Ultraviolet unimolecular reaction

Ordinarily, B has at least one absorption band appearing at longer wavelength than the absorption bands of A. The activating radiation generally is in the ultraviolet (UV) region (300-400 nm) but may be in the visible (400-700nm). The most prevalent photochromic systems are based on unimolecular reactions (A->-B), and those described in this book correspond to this type. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

A unimolecular elementary reaction occurs when one molecule or ion reacts. For example, when one molecule of chlorine absorbs ultraviolet light, the Cl—Cl bond breaks. The product is two chlorine atoms. 

The kinetic descriptions in this chapter are developed for unimolecular scission of an initiator to yield two radicals (Scheme 3.1), the most common means of generating radicals in industrial systems. Thermal initiation of monomers is an additional mechanism capable of forming primary radicals at higher temperatures, as discussed for styrene in Section 3.2.1.3. Photoinitiators that produce radicals by ultraviolet irradiation are often used to initiate crosslinking and curing reactions these polymer modification techniques are not discussed in this chapter. 

Arynes are very reactive intermediates which enter into a large variety of bimolecular reactions including dimerization. With the exception of apparently slow reactions such as equation (13), no unimolecular decompositions of arynes are known. The direct detection of arynes, therefore, is possible in the gas phase or in dispersed, inert media. Utilizing such conditions the ultraviolet and infrared absorption spectra, ionization potential, enthalpy of formation, rate of dimerization, and approximate lifetimes of arynes have been measured. Most claims for the existence of... 

The group of Reisler has employed ion imaging to study the dynamics evolving on the Sq and Sj excited states of isocyanic acid (HNCO). The unimolecular decomposition of HNCO is equally complex as it involves coupling between different electronic surfaces and competition among different reaction pathways. Following ultraviolet absorption, the HNCO molecule dissociates via the pathways in Equation [4],... 

Pure, dry formaldehyde gas shows no visible polymerization at temperar tures of 80 to 100°C and obeys the ideal gas laws without pronounced deviation . However, its stability is dependent on purity, even a trace of water provoking rapid polymerization. At ordinary temperatures the dry gas polymerizes slowly, building up a white film of polyoxymethylene on the wails of the containing vessel. Kinetic studies indicate that this transformation takes the form of a surface reaction, unimolecular at high pressures and polymolecular below 200 mm. It is not accelerated by ultraviolet light -. In the presence of water vapor and other polar impurities formaldehyde gas is stable only at pressures of 2 to 3 mm or concentrations of about 0.4 per cent at ordinary temperatures. 

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