Unpaired electron spin distribution

By far the most NMR studies have been concerned with the amine adducts of Ni(R-dtp)2 complexes. The green complexes formed with primary (mono-and bidentate) and heterocyclic (mono- und bidentate) amines and the yellow-brown five-co-ordinate adducts with secondly amines exhibit paramagnetic shifts in their NMR spectra. The use of paramagnetic NMR shifts in mapping unpaired electron spin distributions of paramagnetic complexes has been reviewed 378-381) jhe upfield paramagnetic shifts of NH protons are diagnostic... 

Figure 3 presents the NMRD curves of Pu " " (5/ , Hs/2) and Np (5/, %/2)-These two ions have non-spherically symmetric distribution of their unpaired electronic spins. Also included is the NMRD curve of Pr which is the lanthanide analog of Pu " " (12). All the ions are very poor relaxation agents... 

Unlike the lanthanides, the actinides U, Np, Pu, and Am have a tendency to form linear actinyl dioxo cations with formula MeO and/or Me02. All these ions are paramagnetic except UO and they all have a non-spherical distribution of their unpaired electronic spins. Hence their electronic relaxation rates are expected to be very fast and their relaxivities, quite low. However, two ions, namely NpO and PuOl", stand out because of their unusual relaxation properties. This chapter will be essentially devoted to these ions that are both 5/. Some comments will be included later about UOi (5/°) and NpOi (5/ ). One should note here that there is some confusion in the literature about the nomenclature of the actinyl cations. The yl ending of plutonyl is often used indiscriminately for PuO and PuOl and the name neptunyl is applied to both NpO and NpOi. For instance, SciFinder Scholar" makes no difference between yl compounds in different oxidation states. Here, the names neptunyl and plutonyl designate two ions of the same 5f electronic structure but of different electric charge and... 

For closed-shell molecules (in which all electrons are paired), the spin density is zero everywhere. For open-shell molecules (in which one or more electrons are unpaired), the spin density indicates the distribution of unpaired electrons. Spin density is an obvious indicator of reactivity of radicals (in which there is a single unpaired electron). Bonds will be made to centers for which the spin density is greatest. For example, the spin density isosurface for allyl radical suggests that reaction will occur on one of the terminal carbons and not on the central carbon. 

Electron spin resonance (esr) 10 2 to 1 Excitation of unpaired electron-spin orientations in a magnetic field Electron distribution in radicals, electron-transfer reactions (Section 27-9)... 

Other molecular properties and phenomena that can benefit from the aid of visualization are the distribution of unpaired electron spin in radicals and the changes in orbitals and charge distribution as a reaction progresses. These and many other... 

Here /, is the 13C nuclear spin, S is the unpaired electronic spin, and A j- is the Fermi contact hyperfine coupling tensor. This coupling is identical for all 13C nuclei as long as the C60 ion is spherical, but becomes different for different nuclei after the Jahn-Teller distortion leading to an inhomogeneous frequency distribution. The homogeneous width of the 13C NMR lines is, on the other hand, mainly determined by the electron-nuclear dipolar interaction... 

This spectrum is dominated by fundamentals, combinations and overtones of totally symmetric vibrations. The intensity distributions among these bands are determined by the Franck Condon factors (vibrational overlap integrals) between the Sl state of the molecule and the ground state, D0, of the ion. (The ground state of the ion has one unpaired electron spin and is, therefore, a doublet state, D, and the lowest doublet state is labelled D0.) The... 

If we confine attention to molecules in a 2S11 n electronic state, there are four magnetic hyperfine parameters, a,b ,c and d. The first of these describes the strength of the nuclear-spin/electron-orbital interaction and gives information on the spatial distribution of the unpaired electrons. The other three parameters give information on the electron spin distribution within the molecule. Though often similar, these two distribution functions are not identical. 

Trivalent gadolinium with f7 configuration has isotropic distribution of electrons and hence cannot produce pseudo contact shift. However, when the Lewis acid-base interaction is partly covalent, the unpaired electron spin density influences the molecular framework of the base and causes an LIS known as contact shift. Gd(III) is used to ascertain the contributions of contact shift to the measured LIS. 

The overall picture describing the electron delocalization in Cua resembles that found in Type I sites most of the delocalized unpaired spin density is found on the Cys ligands. The electron spin density on each P-CH2 Cys proton is about half of that observed on the equivalent protons in blue copper proteins (Bertini etal., 1996, 1999). The unpaired electron is distributed over the two copper ions and the two Cys ligands. The observed values for the hyperfine shifts are consistent with the fact that the hyperfine couplings found in the P-CH2 Cys protons in Type I sites are twice as large as those observed in Cua centers. However, as already discussed, the Cu(H)-Cys covalency in Type I sites can be severely altered by the strength of the Cu(H)-axial ligand interaction. This... 

Even in the meta-phenylenevinylenes 22 the neighboring units can be charged to the biradical form. This is in line with the observation of spin localization in the monoradical, where the unpaired electron is distributed in one stilbene unit only. Upon further charging of e.g. 22b the biradical formation is obtained [44, 434]. 

Figure 15 Comparison of the unpaired electron density distribution of the protona ted intermediate B of (a) GOase and (b) the biomimetic compound (contour at 0.008 e/au ). Yellow and magenta refer to a- and /3-spin densities, respectively. (Reproduced with permission from ref. (1591, Copyright 2000 Springer.)...

Preliminary results for Li salts are shown in Table XII.It is seen that for the neutral radical TTBP the Li enhancement is negative, whereas for the radical anion DBSQ it is positive. Both these radicals have a similar electronic distribution, so the different effects of the radicals may be due to the stronger interaction between the positively charged lithium ion and the negative DBSQ ion in forming a transient ion pair, and thus allowing a transfer of some unpaired electron spin density. Although for WBPC the unpaired electron density resides on... 

Electron Delocalization in Cu-Proteins. Fernandez and Vila have prepared a review. Hansen and Led have used proton Ri s to map electron spin delocalization in a type I Cu protein (oxidized plastocyanin from Anabaena variabilis). Their method relies on the dependence of Ri on the spatial distribution of the unpaired electron spin. 

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