Unpaired electron spins

The negative sign in equation (b 1.15.26) implies that, unlike the case for electron spins, states with larger magnetic quantum number have smaller energy for g O. In contrast to the g-value in EPR experiments, g is an inlierent property of the nucleus. NMR resonances are not easily detected in paramagnetic systems because of sensitivity problems and increased linewidths caused by the presence of unpaired electron spins. 

ENDOR transitions can be easily understood in temis of a simple system consisting of a single unpaired electron spin (S=2) coupled to a single nuclear spin (1=2). The interactions responsible for the various... 

An excited state in which unpaired electron spins occur. 

This specfmm is dominated by ftmdamenfals, combinations and overtones of fofally symmefric vibrations. The intensify disfribufions among fhese bands are determined by fhe Franck-Condon factors (vibrational overlap integrals) between the state of the molecule and the ground state, Dq, 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 Dq.) The... 

By trapping PX at liquid nitrogen temperature and transferring it to THF at —80° C, the nmr spectmm could be observed (9). It consists of two sharp peaks of equal area at chemical shifts of 5.10 and 6.49 ppm downfield from tetramethylsilane (TMS). The fact that any sharp peaks are observed at all attests to the absence of any significant concentration of unpaired electron spins, such as those that would be contributed by the biradical (11). Furthermore, the chemical shift of the ring protons, 6.49 ppm, is well upheld from the typical aromatic range and more characteristic of an oletinic proton. Thus the olefin stmcture (1) for PX is also supported by nmr. 

It seems to the author that another possible pathway can coexist with the above-mentioned reversible addition of sulfinyl radicals to olefinic double bonds, that is, the reversible addition by the sulfinyl oxygen, since a substantial amount of the unpaired electron spin-density resides on the oxygen atom. 

The values for the atomic saturation magnetization at the absolute zero, ferromagnetic metals iron, cobalt, and nickel are 2.22, 1.71, and 0.61 Bohr magnetons per atom, respectively.9 These numbers are the average numbers of unpaired electron spins in the metals (the approximation of the g factor to 2 found in gyromagnetic experiments shows that the orbital moment is nearly completely quenched, as in complex ions containing the transition elements). 

There are no data available for testing the predicted numbers 0.22 and 1.22 of unpaired electron spins for chromium and manganese, respectively. 

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine couphng constant (ajj), can be observed in organic radicals, and CIDNP 3... 

The real power of ESR spectroscopy for identification of radical structure is based on the interaction of the unpaired electron spin with nuclear spins. This interaction splits the energy levels and often allows determination of the atomic or molecular structure of species containing unpaired electrons. The more complete Hamiltonian is given in Equation (6) for a species containing one unpaired electron, where the summations are over all the nuclei, n, interacting with the electron spin. 

ESR spectroscopy can be transformed into an imaging method, ESRI, for samples containing unpaired electron spins, if the spectra are measured in the presence of magnetic field gradients. In an ESRI experiment the microwave power is absorbed by the unpaired electrons located at point x when the resonance condition, Equation (10), is fulfilled. 

In the absence of an endogenous centre, the specificity of relaxation data can be improved by introducing in known positions paramagnetic probes. The unpaired electron spin enhances the relaxation rate of the surrounding nuclei in a distance-dependent fashion. In this way, it is possible to evaluate specific interaction sites between a marked polymer and the small molecules. 

Tetrachloro-5,6-diphenyl- and -5,6-di-n-propyl calicene were found to yield intensely colored radical anions when interacted with alkali metals (Na, K)291. As ESR-spectroscopic investigation showed an unpaired electron spin is located on three-ring carbons Cs/C6 and their substituents. 

Spintronics [136-138] is a word used to describe transport in a mesoscopic junction in which the transport medium (or the electrodes) contain unpaired electron spins. There are different aspects of spintronics, but the simplest idea is that one can transport spin without necessarily transporting charge. This leads to the idea of a molecular spin transistor, and other spin phenomena such as spin valves and spin gates. 

The muon and 29Si hyperfine parameters provide compelling evidence in support of the BC model. In the simple molecular-orbital model proposed by Cox and Symons (1986) the muon is located at the center of a Si—Si bond near a node in the unpaired electron spin density, which is... 

The resonance frequency is characteristic of each nucleus for a given field strength. Nmr may hence be used to identify different nuclei in a sample. Since the effective field at a nucleus is modified by other nuclei and electrons in its vicinity, frequency shifts depending on the environment are observed. This is known as a chemical shift, and on the basis of this it is possible to map out the molecular environment of each spin in a system and to reconctruct details of the molecular structure. It is in this area that nmr finds its major application in chemistry. Epr is used to study radicals, i.e. molecules with unpaired electron spins. 

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