Carbon hyperfine splitting constants

As mentioned for the relationship between the PE spectrum of a parent molecule and the electronic spectrum of its radical cation, any close correspondence between the electronic spectra of anions and cations or their hyperfine coupling patterns holds only for alternant hydrocarbons. The anions and cations of nonalternant hydrocarbons (e.g., azulene) have significantly different hyperfine patterns. Azulene radical anion has major hyperfine splitting constants (hfcs) on carbons 6, and 4,8 (flH = 0-91 mT, H-6 ah = 0-65 mT, H-4,8 ah = 0-38 mT, H-2) in contrast, the radical cation has major hfcs on carbons 1 and 3 (ah = 1.065 mT, H-1,3 Ah = 0.152 mT, H-2 ah = 0.415 mT, H-5,7 ah = 0.112 mT, H-6). °°... 

The 4-line spectra shown in Fig. 26 b and 26c were observed for EP and AB. Simulation gave the hyperfine splitting constants given in Table 3. In both cases, the value of a 1 was the same as that of a 3, indicating that the unpaired electron of the propagating radical is completely delocalized over the allylic three carbons. 

In a u radical system, the amount of unpaired down-spin density (pn) ori a hydrogen atom, and hence the hyperfine splitting constant (oh) of the hydrogen atom in a ESR spectrum of the compound, is proportional to the amount of unpaired up-spin density (pc) the carbon atom to which the hydrogen atom is attached [30]. In other words. 

If the arguments in the foregoing paragraph are pursued quantitatively/ we are led to the conclusion that the hyperfine splitting constant should be proportional to 7r-electron density, p, on the carbon atom. This is expressed mathematically by McConnell s relationship ... 

For instance, nitration of naphthalene, azulene, biphenylene, and triphenylene proceeds preferentially in positions with the greatest constant of hyperfine splitting at the hydrogen atom in ESR spectra of corresponding cation-radicals. The constant is known to be proportional to the spin density on the carbon atom bearing the mentioned hydrogen. It is important, however, that the same orientation is also observed at classical mechanism of nitration in cases of naphthalene, azulene, and biphenylene, but not triphenylene (see Todres 1985). 

If the unpaired electron is a tt electron, HMO theory can be applied to the value of the ESR splitting constant. The interaction between an unpaired n electron and a proton (or other nucleus with nonzero nuclear spin) falls off rapidly with increasing separation. Therefore, the hyperfine structure is generally ascribed to interactions involving protons directly bonded to carbons in the tt system a protons) or else separated from the n system by two a bonds protons). The equilibrium position of an a proton is in the nodal plane of the it system, so it is clear that any net spin density at the proton must be only indirectly due to the presence of an unpaired it electron. This indirect effect arises because the unpaired it electron interacts slightly differently with a- and P-spin a electrons on carbon, and so the spatial distributions of these become slightly different in the a MOs, ultimately producing net spin density at the proton the a electrons are said to be spin polarized by the it electron (see Fig. 8-17). 

---