describing the kinetic behavior of an exchange reaction is to treat it as an example of a catalytic reaction where the products inhibit the reaction as they compete on equal terms with the reactants for the available surface. 

Adsorbed ethyl radicals are formed by the dissociative adsorption of ethane. Every ethyl radical may either leave the surface with a deuterium atom to form an ethane molecule or lose one of the three hydrogen atoms of the methyl group to form adsorbed ethylene. The chances of these two events are 1/(1 + P) and P/(l -f P), respectively, P being a constant for a given catalyst. Equal chance is assumed for the loss of each of the three hydrogen atoms of the methyl group in the second process. 

Similar to ethane, propane dissociates by breakage of a C-C bond, which is weaker than the C-H bonds. In molecules such as ethylene or acetylene, with double or triple bonds between carbon atoms, a hydrogen atom is released during thermal dissociation. The following reactions, in particular, thermal dissociation of the ethyl radical (C2H5),... 

The CH bond in propene is weaker than the CH bond of ethane because the allyl radical is stabilized by resonance. The ethyl radical has no such resonance stabilization. The difference between these bond dissociation energies provides an estimate of the resonance stabilization of the allyl radical 13 kcal/mol (54 kJ/mol). 

Reaction (IV) is possible and in fact quite likely. The breaking of a carbon-bromine bond involves the absorption of 58,000 calories and this minimum is close enough to the 55,000 calories to be within the limit of uncertainty of the constants involved. This reaction can account for the experimental results and is in line with spectroscopic evidence that ethyl halides dissociate photo-chemically into the halogen atom and the free radical. 

The +NR+ spectrum of 8 showed a small survivor ion, but differed substantially from the spectra of other C2H5NO isomers, e.g., 6, 7, AT-methylamino(hy-droxy)carbene (9), and N-methylformamide (10). The low intensity of survivor ions in the NR mass spectra of enol imines is due to Franck-Condon effects in collisional reionization that result in vibrational excitation of the resulting cation radical followed by dissociation. Franck-Condon effects were studied for collisional ionization of acetimidic acid, CH3C(OH)=NH, which was one of the neutral dissociation products of 1 -hydroxy- 1-methylamino-l-ethyl radical, a hydrogen atom adduct to AT-methylacetamide [37]. The cation-radical dissociates extensively upon reionization, and the dissociation is driven by a 74 kj mol-1 Franck-Condon energy acquired by vertical ionization. 

The differences in RSE values obtained from Equations 5.3, 5.6 and 5.8 will be illustrated here using the ethyl radical (CH3CH2, 8) and the fluoromethyl radical (FCH2, 9) as examples. Both systems are not burdened by steric effects and the RSE values can thus be interpreted as the consequences of electronic substituent effects. Also, experimentally measured C-H and C-C bond dissociation energies are available for both systems, allowing for a side-by-side evaluation of experimental and theoretical results (Table 5.1). 

By comparison, the thermal dissociation of the ethylgermanes has been proposed to proceed by cleavage of the Ge—C bond (equation 48) followed by hydrogen abstraction by the nascent ethyl radical . ... 

Ethyl radicals, C2H5-, have been reported to assist in the gas-phase transport of the isotopes Po and " Po. One interpretation of this phenomenon suggests the formation of diethyl polonide. However, an alternate explanation involves the dissociation of (C2H5)2Po into volatile H2P0 and ethylene. 

Cool flames were somewhat more difficult to establish with propion-aldehyde than with acetaldehyde and it was necessary to use a higher initial tube temperature (270 C). In its general features, this system resembled that of acetaldehyde, although the second stage flame was less sharply defined. The analytical data were more complex and considerable production of ethylene occurred, presumably via (42), the ethyl radicals being the result of C2H5 CO dissociation. 

The earliest studies of the photolysis of ethane suggested that the dissociation of ethane to two methyl radicals and to an ethyl radical and a hydrogen atom were the primary steps of the process. The more recent work has shown that the photolysis is much more complicated, with several primary steps involved. As a basis for discussion the following reaction scheme is used. 

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