Transition metal complexes hyperfine coupling

Munzarova, M., Kaupp, M., 1999, A Critical Validation of Density Functional and Coupled Cluster Approaches for the Calculation for EPR Hyperfine Couphng Constants in Transition Metal Complexes , J. Phys. Chem. A, 103, 9966. 

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.)... 

Barone et al.76 studied the transition-metal complex CUC2H2 using different density-functional methods. They found that the experimental hyperfine coupling constants were well reproduced by GGA calculations with fairly large basis sets. But all studies indicate that these constants can be calculated accurately only with great care. 

Electron paramagnetic resonance (EPR) spectroscopy [1-3] is the most selective, best resolved, and a highly sensitive spectroscopy for the characterization of species that contain unpaired electrons. After the first experiments by Zavoisky in 1944 [4] mainly continuous-wave (CW) techniques in the X-band frequency range (9-10 GHz) were developed and applied to organic free radicals, transition metal complexes, and rare earth ions. Many of these applications were related to reaction mechanisms and catalysis, as species with unpaired electrons are inherently unstable and thus reactive. This period culminated in the 1970s, when CW EPR had become a routine technique in these fields. The best resolution for the hyperfine couplings between the unaired electron and nuclei in the vicinity was obtained with CW electron nuclear double resonance (ENDOR) techniques [5]. 

For polyatomic radicals in the gas phase the above Spin-Hamiltonian does not apply and four magnetic hyperfine coupling constants a, b, c, d are needed to describe the interaction between a nuelear and ftie elee-tron spin. These are defined and explained in the introduction to the tables on inorganic radicals. Polyradicals and certain radicals on transition metal complexes have unpaired electrons located on different molecular segments k. Their Spin-Hamiltonian is... 

F. Neese. Metal and ligand hyperfine couplings in transition metal complexes The effect of spin-orbit coupling as studied by coupled perturbed Kohn-Sham theory. /. Chem. Phys., 118(9) (2003) 3939-3948. 

At sufficiently low temperatures, usually below 10 20 K for transition metal complexes with 5 = and below 50 K for free radicals, transverse relaxation of electron spins is dominated by fluctuation of dipolar hyperfine fields from distant protons. These fluctuations are driven by coupling among the protons,... 

In powders, frozen solutions and even single crystals, many of the hyperfine and nuclear quadrupole splittings are typically not resolved in the field-swept EPR spectrum due to inhomogeneous broadening effects. In transition metal complexes, for example, often only flic largest hyperfine coupling from the metal ion is observed. This lack of resolution is mainly due to the transition selection rules, which show fliat the number of EPR lines increases multiplicatively,... 

The correlation patterns are more complex if the nuclear quadrupole, the hyperfine, and the nuclear Zeeman interactions are of the same order of magnitude. This situation is often encountered in X-band HYSCORE spectra of weakly coupled nitrogen nuclei in transition metal complexes. A special case, where the spectrum is considerably simplified, is the so-called exact cancellation condition, where Xs 2 coi. Under this condition, the nuclear frequencies within one of the two ms manifolds correspond to the nuclear quadrupole resonance (NQR) frequencies coq = 2Kt], co = K(3 - t]), and cu+ = K 3 + rj) [43], which are orientation independent. Consequently, correlation peaks involving these frequeneies appear as narrow features in the nuclear frequency spectrum. 

Munzarova ML, Kubacek P, Kaupp M (2000) Mechanisms of EPR hyperfine coupling in transition metal complexes. J Am Chem Soc 122 11900-11913... 

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