Time scales of molecular processes

Figures 3 and 4 survey the wavelength ranges and time scales of molecular processes so far as they concern photobiological questions. Furthermore, the regions attainable...

The triplet carbene, with its two nonbonding electrons of the same spin, must produce an intermediate that cannot close to a cyclopropane until a spin is somehow changed. Such spin flips are not impossible, but they are usually slow on the time scale of molecular processes. In particular, they are slow compared with the very rapid rotations about carbon-carbon single bonds. Therefore the stereochemistry of the original alkene need not be maintained in the eventual product of the reaction, the cyclopropane (Fig. 10.48). If rotation about the carbon-carbon bond is faster than the rate at which the spin quantum number of one electron changes (spin flip), it will not matter whether cis or trans alkene is used in the reaction with triplet methylene. The product will be a mixture of cis and trans 1,2-dialkylcyclopropanes in either case. 

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