Primary decomposition steps

Experimental results of the iodine-inhibited reaction show that biacetyl photolysis takes place almost exclusively through free radicals . Since, among the radicals occurring in the system, it is the acetyl radical that is present in highest concentration at low temperatures , it seems reasonable to suggest that the primary process is 

At short wavelengths (2380-3130 A), in the presence of iodine, the main reaction products are CO and CH3I, formed in equal amounts. Accordingly, it is to be assumed that, at least at these wavelengths, methyl radicals are formed directly in the primary process, i.e. one or both of the following occur 

It was suggested that reaction I proceeds in two consecutive steps. According to this suggestion, CH3 and CH3COCO radicals are formed first this is then followed so rapidly by the dissociation into CH3CO and CO of CH3COCO, that this radical does not enter in any other reaction. However, energetic considerations favour the one-step hypothesis. 

Instead of distinguishing the simultaneously occurring primary processes I, I and I , we prefer the conception of Noyes et according to which only acetyl radicals are formed in the primary process, and a fraction a of these radicals retains sufficient excess energy to decompose before undergoing any other reaction. Using a, the stoichiometry of the primary process can be described as 

The a values were determined under various experimental conditions. At 3650 A, a was found to depend on the biacetyl pressure the reported values were between 0.2 and 0.3 at low pressure, and approached 0.5 at high pressure. (There was also a light intensity dependence observed in the low pressure range.) At room temperature, in the wavelength region 3650-2654 A, a increased with increasing frequency of radiation. At 3130, 2700 and 2654 A, the values were around 0.35, 0.5 and 0.6, respectively. 

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The second wastage reaction is that involving the side reaction(s) of the radicals formed in the primary step of initiator decomposition. Some of the radicals formed in the primary decomposition step, for example, in the reaction... 

Alkane elimination has a low yield during the photolysis of liquid n-alkanes (e.g., n-pentane [104,106]). This reaction takes place with high yield only for branched alkanes where it is likely to be a main primary-decomposition step [105,107]. 

Similarly to the fluorescence quantum yields, the yields of individual primary decomposition steps generally show considerable excitation energy dependence the yields of the unimolecular H2 and alkane eliminations and also those of the radical-type decompositions show a continuous variation with photon energy [27,39,42,107,115]. In cyclohexane photolysis the sum of the quantum yields of the two primary decompositions described by Reactions (5) and (6) is practically unity between photon energies 7.6 and 11.6 eV yield decreases with the energy, [Pg.382]

Pressure and laser power dependence of the product 637 distributions in the IRMPD of CCljCHCl. At moderate pressures elimination of HCl found to be the major decomposition channel but at low pressures results indicate that C-Cl bond fission becomes primary decomposition step... 

Pottie, R.F..A.G. Harrison, and F. P. Lossing Free Radicals by Mass Spectrometry XXII. Primary Decomposition Steps in the Mercury-Photosensitized Decomposition of Methanol and Dimethyl Ether. Can. J. Chem. 39, 102 (1961). 

The extensive fragmentation of TNAZ and its reaction products upon electron impact ionization necessitates caution in the assignment of features in the time-of-flight data to specific reaction channels. To identify the primary decomposition step, the laser fluence is reduced until a single... 

Because the majority of the NO appears to emanate from later stage reactions, especially in the gas phase, its relationship to the parent molecular structure in the condensed phase is primarily through the concentrations of NO2 and HONO produced in the primary decomposition steps. First, a major source of NO is the equilibrium (1) which favors NO at... 

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