Radical Pair Mechanism
The generation of CIDNP in a typical photochemical system (target + photosensitizer, flavin in this example) is a cyclic photochemical process shown schematically in Figure 1. The chain of reactions is initiated by a blue light photon, which excites the flavin mononucleotide (FMN) photosensitizer to the singlet excited state. The fluorescence quantum yield of this state is rather low, and approximately half of the molecules undergo intersystem crossing into the long-lived triplet state. Triplet FMN has a remarkable electron affinity. If a molecule with a low ionization potential (e.g. phenols, polyaromatics) is present in the system, the diffusion-limited electron transfer reaction forms a spin-correlated triplet electron transfer state – a radical pair. The actual kinetics are rather complicated and may involve multiple (de)protonations and hence exhibit pH dependence.
The radical pair may either cross over to a singlet electron state and then recombine, or separate and perish in side reactions. The relative probability of these two pathways for a given radical pair depends on the nuclear spin state and leads to the nuclear spin state sorting and observable nuclear polarization.
Read more about this topic: CIDNP
Famous quotes containing the words radical, pair and/or mechanism:
“The world is not dialectical—it is sworn to extremes, not to equilibrium, sworn to radical antagonism, not to reconciliation or synthesis. This is also the principle of evil.”
—Jean Baudrillard (b. 1929)
“The majority of persons choose their wives with as little prudence as they eat. They see a trull with nothing else to recommend her but a pair of thighs and choice hunkers, and so smart to void their seed that they marry her at once. They imagine they can live in marvelous contentment with handsome feet and ambrosial buttocks. Most men are accredited fools shortly after they leave the womb.”
—Edward Dahlberg (1900–1977)
“The two elements the traveler first captures in the big city are extrahuman architecture and furious rhythm. Geometry and anguish. At first glance, the rhythm may be confused with gaiety, but when you look more closely at the mechanism of social life and the painful slavery of both men and machines, you see that it is nothing but a kind of typical, empty anguish that makes even crime and gangs forgivable means of escape.”
—Federico García Lorca (1898–1936)