Electron paramagnetic resonance hyperfine coupling constants

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

The electron spin resonance (E.S.R.) spectra of a paramagnetic organic molecule, e.g. free radical, radical cation or radical anion, is directly related to its unpaired electron distribution (spin density). In the region of a magnetic nucleus the hyperfine interaction between the magnetic moments of the nucleus and the electron is a function of the spin density. It has been shown that, for an atom N, a direct correlation exists between its observed hyperfine coupling constant, and [pa—pP), the unpaired electron population of its atomic orbitals 1). 

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

The paramagnetic d ion VO + is an excellent probe for electron paramagnetic resonance (EPR) spectroscopy. In combination with proton potentiometry and/or electron absorption spectrometry (UV-Vis), species distribution schemes for VO + in the presence of various ligands have been obtained, comparable to those discussed in Section 2.2.1 for vanadate(V) systems derived on the basis of NMR plus potentiometry. EPR also allows, via the anisotropic hyperfine coupling constant in field direction (A or An,... 

Deuterium quadrupole coupling constants can also be obtained from electron nuclear double resonance (ENDOR).19 30 An observation of the hyperfine structure caused by quadrupole coupling in the electron paramagnetic resonance (EPR) spectrum, as for many lanthanide complexes, has not been reported for deuterium. The determination of nuclear quadrupole coupling constants from Mossbauer spectroscopy is not applicable to the deuterium nucleus. 

Electron Paramagnetic Resonance (EPR) can be used to measure the spin-densities in radicals. It is then assumed that the hyperfine coupling constants for the hydrogen atoms are proportional to the spin-density of the adjacent carbon atom [70]. Measurements on the allyl radical [71] give with such an analysis ratio of —0.282 between the spin-densities of the central and the end carbon atom. The CASSCF value is 0.311. One would suspect that methods that include spin polarization of the a skeleton would give better values. The UHF value is, however, — 0.717. What is the reason for this large difference Let us take a closer look at the CAASCF wave function. It contains three terms ... 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1/2) D = uniaxial zfs E = rhombic zfs g =. g-factor with principal components g, gy, and g ge = free electron g-factor a = hyperfine splitting constant A = hyperfine coupling constant for a given nucleus N (nuclear spin / > 0). 

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method for investigating interrelations between electronic and structural features of systems with unpaired electrons, such as radicals, coordination compounds and paramagnetic sites in solids [107-109]. Along with the hyperfine coupling constants, the electronic g matrix (often called g tensor ) is the fundamental quantity furnished by EPR spectroscopy. Nevertheless, it was only in the past few years that g tensors attracted significant attention of the research community that deals with high-level quantum chemical calculations [86,110-113]. 

Koh, A.K., Miller, D.J. Hyperfine Coupling Constants and Atomic Parameters for Electron Paramagnetic Resonance Data. At. Data Nucl. Data Tables 33 (1985) 235. 

The presence of a paramagnetic centre in a molecule may affect the nuclear relaxation times or the chemical shifts or both. If one of the conditions [19], A or > A, is met, where is the electronic spin relaxation, Te is an electronic spin exchange time, and A the hyperfine coupling constant, then it should be possible to observe contact-shifted resonances. The contact shift in some paramagnetic molecules is related to the hyperfine coupling constant by the following equation ... 

Actually, our mixed discrete-continuum model is not limited to the study of UV-vis spectra, but it has been already successfully employed to model solvent effects on several different spectral properties, such as electron paramagnetic resonance (EPR) hyperfine coupling constants, nuclear magnetic resonance (NMR) chemical shifts, and so on [45, 121]. 

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