Nuclear hyperfine coupling

Nuclear hyperfine coupling results in a multi-line ESR spectrum, analogous to the spin-spin coupling multiplets of NMR spectra. ESR spectra are simpler to understand than NMR spectra in that second-order effects normally do not alter the intensities of components on the other hand, ESR multiplets can be much more complex when the electron interacts with several high-spin nuclei, and, as we will see in Chapter 3, there can also be considerable variation in line width within a spectrum. 

As we will see in Chapter 4, g-matrices are often difficult to interpret reliably. The interpretation of isotropic g-values is even less useful and subject to misinterpretation. Thus isotropic ESR spectra should be used to characterize a radical by means of the hyperfine coupling pattern, to study its dynamical properties through line width effects, or to measure its concentration by integration of the spectrum and comparison with an appropriate standard but considerable caution should be exercised in interpreting the g-value or nuclear hyperfine coupling constants. 

The unpaired electron in these two molecules is now in exactly the right orbital for the present purposes, since d —Px bonding is usually invoked in order to stabilise such antibonding or non-bonding orbitals. As we have seen both the nuclear quadrupole coupling constant and the anisotropic electron-nuclear hyperfine coupling constant are dominated... 

It has been shown that triphenyl(p-cyanobenzyl)phosphonium tetrafluoroborate (16), which exhibits a a LUMO level localized predominantly on the heteroatom and benzylic carbon, gives products derived from out-of-solvent cage chemical reactions on direct irradiation (reaction 6). This behaviour is connected with the nuclear hyperfine coupling constant of the heteroatom in triphenylphosphine radical-cation171. 

The most important examples of 2S states to be described in this book are CO+, where there is no nuclear hyperfine coupling in the main isotopomer, CN, which has 14N hyperfine interaction, and the Hj ion. A number of different 3E states are described, with and without hyperfine coupling. A particularly important and interesting example is N2 in its A 3ZU excited state, studied by De Santis, Lurio, Miller and Freund [19] using molecular beam magnetic resonance. The details are described in chapter 8 the only aspect to be mentioned here is that in a homonuclear molecule like N2, the individual nuclear spins (1 = 1 for 14N) are coupled to form a total spin, It, which in this case takes the values 2, 1 and 0. The hyperfine Hamiltonian terms are then written in terms of the appropriate value of h As we have already mentioned, the presence of one or more quadrupolar nuclei will give rise to electric quadrupole hyperfine interaction the theory is essentially the same as that already presented for1 + states. 

Ammonium, phosphonium and arsonium salts may also possess a a LUMO and undergo PET bond cleavage reactions to provide products that depend largely upon the heteroatom nuclear hyperfine coupling constant [93]. 

Ammonium and phosphonium salts with relatively large nuclear hyperfine coupling constants for N and P provide only out-of-cage products, i.e., bibenzyl. While an arsonium salt, on the other hand, provides mainly in-cage radical coupling, i.e., rearrangement, due to a smaller nuclear hyperfine coupling constant... 

For those familiar with NMR spectroscopy it may be helpful to realize that the ESR g-shift is comparable with the NMR chemical shift. Similarly, electron-nuclear hyperfine coupling can be compared with nuclear-nuclear spin-spin coupling in NMR. (In systems containing more than one unpaired electron per molecule, electron spin-electron spin coupling is, of course, important. For doublet-state radicals, this coupling does not arise it is of great importance in triplet state molecules and in many high-spin transition metal complexes.)... 

Atomic Parameters and Nuclear Hyperfine Coupling Constants... 

Group 3 (Sc, Y, La) metallofullerenes exhibit ESR hfs, which provides us with important information on the electronic structures of the metallofullerenes. Typical ESR-active monometallofullerenes are La Cs2, Y Cs2, and Sc C82- The ESR hfs of a metallofullerene was first observed in La Cs2 by the IBM Almaden group (Johnson et al., 1992) (Eigure 15) and was discussed within the framework of an intrafullerene electron transfer. The observation of eight equally spaced lines provides evidence of isotropic electron-nuclear hyperfine coupling (hfc) to La with a nuclear spin quantum number I = 7/2. The observed electron g-value of 2.0010, close to that measured for the Ceo radical anion (Allemand et al., 1991 Krusic et al., 1991), indicates that a single unpaired electron resides in the LUMO of the carbon cage. They also observed hyperfine... 

Marks and co-workers (12) have studied the alkyl substituted compounds 7-16. Assuming that INDO/2 molecular orbital calculations on alkyl radicals can reasonably predict experimental electron-nuclear hyperfine coupling constants, a, they have calculated the a values for each of the alkyl substituents. Taking the ratio of the contact shifts of the ortho positions in 7 and vinylic position in 16 as equal to the ratio of calculated a values and the ratio of the geometry factors as equal to the ratio of pseudocontact shifts, Marks and co-workers could solve for the contact and pseudocontact shifts in 7 and 16. Factoring the... 

If a given nuclear hyperfine coupling tensor on analysis shows that there is a large atomic s-character in the orbital, then this atom is probably a central atom in a a-radical. 

The nuclear hyperfine coupling is the field at the electron due to the nucleus and its magnitude depends on whether the electron which is interacting with the nucleus is in an s or a p or orbital. Since s orbitals have high electron density at the nucleus, the hyperfine coupling constant will be large and since s orbitals are symmetrical, it will be independent of direction. This interaction is called the isotropic hyperfine coupling (Aijo) or the Fermi contact interaction. In p or d orbitals, where there... 

The possible contribution of electron-nuclear hyperfine coupling (HFC) as a mechanism for intersystem crossing between TT encounter pairs of different multiplicity was strongly suggested by recent resonance-Raman-spectroscopic determinations of k2 = 5.3 X 10 Af s and 2.1 X 10 s (Eq. 67)... 

Combines sensitivity of EPR and high resolution of NMR to probe in detail ligand superhyperfine interactions with metal center and to identify specific type of ligand Complementary technique to ENDOR for measuring very small electron-nuclear hyperfine couplings... 

In this type of ENDOR, the amplitude of the field modulation (ABn,od) defines a window in the broad EPR absorption envelope, in which a set of electron spin packets associated with a given set of nuclear orientations resonates. When the rf field induces a nuclear spin transition, the resonant field of the spin packets is shifted by an amount equal to the nuclear hyperfine coupling. A". If A" s, this rf-induced shift of spin packets... 

The most important examples of states to be described in this book are CO+, where there is no nuclear hyperfine coupling in the main isotopomer, CN, which has... 

Table 10.11. Nuclear hyperfine coupling constants (in MHz) for PF and related free radicals [37]...

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