Charge singlet state

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

The Stale Correlation Diagram (SCD) approach introduced by Shaik and Pross96 appears similar in some respects. However, the LUMO, HOMO and the first two exciled stales are considered, (refer Figure 1.5)4,53 Thus, if we consider the interaction of the radical with the olefin in its ground (singlet) state (R + C=C ) and excited (triplet) state (R + C=CJ) and two charge transfer... 

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

The dynamics of photoinduced charge separation, kcs, and charge recombination, kcr (Fig. 2a), have been studied in several families of hairpins containing an Sa linker and a single G C base pair by means of femtosecond time-resolved transient absorption spectroscopy [27, 28]. Both the singlet state and anion radical of Sa have strong transient absorption centered at 575 nm. The difference in the independently determined band shapes for Sa ... 

The positional specificity of these nucleophilic and electrophilic photosubstitutions can be readily understood by examination of the calculated charge densities of some of these molecules in their ground and first excited singlet states. For example, 4-nitrocatechol has the charge densities shown in (82) and (83).<139> Thus on this basis one would predict that 4-nitroveratrole... 

Thus the change in the direction of the spin angular momentum of the electron effectively imparts some singlet character to a triplet state and, conversely, triplet character to a singlet state. This relaxes the spin selection rule since J S St dr is no longer strictly zero. The greater the nuclear charge,... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

K. Rotkiewicz and W. Rubaszewska, Intramolecular charge transfer state and unusual fluorescence from an upper excited singlet of a nonplanar derivative of p-cyano-N,N-dimethylaniline, J. Lumin. 27, 221 (1982). 

Calculated Charge Densities for the Ground State (Sg), the Lowest Excited Singlet State (Sj), and the Lowest Triplet State (Tj) of Azulene (Pariser, 1954)... 

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