Atomic Hydrogen Reactions

Another common method of producing atomic hydrogen is based on photochemical sensitization. Hydrogen (or its mixture with another gas) saturated with mercury vapour and illuminated by a quartz mercury arc yields excited mercury atoms Hg ( Pj) as a result of the 2537 A resonance line absorption by mercury vapour. Excited Hg atoms interacting with H2 molecules dissociate the latter to atoms. 

Species releasing hydrogen atoms upon irradiation are also used as sources of atomic hydrogen. For instance, hydrogen iodide irradiated with UV light (X 3 100 A) decomposes as HI + hv - H + I. The iodine atoms that are considerably less active than hydrogen atoms only slight distort the reaction mechanism. 

Certain authors obtained hydrogen atoms by the dissociation of Hg molecules on an incandescent tungsten wire (e.g. [423]). 

The simplest reaction of atomic hydrogen is that with molecular hydrogen. It can be studied by replacing usual hydrogen, itself a mixture of the para- and ortho-modifications, by para-hydrogen, or by using deuterium. 

The interaction of H with O2 can occur both as a bimolecular process 

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Other atomic hydrogen reactions mentioned here are those with halogens and hydrocarbon halides. These reactions are very fast and the primary processes are... 

Neuhauser D, Baer M, Judson R S and Kouri D J 1989 Time-dependent three-dimensional body frame quantal wavepacket treatment of the atomic hydrogen + molecular hydrogen exchange reaction on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) surfaced. Chem. Phys. 90 5882... 

A process resulting in a decrease in touglmess or ductility of a metal due to absorjDtion of hydrogen. This atomic hydrogen can result, for instance, in the cathodic corrosion reaction or from cathodic protection. 

Hydrogen atoms ate thought to play a principal role in the mechanistic steps of many reactions, including hydrocarbon thermolysis (119). Some reactions of atomic hydrogen with olefins and paraffins ate the following (120—122) ... 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

By indirect embrittlement (reaction by-product, atomic hydrogen diffusing into the lattice of the steel)... 

Nascent atomic hydrogen released at metal surfaces by chemical reactions between the process environment and the metal (corrosion or cathodic protection reactions)... 

Nascent atomic hydrogen released by a process reaction such as catalytic desulfurization... 

Aliphatic sulfonyl chlorides that have a-hydrogen substituents, react with simple tertiary amines, such as trimethylamine, to generate sulfenes or perhaps their amine adducts 446). These species are suggested by the incorporation of one (but not more) deuterium atoms on reaction of sulfonyl chlorides with deuterated alcohols and triethylamine (447-450). A 2 1 adduct of sulfene and trimethylamine with proposed sulfonyl-sulfene structure could be isolated (451). 

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

The production of allyl chloride could be effected by direct chlorination of propylene at high temperatures (approximately 500°C and one atmosphere). The reaction substitutes an allylic hydrogen with a chlorine atom. Hydrogen chloride is a by-product from this reaction ... 

A third reaction, which goes in parallel with the desorption reaction, is the entry of atomic hydrogen into the steel from the surface adsorbed state ... 

If the acid contains certain impurities such as arsenic, the arsenic raises the overvoltage for the hydrogen evolution reaction. Consequently, the amount of atomic hydrogen diffusing into the steel, and the brittleness, increase. 

Table III lists the kinetic equations for the reactions studied by Scholten and Konvalinka when the hydride was the catalyst involved. Uncracked samples of the hydride exhibit far greater activation energy than does the a-phase, i.e. 12.5 kcal/mole, in good accord with 11 kcal/mole obtained by Couper and Eley for a wire preexposed to the atomic hydrogen. The exponent of the power at p amounts to 0.64 no matter which one of the reactions was studied and under what conditions of p and T the kinetic experiments were carried out. According to Scholten and Konvalinka this is a unique quantitative factor common to the reactions studied on palladium hydride as catalyst. It constitutes a point of departure for the authors proposal for the mechanism of the para-hydrogen conversion reaction catalyzed by the hydride phase.

Two thermocouples, Em at x = 0 and Ex at a distance x, permit the monitoring of the atomic hydrogen concentration change along the side-tube. The atoms recombining on the thermocouple tip covered by the active catalyst evolve the heat of reaction and thus the thermoelectric power becomes a relative measure of the concentration of atoms in the gas phase. Finally, one obtains for the direct use in an experimental work the following equation... 

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