Triplet ground state singlet carbenes

It has been possible to record the IR and UV spectra of several derivatives of the carbene [75] - tetrachlorocyclopentadienylidene [80], indenylidene [81] and fluorenylidene [82] (Bell and Dunkin, 1985). These carbenes were formed by UV photolysis of the corresponding diazo precursors frozen in inert matrices and have a triplet ground state. The carbenes [80]-[82] react with CO in inert matrices at 30 K, but exhibit a lower reactivity than the carbene [75]. Furthermore, they were stabilized in a pure CO matrix at 12 K, whereas the free carbene [75] could not be detected under these conditions. The different reactivity towards CO between [75] and [80]-[82] may be associated with the different steric shielding of the carbene centres and with the different triplet-singlet gap as well. 

By analogy with the mechanism proposed for the reaction with alkenes, C—H insertion product formation can be explained in terms of a H abstraction-recombination process of triplet arylcarbenes. The observations that ground-state singlet carbenes, for example, chlorophenylcarbene (67), produce only O—H insertion... 

The role of spin in the reaction is an especially interesting and important issue in carbene chemistry. Apparent violation of the spin selection rule, such as the reaction of ground state singlet carbene with triplet molecular oxygen in matrices as well as in solution, and the reaction of triplet ground-state carbenes with (singlet) CO, CO2, and N2 in matrices, are challenging issues for the near future. 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

The susceptibility of a carbene to reaction with oxygen can be used as a probe of its spin multiplicity. If a carbene is insensitive to oxygen, this is taken to be an indication that it is a ground-state singlet and that the singlet does not readily form the triplet carbene. 

In the case of carbenonitrenes, the nitrene subunit has an intrinsic preference for a triplet configuration, and, thus, for a A" ground state, a carbene with a very strong preference for a closed-shell singlet ground state is required. Such a case has not been realized yet. On the other hand, if both subunits are carbenes, then. 

Carbenes with a triplet ground state well separated from the singlet (A st > 5 kcal/mol) react in a multiplicity determined by the method of generation. 

This law means that the intensity becomes smaller as the temperature is raised. Deviations from linearity could indicate a temperature-dependent equilibrium between a triplet and a singlet. Conversely, a linear Curie plot is taken as evidence for a triplet ground state far below the singlet, but such observations require that the carbene be stable to a certain extent at elevated temperature. 

Such solvent stabilization effects on the reactivity of singlet carbenes in equilibrium with their triplet ground states have also been observed experimentally in other arylcarbene derivatives. ... 

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