Electron hyperfine coupling

Another mechanism to provide splitting of the S manifold is the hyperfine coupling between the unpaired electron and the metal nucleus. For example, at zero magnetic field an S = Vi / = V2 system gives two sets of levels of degeneracy 3 and 1, separated by A (see Appendix III) where A is the metal-nucleus-unpaired-electron hyperfine coupling. The effect of this splitting is... 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

Compute the isotropic hyperfine coupling constant for each of the atoms in HNCN with the HF, MP2, MP4(SDQ) and QCISD methods, using the D95(d,p) basis set Make sure that the population analysis for each job uses the proper electron density by including the Density=Current keyword in the route section. Also, include the 5D keyword in each job s route sectionfas was done in the original study). 

The electron densities for a spin electrons and for spin electrons are always equal in a singlet spin state, but in non-singlet spin states the densities may be different, giving a resultant spin density. If we evaluate the spin density function at the position of certain nuclei, it gives a value proportional to the isotropic hyperfine coupling constant that can be measured from electron spin resonance experiments. 

In standard Hiickel n -electron theory, the highest occupied orbital has a node through the position of C3 and so we might expect a zero proton hyperfine coupling constant, even after using McConnell s argument. 

The and operators determine the isotropic and anisotropic parts of the hyperfine coupling constant (eq. (10.11)), respectively. The latter contribution averages out for rapidly tumbling molecules (solution or gas phase), and the (isotropic) hyperfine coupling constant is therefore determined by the Fermi-Contact contribution, i.e. the electron density at the nucleus. 

In order to explain qualitatively how CIDNP arises, the simple case of a radical pair in which only one component contains a nucleus (spin J) coupled to an electron (spin through a hyperfine coupling constant will be considered. 

Kasai and McLeod (57, 58) also studied a series of bimetallic diatomics, AgM (M = Mg, Ca, Sr, Be, Zn, Cd, or Hg), by ESR spectroscopy. For all of these species, the hyperfine coupling to the Ag nucleus was found to be isotropic. It was shown that the unpaired electron resides in an orbital resulting essentially from an anti-bonding combination of the valence s orbitals of the Ag and M atoms. A typical spectrum is shown in Fig. 13. 

Hyperfine coupling constants provide a direct experimental measure of the distribution of unpaired spin density in paramagnetic molecules and can serve as a critical benchmark for electronic wave functions [1,2], Conversely, given an accurate theoretical model, one can obtain considerable information on the equilibrium stmcture of a free radical from the computed hyperfine coupling constants and from their dependenee on temperature. In this scenario, proper account of vibrational modulation effects is not less important than the use of a high quality electronic wave function. 

Here this work is continued and extended to C70 where a considerable amount of experimental work is currently in progress. The observation [4] of three electron-muon hyperfine coupling constants in not unexpected since there are five chemically distinct sites for muon to attack. The lower symmetry of C70 makes the molecule much more interesting than Cgo-... 

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