Equilibrium constants triplet carbenes

The rate constants kTS and kST define an equilibrium constant (ATeq) connecting the singlet and triplet carbenes. An estimate of Ktq, and hence AGSX, for BA can be obtained from the experiments described above. The time resolved spectroscopic measurements indicate that BA reacts with isopropyl alcohol with a rate constant some five times slower than the diffusion limit (Table 7). This, in conjunction with the picosecond timescale measurements, gives a value for ksr. The absence of ether formation from the sensitized irradiation, when combined with the measured rate constant for reaction of 3BA with isopropyl alcohol, gives an upper limit for k-. These values give Keq and thus AGST 2 5.2 kcal mol-1 (Table 8). 

When monitoring the transient due to triplet carbenes is difficult because of the inherent weak nature of the bands and/or severe overlapping with the absorption bands of the parent diazo compounds, it is more convenient to follow the dynamics of the triplet carbene by measuring the rate of the products formed by reaction of triplet carbenes with quenchers such as radicals (Section 5.3) and carbonyl oxides (Section 6.5). In this case, note that the observed rate constant (feobs) of a triplet carbene reaction is the sum of the decay rate constants of the triplet. These may include decay via an associated but invisible singlet with which the triplet is in rapid equilibrium. Thus in general. 

The final carbene in this group is dimethylsila-anthrylidene, SA. Low temperature spectroscopy identifies the triplet as the ground state (Sekiguchi et al., 1982). This carbene appears to react rapidly with methanol and with cyclohexane (Sugawara et al., 1983b). This pattern of reactivity and the reported rate constants fits quite well within the equilibrium model. However, the products of these reactions have not yet been identified. Nonetheless, it is possible to conclude tentatively that ACsj is smaller for SA than for DPM. If true, this means that the electronic effects of the silicon substituent outweigh any increase in the carbene-centre bond angle. 

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