Carbon 13 hyperfine splittings

CCC bond angle is larger than 120°. Analysis of the anisotropic components of the carbon-13 hyperfine splitting confirms this and shows that... 

In contrast no hyperfine structure from the alkali metal is observed when 00 is trapped in a matrix of water. This effect is attributed to heavy solvation of the two ions which results either in an actual physical separation of the two ions, or in stabilization of the ionic structure so strongly that the contribution of a non-ionic structure is negligible. The gf-factors and carbon-13 hyperfine splittings are the same as those for CO trapped in carbon dioxide, which shows that the basic structure of the anion is not influenced by the surrounding matrix. 

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). 

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 diphenyl derivative is more stable than the radical anion of its carbon derivative, 9,9-diphenylfluorene, but, after extended periods of reduction, the spectrum of the biphenyl radical anion begins to grow in intensity. The 5,5-dimethyl derivative appears to be stable under these conditions. The enhanced stability of the silicon derivative might be due to stabilization of the carbon-silicon bonds by delocalization of charge into available d-orbitals 81). Methyl proton hyperfine splitting observed for the anion radicals of the 5,5-dimethyl- and 5,5-diethyldibenzosilole has been cited as evidence for d-7r interaction (56). 

ESR spectra of the three cyclic carbenes fluorenylidene, indenyl-idene, and cyclopentadienylidene have been studied.40,42 They all display D values around 0.4 cm-1, while their E parameters and the observed 13C hyperfine splittings indicate that the angles between the bonding orbitals of the divalent carbons are 140-145°. Such a value is obviously too large for a five-membered ring, leading to the suggestion that the bonds may be bent in the sense proposed by Coulson and Moffitt.43... 

The ESR spectrum of 3-CP irradiated and measured at LNT is shown in Figure 2. The spectrum is almost identical to that of the PVC/TP glass with the same hyperfine splitting, intensity ratio, and g-value. This result should not be surprising, since carbon-chlorine cleavage was expected to be the major process in each, and both molecules can be considered as R-CH2-CHCI-CH2-R. 

First, the hyperfine splitting conferred on the ESR spectrum of a 7t-radical by interaction of the electron spin with a proton is proportional to the 7t-spin population p, at the atom bearing the proton (the McConnell relationship).17 Thus, for a proton attached to carbon atom i the hyperfine splitting a(H),-is given by Eq. (1), where QCH is the spin polarization parameter for the C—H bond. 

The hyperfine splitting of naturally abundant carbon-13 atoms in the free valence position has been observed in the spectrum of the fully deuteriated radical. The isotropic value (129 G) is less than that predicted by Pople (152 G) which suggests that the s-character of the orbital of the unpaired electron is less than that in an sp -orbital and that the... 

N hyperfine splitting is 13.00 gauss. The radical ion can be made more stable for calibration by making up the aqueous solution in 0.05M sodium carbonate. 

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