Character, spin-polarized

As increasing the time interval between the single-shot DNP operation and the aH NMR measurement, the intensity of the magic echo was found to decrease. Instead, the intensity of the 1H Hahn echo grew up, as demonstrated in Figure 19B-D). The fact that the inhomogeneous character in the aH spectra increased with time shows that the spin polarization had been transferred to the place where the H spins were dilute, that is surrounded by the 2H spins in the heavily deuterated host material. 

This electron distribution follows the semiempirical model proposed by Hoffman et al. in the late nineteen-seventies, with some variation because of the spin polarization [51]. In this example one can clearly differentiate these orbitals as having either iron or oxygen character [52] (although a small dZ2 component appears in the third orbital, 7r s, its relative contribution is very small). It follows from the above orbital assignment (A) that the Fe-02 bond can be formally described as FeIH-02. 

This effect is essentially similar to the spin-polarization effects which are responsible for the small s-character nearly always detected for radicals and transition metal ions having electrons primarily in p or d atomic levels. It has been discussed by McConnell et al. (1960) who explain why the spin density on the proton is negative when the major... 

The description of the chemisorption in terms of cycloaddition reactions is useful if it leads to reliable predictions of the reaction products for most reactions, a variety of products are possible, yet only one will result from a particular cycloaddition mechanism. Central to the applicability of such schemes is the notion that the silicon dimers contain a weak 7r bond responsible for the enforced concerted motion of the two electrons involved. However, in reality there is little evidence to support the presence of even a weak 7r bond within the dimers. While DFT calculations that enforce spin pairing depict the bond as a singlet biradical [32], spin-polarized calculations predict a triplet ground state for the unbuckled dimer [33] with no 7T character whatsoever. The decoupling of the two silicon electrons means that their motion is not likely to be concerted so that a [2+2] cycloaddition reaction becomes better represented as an independent [1 + 2+1] process, a notation that recognizes the independence of the silicon free radicals. This mechanism is also illustrated in Fig. 3. In practice such a reaction is unlikely to proceed in a concerted fashion, and a key signature for it would be the... 

The only atomic wave-functions that do not have a node at the nucleus are s-functions. The isotropic coupling constant is thus a measure of the s-character of the wave-function of the unpaired electron at the nucleus in question. The coupling constant for an atomic s-electron can be either measured experimentally or calculated from Hartree-Fock atomic wave-functions so that, to a first approximation, the s-electron density may be calculated from the ratio of the experimental and atomic coupling constants. Should the first-order s-character of the wave-function of the unpaired electron be zero, as for example in the planar methyl radical, then a small isotropic coupling usually arises from second-order spin-polarization effects. The ESR spectra of solutions show only isotropic hyperfine coupling. 

From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption. 

Barbel Fromme and Eberhard Kisker have performed spin-polarized EELS measurements, which allow an assignment of the spin character of all states via the control of spin-polarization in the scattering conditions [44, 45]. The assignment and a spin-polarization measurement have been superimposed on the spectra. The important point here is that there are additional spectroscopic fea-... 

Since only an s orbital has a finite density at the nucleus, the occurrence of hyperfine splitting requires that the unpaired electron is either in a pure a oribtal (as in the hydrogen atom, for which = 507 G) or in an orbital which has some s character. It is therefore not immediately apparent how splitting arises in a radical in which the unpaired electron is in a ir-orbital (e.g. the benzene radical-anion, for which as = 3-75 G). The mechanism by which it does so is described as spin polarization thus, in a fragment in which the unpaired electron is in a p orbital,... 

We carried out non-spin-polarized molecular orbital calculations using a DV-Xa code called SCAT (7). In the DV-Xa method, matrix elements in the secular equation are derived from the weighted sum of integrand values at sampling points. Hence numerical atomic orbitals can be used as basis sets. These atomic orbitals were generated on each iteration in atomic-like potentials derived from spherically averaging the molecular potential around each nucleus (7). The numerical wave functions thus obtained can be used efficiently for the molecular orbitals and have a practical character making them suitable for... 

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