Sign of spin density

Alternating sign of spin density proven experimentally for this and the methylated allyl radicals. ... 

Oxidation of the porphyrin 7r-system of iron(IH) (45) and iron(IV) tet-raarylporphyrins (8) leads to large chemical shifts for the meso-aryl protons. This is a result of spin density on the raeso-carbon, which is delocalized into the aryl rings via a 7r-spin delocalization mechanism (50). Accordingly, the chemical shifts of ortho-, meta-, and para-protons have alternating signs. Coupling of the porphyrin spin to the metal spin in both a ferro- and antiferromagnetic manner has been demonstrated (45). 

Subsequently, the H NMR of 1,1, 3,3, 5,5, 7,7 -octamethyl-uranocene was analyzed in a similar manner (6). The contact shifts for the ring and ot-protons were found to be similar in magnitude, but opposite in sign, implying spin density in a 7T-MO, and transfer of spin density via a spin polarization type mechanism (entries 3 and 4 in Table II). In this paper, a new, significantly smaller, value for the pseudocontact shift in uranocene... 

Application of the amplitude sweep depicted in Fig. 31a (left trace) to a coupled spin pair will therefore lead to a change of sign of the density operator during the course of the experiment (p(r) = —p(0), see above), while the final density operator after the sweep shown in Fig. 31a (right trace) will be identical to the initial density operator (p(r) = p(0)). It follows that subtraction of the first experiment from the second will result in a 2p(0) difference for a pair of coupled spins. The effective Hamiltonian for uncoupled spins however, conunutes at all times with the initial density operator p(0), so... 

McLachlan calculation of spin densities including perturbation of all a-electrons. ) Sign determined by CIDNP [73Barl],... 

The CIDNP spectrum is shown in figure B 1.16.1 from the introduction, top trace, while a dark spectrum is shown for comparison in figure B 1.16.1 bottom trace. Because the sign and magnitude of the hyperfine coupling constant can be a measure of the spin density on a carbon, Roth et aJ [10] were able to use the... 

Active Figure 10.4 The spin density surface of the allyl radical locates the position of the unpaired electron (blue) and shows that it is equally shared between the two terminal carbons. Sign in at www. thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

For aromatic hydrocarbon radical anions, this approach works pretty well. Figure 2.7 shows a correlation plot of observed hyperfine splitting versus the spin density calculated from Hiickel MO theory. It also correctly predicts the negative sign of aH for protons attached to n systems. 

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