Mercury atoms, excited, reactions

Gray [215] studied the photooxidation of methane and ethane at room temperature in the gas phase using Hg as a photosensitizer. Hydroperoxide was found as the product of oxidation. Along with hydroperoxide, ozone was also found as the product of photooxidation. It was supposed to be due to the reaction of excited mercury atoms with dioxygen. 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

Since monoisotopic photosensitization studies are usually carried out with substrates which react with the excited mercury atoms to form mercury compounds, it follows that mercury vapor is consumed in the course of the reaction and, therefore, experiments have to be carried out... 

Even when conditions can he found which result in a distribution of isotopes in upper states, different from that in AIIg a reaction is still required which fixes this upper state distribution as stable compounds of mercury. Therefore, a developer reaction is required, which usually involves substrate molecules which contain Cl, O, OH, etc. Such reactive fragments, however, react not only with excited, but also with ground-state mercury atoms. Isotopic fractionation will therefore occur only in an isotopieally-specific primary process of the type ... 

Any mechanism proposed for the detailed reaction path in the primary interaction of excited mercury atoms which 1T(T molecules and for the... 

Reactions sensitized by monatomic gases have been very extensively studied. The majority of the cases involve mercury vapor. Direct reaction of excited mercury atoms with other gases has also received considerable attention of late. 

If it is desired to calculate relative rates of the various reactions it now becomes necessary to evaluate [1Hg]. If the concentration of M can be maintained sufficiently high to prevent diffusion of radiation, i.e. if essentially every excited mercury atom collides effectively with a molecule M before it emits, and if the concentration of X can be kept so low that reactions between it and Hg may be neglected, the average rate of formation of excited mercury atoms per unit volume will be... 

An interesting case is the action of excited mercury on oxygen18 19. The energy required to dissociate the oxygen molecule is 5.13 electron volts or 118 kcal.mole-1. 63Pi mercury atoms are thus unable to cause this dissociation. However, the reaction... 

The addition of deuterium atoms (produced by mercury vapor atoms excited with ultraviolet light of 2537 A) to the reaction... 

Recent investigations on ethane formation in the photolysis of acetaldehyde indicate that decomposition into methyl and formyl radicals occurs from the triplet state which is also removed by first-order internal conversion and, to some extent, by second-order deactivation. In the mercury-photosensitized reaction methyl radicals are formed by direct dissociation of the excited aldehyde molecules, as well as by collision of excited mercury atoms . 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

Two basic schemes have been used for initiating intracluster processes. In the first the reaction is induced by electronic excitation of an atom inside the complex. Jouvet and Soep used this scheme for the study of several reactions of electronically excited mercury atoms Hg( Pi). In the reaction with CI2 they obtained conclusive evidence of the formation of a charge transfer intermediate in the process. In the reaction with Hj a strong dependence of the reactivity on the geometry was observed. It was found that the reaction occurs on the n surface with a C2 symmetry. 

The importance of photosensitization derives f rom the f act that reaction is produced in the presence of the sensitizer in circumstances where the direct photochemical dissociation is not possible. The example just cited is a case in point. Radiation of wavelength 253.7 nm was absorbed by a mercury atom. The excited mercury atom dissociated a molecule of hydrogen by transferring the excitation energy in a collision. The mercury atom had 471.5 kJ of this 432.0 kJ were needed for the dissociation 39.5 kJ are left over and go into additional translational energy of the two hydrogen atoms and the mercury atom. If the attempt is made to dissociate H2 directly by the process... 

A number of reactions of excited mercury atoms have been studied... 

A particularly large amount of research has been devoted to mercury-sensitized photochemical reactions of hydrogen compounds (H2, hydrocarbons, etc), especially to decomposition and oxidation reactions. The secondary process of interaction between an excited mercury atom and a reactant molecule may be involved in these reactions by proceeding two paths Hg + HH Hg -f- R + H and Hg + RH -> HgH + R. 

Consider now the interaction of excited mercury atoms with O2 molecules which is very essential in connection with the mercury-sensitized photochemical oxidation reactions. 

Mercury-sensitized photochemical oxidation has been studied by many authors for hydrogen, methane, ethane, propane, etc. The main product of hydrogen oxidation at 14 °C has been found to be [24] H2O2 in a yield > 85%. This supports the conclusion that oxygen atoms are not the primary products of the interaction between an excited mercury atom and an O2 molecule since the main reaction product would then be water. 

Palladium, platinum, silver, gold, and mercury atoms are formed by LMCT excitation of suitable complexes such as [Pd(N3)4]2-, [Pt(N3)4]7-, Ag(PPh3)N3, [Au(N3)2]-, and [Hg(N3)3] [27,106,145]. Generally, the metal atoms are rather energy-rich. They agglomerate usually to larger particles which appear as colloids, suspensions, or thin films. In the presence of appropriate compounds the metal atoms participate in addition or insertion reactions [106], e.g., cf. equations (52, 53) ... 

This transfer is frequently referred to as a photosensitization reaction, where the quencher Q is sensitized to emission by excitation from M. In certain cases where it is not possible to generate the excited state Q by direct absorption of a photon from an incident light source, it is possible to access Q by energy transfer from a sensitizer. Mercury atoms can in selected cases be used as the sensitizer M. Table 1.2 demonstrates the triplet energies of a series of compounds that are frequently... 

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