Hyperfine coupling isotropic

The energy of an unpaired electron will now not only depend on the interactions of the unpaired electron (Zeeman level) and the nucleus (Nuclear Zeeman levels) with the applied external magnetic field, but also on the interaction between the unpaired electron and the magnetic nuclei. To explain how one derives the energy terms for such a system, a simple two-spin system (S = 1/2, I = 1/2) will be considered. The simplified spin Hamiltonian for this two-spin system (S = 1/2, 

For simplicity, the electron and nuclear Zeeman energy terms can be expressed in frequency units giving  

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Determination of relative signs of isotropic hyperfine coupling constants J. Chem. Rhys. 63 3515-22... 

Gaussian computes isotropic hyperfine coupling constants as part of the population analysis, given in the section labeled "Fermi contact analysis the values are in atomic-units. It is necessary to convert these values to other units in order to compare with experiment we will be converting from atomic units to MHz, using the following expressions ri6ltYg ... 

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 the following, all isotropic hyperfine coupling constants were calculated using the BLYP functional and the EPR-II basis set. A full geometry optimization was done in all cases. 

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. 

As can be seen from Table 2, the agreement between measured and calculated isotropic hyperfine coupling parameters is good, confirming previous interpretations [9] of ESR data. 

Isotropic Hyperfine coupling constants an are related to the spin densities p(rN) at the corresponding nuclei by... 

The EPR spectra of electrolytically produced anion radicals of Q -aminoanthraquin-ones were measured in DME and DMSO. The isotropic hyperfine coupling constants were assigned by comparison with the EPS spectra of dihydroxy-substituted antraquinones and molecular-orbital calculations. Isomerically pure phenylcarbene anion (PhCH ) has been generated in the gas phase by dissociative electron ionization of phenyldiazirine. PhCH has strong base and nucleophilic character. It abstracts an S atom from and OCS, an N atom from N2O, and an H atom from... 

Fig. 4. Effect of (A) axial zero field splitting for the spin systems S = 1,3/2,2, and 5/2 (with Bo applied along the z direction of the ZFS tensor), and (B) isotropic hyperfine coupling with the metal nucleus for systems with I = 1/2, S = 1/2 and I = 3/2, S = 1/2.

The thymine anion is only a weak base = 6.9) [22]. This means that protonation of the anion may depend on the specific environment. The primary reduction product observed in the solid state in thymine derivatives is the C4-OH protonated anion [17]. This species exhibits significant spin density at C6 and 04. Here one must distinguish between two different situations. In single crystals of thymidine, the C4-OHp proton is out of the molecular plane which gives rise to an additional 33.1-MHz isotropic hyperfine coupling [31]. A similar situation is observed in single crystals of anhydrous thymine [32]. In 1-meThy, however, the C4—OHp proton is in the molecular plane. Consequently, the proton coupling is very small. 

The adenine radical cation was observed in a single crystal of adenine hydrochloride hemihydrate [43]. In this crystal, the adenine is protonated at Nl. After electron loss, the molecule deprotonates at Nl, giving Ade(Nl -l-H, Nl-H). This produces a radical that is structurally equivalent to the cation of the neutral adenine molecule with spin density on C8 and N6 [p(C8) = 0.17 and p(N6) = 0.25]. The adenine radical cation is strongly acidic (pi a< 1) [22]. This strong driving force makes the reaction independent of environmental conditions. In single crystals of adenosine [42] and anhydrous deoxyadenosine [44], the N6 deprotonated cation [Ade(N6-H) ] is observed which is characterized by p(C8) = 0.16 and p(N6) = 0.42. The experimental isotropic hyperfine couplings are N6-H = 33.9 MHz and C8-H = 12.4 MHz. 

This property of the — SiMes group has also been quite clearly demonstrated in an extremely elegant manner by Bedford et al. (77). It has been amply demonstrated that in an electron spin resonance spectrum the isotropic hyperfine coupling constant, an, of a hydrogen atom attached to an sp2 hybridised carbon atom having an unpaired electron in the 2p—orbital is given approximately by an Equation (3) due to McConnel (18)... 

Cramer, C. J. 1991. Dependence of Isotropic Hyperfine Coupling in the Fluoromethyl Radical Series on Inversion Angle J. Org. Chem., 56, 5229. 

Table 1. Isotropic hyperfine coupling constants of flavin semiquinones...

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