AGst values, 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). 

It is interesting to note here that the value of the rate constant for DPC (14a), 1-NC (a-12), methoxycarbonylphenylcarbene (53), and FL (23) with O2 are 5 X 10 , (3.5 0.7) X 10 , 8.6 x 10 and (1.4 0.2) x 10 AT s-, respectively. The difference in the rate constants is not as large as that observed for the hydrogen atom abstraction rate constants for those carbenes (Table 9.9) and do not reflect the difference in the magnitude of AGst- The reason is probably because the rate constant of triplet carbenes is very fast and because the singlet states do not interact with triplet oxygen because of the spin restriction. 

---