Simple radical reactions, molecular

Firstly, the classical theories on radical reactivity and polymerization mechanism do not adequately explain the rate and specificity of simple radical reactions. As a consequence, they can not be used to predict the manner in which polymerization rate parameters and details of polymer microstructurc depend on reaction conditions, conversion and molecular weight distribution. 

Molecular Orbital Correlations of Some Simple Radical Reactions... 

Instead of radical reactions, models based on molecular reactions have been proposed for the cracking of simple alkanes and Hquid feeds like naphtha and gas oil (40—42). However, the vaUdity of these models is limited, and caimot be extrapolated outside the range with confidence. With sophisticated algorithms and high speed computers available, this molecular reaction approach is not recommended. 

This simple oxidoreduction reaction involves complex OH - water molecules interactions whose the spectral signatures are assigned to Charge-Transfer-To-Solvent states (CTTS states). Indeed, the anionic precursor of the hydrated OH radical represents an interesting system for the direct investigation of elementary redox events in a protic molecular solution. 

Free-radical reactions, for so long the Cinderellas of organic chemistry because of lack of control over their selectivity, have in recent years frequently emerged as simple, efficient, and novel means for effecting molecular transformations, and they are now at the forefront of synthetic methods. Previous emphasis was on the physicochemical investigations of reactions from which developed appreciation of such factors as the regioselectivity of... 

A theoretical analysis of the reaction kinetics was given in the study by Jodkowski et a/.31 A hydrogen-bonded molecular complex was found for the hydroxymethyl reaction channel. The formation of this complex may be a ratedetermining process in the two-step reaction mechanism of the H-abstraction from methyl group. On the other hand, the reaction channel which produces methoxy radicals is a simple metathesis reaction. The profile of the potential energy surface obtained by Jodkowski et al at the G2 level is shown in Fig. 14. 

Let us also mention that interest in atmospheric chemistry and combustion chemistry of PhO led to a number of theoretical studies of its reactions with simple radicals such as atomic oxygen , HOO radical , NO and NO2 radicals and molecular oxygen . In all cases, computation of the potential energy surfaces has helped a great... 

In this chapter, we have shown how the recent advances in the crossed molecular beam technique allow us to study complex polyatomic reactions of relevance in astrochemistry. The focus was on the CN radical reactions with simple alkynes, but the same approach has been also applied to the study of other CN radical reactions with unsaturated small organic molecules, such as ethylene, benzene, and allene, which are of relevance in astrochemistry as well [77,81,84]. 

Some reactions occur thermally at temperatures too low for homolysis of any of the covalent bonds present to provide enough radicals to start the reaction. For example, mixtures of fluorine and methane explode at room temperature, and many hydrocarbons are oxidized slowly by molecular oxygen (see below). It has been postulated that the bimolecular reactions (6.10) and (6.11) are responsible. In (6.10), the formation of the very strong H-F bond makes this reaction much less endothermic than the simple homolysis of the fluorine molecule. In (6.11), a strong O-H bond is formed in the hydroperoxyl radical 18, whereas two relatively weak bonds are broken, the 0=0 n bond and the C-H bond in 16 which is weakened by the stabilization of the product benzylic radical 17. The occurrence of these molecule-induced homolysis reactions is difficult to prove because the compounds formed tend to be swamped by those from the subsequent radical reactions. 

Hydroperoxide decomposition in the presence of transition metal compounds may be, in effect, more complicated than the above simple radical scheme suggests. It may involve, for example, the formation of intermediate complexes between the catalyst and hydroperoxide. The reaction may result in the formation of molecular products without the participation of free radicals or proceed itself by a radical chain mechanism [15]. 

Pyrolysis is a technique that uses heat to transform macromolecules into a series of lower molecular weight volatile products characteristic of the original sample. The initiation reactions involved in the pyrolysis of most organic materials are the result of free radical reactions initiated by bond breaking or by the unimolecular elimination of simple molecules such as water or carbon dioxide [172-175]. The products of pyrolysis are identified by gas chromatography, often in combination with mass spectrometry. 

More recently, photo-induced generation of the hydrides has been reported utilizing UV-irradiation of the sample in an aqueous medium spiked with low molecular weight acids such as formic, acetic, ma-lonic, etc. The efficiency of the process is greater than 70% for Se and, depending on the species in solution, radical reactions can give rise to various products, including the simple hydrides or alkylated derivatives, as outlined below ... 

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