Carbon excited, from

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Recent studies of the photolysis of bicyclic ketones have led to the preparation of unusually strained systems by the eUmination of carbon monoxide from excited ketone molecules. Irradiation of Formula 286 gives bicydo [2.2.0[hexane (Formula 287) in low yield (118). Mercury-photosensitized decomposition of bicyclo [2.2.1 ]heptan-2-one (Formula 258) gives bicyclo [2.2.1 [hexane (Formula 289) in 20% yield (119). Camphor (Formula 290) undergoes a similar mercury photosensitized decomposition to 1,5,5-trimethylbicyclo [2.2.1 [hexane (Formula 291) (10%) (119). 

Carbonyls, compounds with carbon-oxygen double bonds, also absorb light in the UV region. For instance, acetone has a broad absorption peak at 280 nm. In this example, the electron can be excited from an unshared pair into a nonbonding -orbital, (ji... 

Suppose that we are talking about a double-quantum transition in which both the proton and carbon change from the a state to the p state. This transition is thus from the aH c state to the PuPc state ol l lc two-spin, four-state system. This transition corresponds to DQC. Likewise, if the proton flips from ft to a while the carbon simultaneously flips from a to P, we have a zero-quantum transition (P ac to a Pc) because the total number of spins in the excited (ft) state has not changed. This transition corresponds to ZQC. What can we say about these mysterious coherences In Section 7.10, we encountered ZQC and DQC as intermediate states in coherence transfer, created with pulses from antiphase SQC ... 

In C NMR spectroscopy, the carbene carbon resonates at ca. 140-400ppm to low field of SiMe4. This is probably a result of the existence of low-energy electronic excited states for the complex, which leads to a large paramagnetic contribution to the shift. A proton substituent at the carbene carbon resonates from 4-10 to 4-20 ppm. 

It is also useful to consider the bonding among the carbon atoms in diamond in terms of the MO model. Energy-level diagrams for diamond and a typical metal are given in Fig. 16.27. Recall that the conductivity of metals can be explained by postulating that electrons are excited from filled levels... 

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