Singlet radicals

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. 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

When uncomplexed Bp is irradiated in the presence of DMA, it produces Bp + DMA (Scheme 2.1). The fate of Bp is then determined by the concentration of DMA as well as the nature of the solvent either Bp may undergo intersystem crossing, kisc, to form Bp or be reduced to the radical ion, Bp, by electron transfer from DMA. The kinetics of the singlet electron-transfer process, kset, to produce a singlet radical ion pair + DMA jip are governed... 

Note that the error estimates obtained from Eq. 19 are, in the majority of cases, less than 2kcalmol This indicates that the computational method is converging well from the lowest level of perturbation theory through to the highest. The only exceptions to this are the divalent singlet radicals (compounds of the form XSnY), in which there is a nonzero correction for UHF instability. In these cases, the ad hoc error estimation method indicates a higher level of uncertainty because the presence of UHF instability is an indication that the MP4(SDTQ) level of theory is not fully adequate to describe the electronic ground state of the molecule (the same is true of a nonzero spin contamination correction). 

The third intra-pair reaction to be discussed involves bond formation between radical anion and cation without intervening transfer both singlet and triplet radical ion pairs can couple. For example, the bifunctional radical cation 24 generates two chloranil adducts, most likely via zwitterions (e.g., 74 and 75 ), initiated by forming a C O bond. The CIDNP results indicate that 74 and 75 are formed from a singlet radical ion pair. Adduct 75 is a minor product, as the major spin density of 24 + is located in the allyl function which, therefore, is expected to be the principal site of coupling. 

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... 

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