Atomic carbon spin preferences

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. 

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). 

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...

This latter interpretation follows a model developed by J.W. Linnett in 1964 (ref. 107) in which the orbital concept is largely ignored in favour of spin correlation which is a consequence of the antisymmetrization of the total wavefunction demanded by the Pauli principle. In such a model, what matters are the most likely relative positions of the electrons. It can be shown that, with an antisymmetric wavefunction, electrons having parallel spins tend to be as far apart as possible around the nucleus of an atom. Let us take the carbon atom as an example. For its excited valence configuration 2s, 2p, the four electrons have preferably parallel spins (extension of Hund s rule to excited configurations) and, among the infinity of spatial arrangements, the most likely ones are those in which the four electrons define the vertices of a tetrahedron centred at the nucleus. In particular, for... 

Morishima et al. (171) measured NMR contact shifts for Ni (2-I-) (acac)2 complexes of piperidine(I), 4-methylpiperidine (II), iV-methyl-piperi-dine(III), 1,4-dimethylpiperidine(IV), and quinuclidene(V). The P carbon atom of the ligand in complexes (III) and (IV) show an attenuation of the contact shift relative to this same carbon in (I) and (II). The authors rationalized this discrepency on the basis of the orientation of the lone pair on nitrogen. For complexes (I) and (II) the lone pair prefers an equitorial position, whereas, in (III) and (IV) it prefers an axial position. For (I) and (II) a zig-zag path of distribution is necessary. Apparently (see data in Table LVII) the zig-zag path is more favorable. For quinuclidene (V) the y carbon shows a downfield shift (positive spin density) (opposite to I-IV). The downfield shift may be accounted for by spin delocalization through space involving the lone-pair electrons. 

Fig. 12.17. Is the proton-proton coupling constant through two bonds (H-C-H), i.e., Juu positive or negative Recall that Jhh >0 (shown in Fig. 12.14a), where the induction mechanism is described. The interaction through two bonds depends on what happens at the central carbon atom are the spins of the two electrons there (one from each bond C-H) parallel or antiparallel Hund s rule suggests they prefer to be parallel This means that the situation with the two proton spins parallel is more energetically favorable, and this means JhH < 0. This rule of thumb may fail when the carbon atom participates in multiple bonds, as in ethylene. For more information, see the section Fwm the Research Fnmt," later in this drapter.

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