Mercury singlet state

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. 

The data in Tables II-VI may not be used, therefore, to indicate precisely which absorption bands will be important with various mercury lines. It is evident on inspecting these tables that not only the vibrational energy but the modes of vibration in the upper singlet state will depend on the spectral characteristics of the absorbed light. Not enough data are at present available to relate mode of vibration to behavior but studies of this type should be made. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

Thietanes are photochemically unstable and should be protected from light if they are to be stored for any length of time. Short-lived (hot) biradical intermediates, for example, 98a, appear to be formed and can undergo a variety of reactions, as shown for thietane. " Mercury-sensitized photolysis gives triplet biradicals that are longer lived than the singlet biradicals formed on direct excita-tion. " Some cyclopropane product is produced in the sensitized photolysis. The second excited singlet state of thietane decomposes either to ethylene and thio-formaldehyde via a 1,4-biradical or to elemental sulfur and cyclopropane. ... 

Very little is known regarding the nature of the electronic state responsible for the decomposition of other ketones. The low quenching efficiencies observed in the photolysis of 2-hexanone may be interpreted by the short-life-time of the triplet state of 2-hexanone. The similar distribution of the butene isomers found in the photolysis of 4-methyl 2-hexanone at 2537 A and in its mercury-sensitized decomposition, indicates the important role played by the triplet state in reaction II even at short wavelengths. Finally, it is to be mentioned that the presence of secondary H atoms in the y position seems to enhance the contribution of the excited singlet state to the decomposition step II. 

Cycloaddition of carbon-carbon double bonds can also occur intramolecularly. Direct irradiation of simple dienes leads to cyclobutanes. This is a singlet-state process and is concerted. The stereochemistry of the cyclobutane can be predicted on the basis of orbital-symmetry rules (Part A, Section 10.1). Nonconjugated dienes can also undergo photochemical cyclization employing mercury or carbonyl compounds as sensitizers. Cyclobutane formation is usually unfavorable with 1,4-dienes because it would result in a very strained ring system. When the alkene units are separated by at least two carbon atoms, cyclization becomes more favorable sterically ... 

The addition of the methylmercuriacetylcarbene to cis- and //ms-butene was found to be completely stereospecific, suggesting that this carbene has a singlet ground state (with the heavy mercury atom, relaxation to the ground state should be rapid). 

The allowed transitions are between adjacent columns of energy states. The singlet and triplet manifolds are separated as they are forbidden by spin selection rules. Under certain conditions they do occur with reduced efficiency, as for example, the transitions between 6 S and 6 states of mercury. They are indicated by dashed lines in the diagram. The wavelength associated with each transition is indicated in A units. 

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