Carbon spin preferences

A number of diradicals (also called biradicals) are known,and the thermodynamic stability of diradicals has been examined. Orbital phase theory has been applied to the development of a theoretical model of localized 1,3-diradicals, and used to predict the substitution effects on the spin preference and S-T gaps, and to design stable localized carbon-centered 1,3-diradicals. When the unpaired electrons of a diradical are widely separated, for example, as in CH2CH2CH2CH2, ... 

Prototypical spin preferences. A. At all separations, r, molecular hydrogen shows a singlet ground state. B. Atomic carbon shows a triplet ground state because of a cancellation of regions of positive and negative overlap of the 2p atomic orbitals. C. Possible spin states for the two-electron system. 

A preferable system is poly(p-fluorostyrene) doped into poly(styrene). Since rotations about the 1,4 phenyl axis do not alter the position of the fluorine, the F spin may be regarded as being at the end of a long "bond" to the backbone carbon. In standard RIS theory, this polymer would be treated with dyad statistical weights to automatically take into account conformations of the vinyl monomer unit which are excluded on steric grounds. We have found it more convenient to retain the monad statistical weight structure employed for the poly(methylene) calculations. The calculations reproduce the experimental unperturbed dimensions quite well when a reasonable set of hard sphere exclusion distances is employed. 

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

Any molecular entity possessing an unpaired electron. The modifier unpaired is preferred over free in this context. The term free radical is to be restricted to those radicals which do not form parts of radical pairs. Further distinctions are often made, either by the nature of the central atom having the unpaired electron (or atom of highest electron spin density) such as a carbon radical (e.g., -CHs) or whether the unpaired electron is in an orbital having more s character (thus, radical molecular entity in a manuscript, the structure should always be written with a superscript dot or, preferably, a center-spaced bullet (e.g., -OH, -CHs, CF). 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Fig. 4a Preferred configuration of electron spins in the a orbital connecting a hydrogen atom to an sp2-hybridized carbon atom bearing unpaired 7t spin density, b Molecular n orbital consisting of two carbon pz orbitals and an H2 group orbital generated by hyperconjugative interaction of an, fp2-hybridized C atom bearing unpaired spin with a CH2—R group...

Consequent upon the orbitals preferred for the unpaired electron, a high spin population occurs at C-4 in each of 1,52, and 53. These are the anion-radicals found to exhibit the least persistence.35 Relative reactivities of the anion-radicals have been estimated by a cyclic voltammetric method as 1 > 1,3,5-triazine anion-radical > 52 > 53 > 22 > 54, which corresponds with qualitative observations on persistence and accords very closely with the ranking order of maximum spin populations at carbon.119 120 Dimerization of the radicals at positions of high spin population at carbon is proved for 1 and 52, although the mechanism is by no means clear for 52 and other modes of reaction can also occur, e.g., proton abstraction from solvent or adventitious water.30,35,121-127... 

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