Absolute rate singlet carbenes

Table 3. Absolute Rate Constants for Reactions of Singlet Carbenes with Proton...

Calculations predict that the lowest state of PN has an open-shell electronic configuration." " The Salem-Rowland Rule for ISC promoted by spin-orbit coupling (SOC) predicts that singlet to triplet relaxation will have its maximum rate when the singlet state is closed-shell. This is the case with diaryl carbenes where the absolute rate constants of ISC are in the order of Michl has recently pointed out the importance of donor-... 

Simon and Peter (1984), and Griller et al. (1984) have reviewed laser flash photolysis of carbene formations. Absolute rates of singlet-triplet interconversions /tst and have been summarized for carbenes by Eisenthal et al. (1985) and by Schuster (1986). 

At that time, it was not possible to measure any of the rate constants of Scheme 1 directly but in some cases it was possible to measure ratios of rate constants or to determine if spin equilibration was much faster or slower than intermolecular reactions. Organic chemists could then only dream of determining the absolute rate constants of Scheme 1. This would become possible around 1980 with the invention of laser flash photolysis with nanosecond (ns) time resolution. But successful application of this tool would require knowledge of the electronic spectra of singlet and triplet carbenes. Low temperature spectroscopy was enormously helpful in this regard. 

The effect of multiplicity of carbenes on their reactivity is most vividly marked in the following features rationalized by Skell et al. from experimental data [37-39]. First, the reaction of carbenes occurs in the singlet electron state at a much faster rate than in the triplet, with the absolute rates of typical reactions of addition to multiple bonds and of insertion into the C—H bonds exceeding, under normal conditions, the rate of intercombination conversion. Secondly, the singlet carbenes are characterized by one-step stereospecific addition to double bonds, as, for instance, in the cyclopropanation reaction, while the triplet carbenes react in a nonstereospecific way to form first an intermediate biradical through addition to one of the atoms of the double bond. The formation of a trimethylene radical, in the course of reaction of triplet methylene ( B ) with ethylene, has been confirmed by semiempirical [40, 41] and ab initio [42, 43] quantum chemical calculations. 

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