Mercury, triplet state, reaction, with

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... 

Generally speaking, atoms in doublet and triplet states are reactive and tend to form chemical bonds either with themselves or with other atoms or molecules. Hence the photochemist is concerned in practice with gas phase photochemical reactions of mercury, cadmium, zinc, and the noble gases whose atoms exist normally in singlet states. 

It should be mentioned in passing that both mercury and benzene sensitized Norrish Type II reactions have been shown to occur. Under the proper conditions the triplet states of the substrate molecules would be preferentially excited. This is further proof that Type II reactions may, with certain compounds, occur through the triplet state. 

Emission (AmaT = 380 nm), assigned to that from the lowest triplet state of water, has been observed following either y-irradiation or mercury sensitization of crystalline H20 or D20.439 For the mercury sensitization a triplet energy-transfer mechanism appears to be operative, while for the y-irradiation the excited state is formed via reaction (88). Photolysis of water adsorbed on various organic materials is observed with short-wavelength u.v. light (A < 250 nm).440... 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

Furans are able to undergo photocycloaddition of the [W2S+ 2S] and the [W4S+ 4S] type to suitable substrates. With benzene (80JCS(P1)2174) five 1 1 products are obtained. The relative proportions of these products are highly variable and depend on the relative concentration of the reactants, the irradiation time, the light intensity and the temperature of the solution. For the shortest irradiation time with a low-pressure mercury lamp at 15 °C, the relative proportions are 1 1 10 40 2. The major product is the 2,5 l, 4 -adduct (301) and the next most prolific is the 2,3 l, 2 -adduct (302). Adduct (301) is unreactive to dienophiles but gives adduct (302) by Cope reaction at 60-70 °C. This reaction can also be achieved by irradiation of a cyclohexane solution of (301). Adduct (302) reacts readily with dienophiles in ethereal solution to form Diels-Alder adducts. The minor adducts possess structures (303), (304) and (305). The reaction is thought to involve the first excited triplet of benzene or an excited state complex. A [ .4s+ .4g] photoadduct (306) is formed... 

The reactions of reactive ground state atomic triplets with organic compounds have been studied most extensively with oxygen atoms. Direct photolysis of oxygen or nitrogen dioxide yields O P) atoms, but Cvetanovic s method of photosensitizing the decomposition of N20 with excited mercury triplets9 in the presence of various pressures of... 

Sensitization by mercury is useful for performing the photoreaction on all those 1,4-dienes that lack chromophoric groups, e.g. alkyl and/or cycloalkyl substituted systems. These reactions appear to proceed by triplet energy transfer from the mercury to give vibrationally exited diene triplets. Thus, at variance with solution phase photoreactions, the exited states possess greater vibrational energy and substantially different products are possible. While such comparison is impossible for the parent system because no solution phase photochemistry has been reported, the di-ir-methane product (4) was not observed when irradiation of the 3,3-dimethyl-substituted diene (3) (equation 3) was carried out under acetone sensitization in solution. Hydrocarbon (4) was the major component under mercury-sensitized conditions in the vapor phase. ... 

Thus, Reaction 1 produces sulfur atoms in their lowest excited singlet-D2 state with 26.4 kcal excitation energy while Reaction 2 results in their ground, triplet-P2.i.o states. Alternative sources of triplet sulfur atoms are the triplet mercury photosensitization of COS and the direct photolysis of COS in the presence of excess CO2. Carbon dioxide is a quencher of singlet, excited sulfur atoms... 

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