Carbenes singlet-triplet energy separation

It is well known that double bonds have a different effect on the singlet-triplet energy separation and thus on the reactivity of the carbene than alkyl groups. The description of the electronic structure of such carbenes is rendered more difficult by the fact that several low-lying electronic states are possible. For... 

Fig. 3 The estimated singlet-triplet gap (in kcal/mol) on the basis of the sum of the spin-potentials ( is + fis) plotted against the corresponding vertical singlet-triplet energy separations (A v i at the singlet geometry and AE, 2 at the triplet geometry (in kcal/mol)) for the investigated carbenes, silylenes, germylenes, and stannylenes (Reproduced with permission from J. Phys. Chem. A, 2004, 108, 490. Copyright 2004 American Chemical Society).

Tbble 4 Singlet-Triplet Energy Separation of Simple Carbenes and Stabilization Energies vs. Methylene at B3LYP/6-31H-G ... 

Carbene structures and properties can now be computed with chemical accuracy , despite the difficulties associated with multi-reference species such as singlet carbenes. This is particularly encouraging since the determination of singlet-triplet energy separations and accurate structures of carbenes, which very often are, at best, fleetingly observable intermediates, is extremely difficult experimentally. Transition structures also can be computed apparently with good accuracy and complement experimental studies of the reactions as well as the complex rearrangements commonly found in carbene chemistry. 

Carbenes with a triplet ground state with a small energy separation (A st < 3 kcal/mol) react in the singlet state regardless of the method of generation. 

The triplet dimer diradical DR2(Ti) finally will relax into thermal equilibrium (kT) with its singlet ground state DR2(So). As we have seen from the ESR spectra (see Fig, 10) the energy separation between the singlet and triplet diradical states is very low and thermally activated transitions occur even at low temperatures. Furthermore the ESR spectra have revealed an admixture of about 10% carbene character with the diradical intermediates. This carbene character may be important in determining the probability x of the side reactions (see Eq. (19)) for the DR -+ AC chain termination reaction. It surely is not, however, the only essential factor, otherwise there should be no difference in the optical and thermal termination reaction steps. Up to now a direct observation of the metastable triplet state Ti(M) has been possible only in two specific crystals where the polymerization reactions are very weak. 

In a singlet state a or p carbene, the electron-electron coulomb repulsion would be severe, since two electrons are constrained to the same small MO. On the other hand, the triplet configuration is stabilized by relief of the coulomb repulsion and exchange repulsion however, the separation of electrons into different MOs does not come without a cost. Thus, the magnitude of the energy difference between the triplet and singlet states (the singlet-triplet split-... 

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