Gas phase homolysis

Hiob, R. and Karelson, M. (2002) Quantitative relationship between rate constants of the gas-phase homolysis of N—N, 0—0 and N—O bonds and molecular descriptors. Internet Electron.]. Mol. Des., 1, 193-202. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

It is seen that the values of kd are very close. Hence, the reaction of POOH with the C—H bond is not the main initiation reaction. If the breakdown is a monomolecular process, the rate of O—O bond homolysis in polymer must be close to that in the gas phase. 2,2-Dimethylethyl hydroperoxide breaks down in the gas phase with a rate constant of 1.6 x 1013 exp(— 158/i 7) = 5.3 x 10 x s 1 (398 K, [4]), that is, by four orders of magnitude more slowly than in polymer. Hence, the decomposition reactions in the polymers are much faster than the monomolecular homolysis of peroxide. Decomposition reactions may be of three types (see Chapter 4), such as the reaction of POOH with a double bond... 

Photochemical behaviour of compounds 83-86 [33] in the gas phase has been reported, in order to distinguish between silyl radical and silylene formation. Photolysis of the noncyclic precursors 83 and 84 gave products derived from silyl radicals, which come from a direct Si—Si bond homolysis, with a little evidence of silylene formation. In contrast, dimethylsilylene (Mc2Si ) was observed as a direct photoproduct from the cyclic precursors 85 and 86. The reaction sequence including a Sni step shown in Scheme 6.18 for the formation of dimethylsilylene was proposed to explain the different observations for cyclic and noncyclic systems. 

An interesting consequence of the R—X avoided crossing in solution is that its very existence in certain systems may actually depend on the magnitude of solvation. Since in the gas phase dissociation tends to be homolytic and in solution heterolytic, it is conceivable that in certain cases the solvent may determine whether homolysis or heterolysis takes place. 

It is seen that the values of kA are very close. Hence, the reaction of POOH with the C—H bond is not the main initiation reaction. If the breakdown is a monomolecular process, the rate of — bond homolysis in polymer must be close to that in the gas phase. 

The photodecomposition of oxiran and alkyloxirans both in the gas phase and in solution has been extensively investigated. Processes arising by carbon-oxygen bond homolysis and hydrogen abstraction have been reported, and the subject has been reviewed in detail elsewhere.48 The most recent studies include the photoaddition of methanol to alicyclic epoxides, a process that appears to be promoted by acid,49 and the interesting if unusual photochemically induced conversion of the epoxyalcohol (56) to sugiresinol dimethyl ether (57).50... 

Bond Homolysis. A substantial number of gas-phase bond homolysis rate constants and free-radical enthalpies of formation have been determined (1,11) and a far greater number may be reliably estimated. However, two factors must be considered when applying gas-phase bond homolysis rate constants to condensed-phase systems. First, any selective solvation of product radicals will tend to increase (k /kg). However, solvation effects on free radical reaction rates are generally the... 

The above considerations imply that over a wide range of conditions gas and solution phase homolysis rate constants are, to a good level of accuracy, independent of phase or solvent. 

Whereas the activated complex for monomolecular homolysis [Eq. (5-59a)] has no dipolar eharaeter and is nearly solvent-independent [ r(H20)//rr(toluene) = 7], inereases moderately with increasing solvent polarity [ H(H20)/ H(toluene) = 59], This seems to be due to the development of dipolar character in the corresponding activated eomplex, whieh involves preformed dipolar acetone molecules [Eq. (5-59b)]. In the gas phase, the normal free-radical producing 0-0 homolysis is the preferred reaction route [564],... 

The photolysis of Mu2(CO)io is representative of the behavior of many bimetallic carbonyl compounds. In solution and in the gas phase strongly wavelength dependent photochemistry has been observed. At low energy, Mn-Mn bond homolysis predominates while at high energy CO-loss is observed. 

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