Hyperfine Structure of ESR Spectra

Since the free electrons are attached to molecules, they will sense not only the presence of the applied external magnetic field but will show magnetic interactions with nuclei of surrounding atoms which have a magnetic moment. These interactions give rise to the splitting that can be used to identify particular radicals. Equation (6.3) is accordingly transformed into 

Here mij is the spin quantum number of the interacting nucleus i, Aj the isotropic part of the magnetic coupling between radical electron and nucleus i, and Bi the anisotropic part which depends on the orientation of the radical within the external magnetic field. 

For the important class of hydrocarbon radicals, B is different from zero only for the a-protons, i.e. for the nuclei of hydrogen atoms directly bound to the radical C atom. In well ordered systems (highly oriented polymers, single crystals) these radicals do have a strongly orientation dependent hyperfine structure. 

---

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

For radicals with magnetic nuclei, the hyperfine structure of ESR spectra is produced by the interaction of the electron magnetic moment with the nuclear spin of those nuclei covered by the molecular orbital of the unpaired electron. This interaction splits further the two spin levels in a magnetic field. The hyperfine coupling is often given by the Hamiltonian HgN ... 

The most important characteristic of ESR spectra is their hyperfine structure. The number, position and width of the lines in the spectra depend on the nature and on the conformation of the radical. The hyperfine structure of the spectra results from interaction of the unpaired electron with neighbouring atoms possessing resultant nuclear spin. When these are only hydrogen atoms... 

The analysis of the experimental results is further facilitated by major advances in computational power and with the rapid development of theoretical methodology1,2. This had allowed more accurate results than before, and thus the theoretical investigation of ET has been significantly pushed ahead, Quantum-chemical calculations of spin densities in radical ions have proved to be important for interpretation of experiment results related to the hyperfine structure of the ESR spectra of the radical ions. 

In the case of those complexes which are paramagnetic, additional information can be obtained from (a) the fine structure and occasionally (b) hyperfine structure of the observed electron-spin-resonance (ESR) spectra. 

Figure 4. N,N -Disubstituted pyrazine cation radicals with assignments for hyperfine structures of the ESR spectra in the Maillard reaction mixtures.

We report an electron spin resonance (ESR) study on a C60 anion and a metal (M) encapsulated in fullerene (C ) (a metallofullerene M C ). The anisotropy components of the g-factor of Cg0 were determined accurately from the analysis of angular-dependent ESR spectra of single crystal Cg0 salt. The evaluation of the g-factor was performed according to the classification of symmetry of the C60 geometry. It was found out from the evaluation that the molecular structure of Cg0 should he distorted to lower symmetry, C2h or C,. The variety of ESR spectra of metallofullerenes of La C s was obtained in terms of a g-factor, a hyperfine coupling constant, and a line width. In the case of the isomer I of La C80 and the isomer II of La C84, an abnormally large line width was measured. The molecular structure with high symmetry would reflect on the specific spin dynamics. 

The ESR method combined with a flow system should be very powerful for studying short lived transient radicals during vinyl polymerization in solutions. As will be made clear later, however, the conditions for the reaction occurring in the flow cell are quite different from the conventional solution polymerization studied during a steady-state process. Nevertheless, the hyperfine structure of the ESR spectra observed by the flow technique, can provide straightforward information on the structure, concentration, reactivity, and even the steric conformation of the transient radicals involved, particularly at the initial stage of the polymerization. 

In the present experiment, we are concerned with the hyperfine structure of the ben-zosemiquinone radical anions. The delocalized unpaired tt electron is of course distributed over the entire molecular frame of six C atoms and two O atoms. With R = H, by symmetry, it is clear that the four protons are all equivalent in the para species hence five hyperfine lines with relative intensities 1 4 6 4 1 are expected in the ESR spectrum of this radical. By contrast, when R is not a proton, the three ring protons are not related by symmetry, and thus each may be expected to possess a different splitting constant. A hyperfine structure pattern of eight unequally spaced lines of equal intensity is expected. The line splittings and relative intensities in ESR spectra thus convey information about the geometric arrangement of the atoms. 

Parallel to the generation of the DR oligomers carbenoid AC oligomers (see Table 2) are formed upon UV-irradiation of the monomer crystals at low temperatures. The high field part of the AC triplet ESR spectra with the lines 1, 2, 3, and 4 is shown in Fig. 12. The unspecified lines are not relevant for our purposes they are attributed to the doubling of the unit cell in the low temperature phase of the TS-6 crystals In contrast to the DR triplets, the AC triplets are in their electronic ground state. This follows from the 1/T Curie law temperature dependence of the ESR intensities. The fine structure splitting and the pattern of the hyperfine structure of the DR and... 

Some phenomena, like the existence of a hyperfine structure of the ESR spectra of free radicals, which is due to coupling with the proton spins, are completely outside the frame of naive n considerations and should be explained in terms of more general theories. However, most physical properties of molecules are not seriously sensitive to correlation effects they can be understood in terms of the MO theory, and, in the particular case when the given molecule is planar, within the frame of the a—n separation. 

The characteristics of ESR spectra for d VO complexes are similar to the ESR spectra for d Cu (where the single-hole formalism is equated with a single unpaired electron) except that each principal axis is split into eight hyperfine lines by the vanadyl nucleus (/ = 7/2). Comparison of g values and hyperfine coupling constants with values for complexes of known structure provides information on the donor atoms coordinated to in an experimental system. The intensity of aquated... 

Visible absorption and ESR spectra of the aqueous solution of the Cu/3 complex are shown together with those of the corresponding monomeric Cu/l-ethylimidazole complex in aqueous solution and in toluene (Figure 1). The visible absorption maximum and the hyperfine structure of the perpendicular ESR spectrum of the Cu/3 solution do not agree with those of the Cu/ethylimidazole complex in aqueous solution, but they are close to or agree with those in toluene. Tiiese results suggest that the Cu complex bound to 3 is situated in a rather hydrophobic environment like that of toluene even in an aqueous medium. 

U of about 2.5 eV was proposed to account for the observed spin-density distribution. The existence of the resolved hyperfine structures of the ESR spectra shows that the motion of the spin is much slower than the timescale of hyperfine coupling, which was attributed to the role of ring motion. 

Compute from this both the distribution of the sum of all the tt electrons, i.e. the distribution iv of the overall electron density over the three carbon atoms in the radical, as weU as the distribution qr of charge over the ions. Calculate the distribution Qr of electron spins in the radical. This quantity is called the spin density g>r- It can be determined for example from the hyperfine structure of electron-spin resonance (ESR) spectra. 

---