Geminate polarization

The nature of the cyclopropane radical cation was first characterized unambiguously by CIDNP effects of a 1,2-disubstituted derivative. The pattern of benzylic and geminal polarization observed during the reaction of chloranil with cis- and tru 5-l,2-diphenylcyclopropane (Figure 16) supported radical ions with spin-density on the benzylic carbons [107]. 

The determination of surface accessibilities in a protein is by now a well-established application of photo-ClDNP. Its operating principle is that a sensitizer in the bulk solution is photoexcited, forms a radical pair with an amino acid that is exposed to the solution, and so causes CIDNP to arise amino acids not accessible to the dye remain impolarized. To avoid disruptions of the structure the photore-action(s) must be cyclic. As this leads to exchange cancellation, one either observes the polarizations that remain because of relaxation in the free radicals or samples the geminate polarizations in a time-resolved CIDNP experiment. The latter appears preferable for quantitative conclusions as it also removes other artefacts, but cannot be applied to the pulse-labelling and related experiments described below. Commonly employed dyes are 2, 2 -dipyridyl 16 or flavins 17. As already mentioned in the preceding section, only tryptophan 10, tyrosine 11, and histidine 12 are polarizable. However, the reduction of the information content in a crowded protein spectrum by this selectivity is a much desired blessing rather than a drawback. 

From the above definitions, the geminate polarization at t 0 is and the concentration of A - radicals. 

Continuous generation of radicals and therefore of geminate polarization can be included in equations (20) and (21) by the addition of zero-th order terms in which k moles of geminate radical pairs are formed per second, hus ... 

The Photo-reaction of Flavin with Tryptophan. Typical spectra obtained from the flavin/N-acetyl tryptophan (Trp) system in D2O are shown in Figure 17. At long delays (100 ms) the flavin peaks (labelled F) which were strong shortly after the laser flash have very nearly disappeared. In the corresponding spectrum obtained with c.w. laser illumination, (Figure 11) the flavin peaks are also very weak relative to those of tryptophan. Thus under steady state conditions a lot of information obtainable in a fast time resolved experiment is actually lost. As we shall see these effects are due to a rapid partial cancellation of geminate polarization. 

Closs G L and Miller R J 1979 Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP separation of geminate and random-phase polarization J. Am. Chem. Soc. 101 1639—41... 

There is an additional problem that must be considered for radicals that escape an original geminate radical pair. Such radicals are polarized. If they form new (encounter) radical pairs, they could yield combination products after undergoing T-S mixing again. However, because of the rapidity of nuclear relaxation in free radicals, the F-type polarization generally predominates. 

Keywords Chemical orbital theory, Cw-stability, Cyclic conjugation. Disposition isomers. Diradicals, Donor-acceptor, Electron delocalization, Geminal bond participation, Inorganic heterocycles. Ring strain. Orbital phase. Orbital phase continuity. Polarization, Preferential branching. Reactivity, Selectivity, Stability, Tautomerism, Z-selectivity... 

Polar functional groups such as carbonyl, cyano, and sulfoxide, as well as silyl and stannyl groups, exert a strong directing effect, favoring proton removal from the geminal methyl group.171... 

Geminal bis(hydrostannation) is attributed to quenching of the bis-2-tributylstannylethyl radical by the thiol, with polarity reversal catalysis (Equation (16)). 

Geminate Recombination. Coulomhic forces attract the Ionisation electron back to Its geminate positive partner (reaction (1) + (2)). In water and other polar media, these forces are modified by the high dielectric constant and the possibility of solvation, giving a high probability of escape. This results In a high yield... 

Martin and Martin 51) have reported similar small (AJ= -0.2 to -0.45 Hz) changes for VHH in a series of alkyl substituted vinyl bromides. The same authors si> have repeated a 1.5 Hz decrease with increasing solvent polarity for the negative geminal coupling constant in allenic ketones of the form RCOCH = C=CH2 where R is methyl or ethyl. Reaction field interactions are suggested. 

Geminal H—H coupling constants always decrease in solvents of increasing polarity (dielectric constant). 

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