Singlet radical pair

Since only singlet radical pairs can give c-products the c-products are enriched in ]3 spin, giving enhanced absorption (A), and e-products are enriched in a spin, leading to emission (E). The immediate result is... 

If A is the probability of recombination during a single encounter of a singlet radical pair (close to unity), then the chance of product formation during the second encounter at time t is A Csn( )Pf(0- additional re-encounters, it can be shown that An(<) = A Csn( )P d therefore recombination during the time interval between t and t + dt has a probability Pn(<)d< given by equation (34),... 

The external magnefic field effect on the PFR of 1-naphthyl acetate has also been reported to be > 1 for a labeled sample ( C) and equal to 1 for a nonlabeled sample ( C) of ester this means that the precursor of the reaction product (2-acetylnaphthol) is a singlet radical pair. 

The manifestations of singlet interactions need not be subtle for example, attempts to prepare 1,3-cyclohexadiene photodimers using naphthalene as a sensitizer will give an infinitesimal yield.156 Moreover, the reported159 use of triphenylene to form triplet radical pairs from azo compounds is incorrect since only singlet radical pairs are formed.160... 

The protons of 12 are polarized due to partial back recombination of the singlet radical pair (Et3Sn CH2CHBrCH2CCl3)s. The analysis of CIDNP effects of trichlorobutene does not preclude the assumption that the polarization of CH2=CHCH2CCl3 is also... 

Incorporation of triethylamine into the reaction medium produced more reduction product presumably due to electron transfer from the triethylamine to the excited alkyl halide. This results in a weakly-bound amine-alkyl halide pair [57]. The alkyl halide radical anion releases X- (Scheme 17). In a related example, it is known that solutions of aliphatic amines in CC14 are unstable to light quickly forming white crystalline precipitates [60]. The initial reaction is formation of a singlet radical pair via excitation of a ground state charge-transfer complex. 

Afree radical (with just one unpaired electron) is described as an electronic doublet because, in an external magnetic field, the electron can only exist in one of two possible spin states ( up or down ). By contrast, a pair or radicals, or a biradical (a species with two unpaired electrons in the same molecule) can exist in either of two electronic states singlet or triplet. In the singlet state the electrons are paired (opposite spin) a singlet radical pair (or biradical) is thus diamagnetic (W5 = I -1 = 0) and not observable by EPR. The radical pair above is shown in the singlet state. 

In many cases an NMR spectrum exhibits both net and multiplet CIDNP effects. In many of these cases the net For A effect is superimposed on the multiplet effect of a given signal. For example, when singlet radical pair 11-14 undergoes recombination to 11-15, the H spectrum of the CH2 group in the product shows an E/A multiplet effect superimposed on an E net effect3 ... 

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