Carbonyl compounds spin-orbit coupling

In this section the effect of spin-orbit coupling on radiative and radiationless intercombinational transitions (transitions occurring between states of different multiplicity) will be investigated. We will be particularly concerned with the use of internal and external heavy atoms to induce spin-orbit coupling. The effect of heavy atoms on intercombinational processes occurring in aromatic hydrocarbons, carbonyl compounds, and heterocyclic compounds will be discussed. 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

If the orbitals involved are centered at the same atom, as is the case for carbonyl compounds, the spin-orbit coupling is particularly large due to the factor Irfp in the denominator of Equation (1.42). The (rr,jr ) and... 

Since one electron is in a different MO in an excited state, the requirement of the Pauli principle for paired electron spins is relaxed. The excited state can therefore exist as either a singlet state Sj (x=0) or a triplet state Tj (i = 1). The reactivity of singlet states and triplet states is generally very different. Intersystem crossing, which is a radiationless transition between states of different multiplicity, is a formally spin-forbidden transition however, spin-orbit coupling removes this constraint in carbonyl compounds. 

These 1,4-biradicals (15) are formed in [2-1-2] cycloaddition of electronically excited carbonyl compounds to alkenes. The conformational dependence of their spin-orbit coupling and... 

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