Calculation of EPR Parameters

in this section we illustrate the performance of our methods on selected carbon radicals. In particular, we choose to use the new (Table 6.1) basis set (with 

PBEO and B3LYP functionals) because it seems to be the best choice for a number of properties geometrical parameters, dipole moments, and magnetic properties. To demonstrate the accuracy that can be obtained in A and g tensor calculation, we selected 27 radicals (aliphatic and aromatic), including a and 7i species (Fig. 6.1). The selected molecules are neutral, cationic, anionic doublet, triplet, quartet and localized and conjugated radicals. 

TABLE 6.1 Functions to be Added to 6-31G Set to Obtain the N07D Basis Set for B3LYP and PBEO Functionals 

as case study, we consider the glycine and glycyl radicals (Fig. 6.2) in solution. As mentioned above, the calculation of magnetic tensors needs to take into account the several factors such as the geometries, environmental effects, and dynamical effects (vibrational averaging from intramolecular vibrations and/or solvent librations). We use an integrated computational approach where the molecular 

FIGURE 6.2 Structures of glycine and glycyl radicals. The orientations of the principal axes of the g tensor are also shown. 

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B3LYP/6-31G B3LYP/6-31+G Conformational analysis, electronic structure, calculation of EPR parameters 2001JA6654... 

CALCULATION OF EPR PARAMETERS TABLE 6.7 Hyperfine Coupling Constants (in Gauss) of Glycine Radical 117... 

Arbuznikov AV et al (2002) Validation study of meta-GGA functionals and of a model exchange-correlation potential in draisity functiraial calculations of EPR parameters. Phys... 

Calculations of EPR parameters were also performed on some of the complexes. Experimental EPR spectra are either axial (gx = gy-, axial type 1 copper proteins) or rhombic (other blue copper proteins). The results indicate that the geometry is more important than the electronic structure for the rhom-bicity of the spectrum the optimized trigonal structure of Cu(imidazole)2(SCH3)(S(CH3)2) and the crystal structure of plastocyanin both give an axial spectrum, while both the crystal structure of nitrite reductase and the other optimized model of Cu(imidazole)2(SCH3)(S(CH3)2)" give a rhombic spectrum, although the latter structure is mainly n bonded with... 

The calculation of magnetic parameters such as the hyperfine coupling constants and g-factors for oligonuclear clusters is of fundamental importance as a tool for the evaluation of spectroscopic data from EPR and ENDOR experiments. The hyperfine interaction is experimentally interpreted with the spin Hamiltonian (SH) H = S - A-1, where S is the fictitious, electron spin operator related to the ground state of the cluster, A is the hyperfine tensor, and I is the nuclear spin operator. Consequently, it is... 

The present contribution is devoted to the development and application of an integrated computational approach to the EPR spectra of organic radicals in solution. Using nitrox-ides as test cases we have shown how the magnetic properties are modulated by structural, environmental and dynamical effects. The use of methods able to provide accurate results for all these contributions is thus mandatory for a reliable calculation of magnetic parameters. 

Overall, in the literature systematic explorations of the performance of post-HF methods in the computation of EPR parameters are less well represented than for the NMR case however, as a rule of thumb, trends that characterize the performance of NMR chemical shielding calculations can often be extrapolated to the corresponding g-tensor computations likewise, many observations relative to spin-spin couplings will typically hold for hyperfine coupling constants as well (since both effects are dominated by Fermi contact interactions). 

Schwerdtfeger, P., Pernpointner, M. and Nazarewicz, W. (2004) Calculation of Nuclear Quadrupole Coupling Constants, in Calculation ofNMR and EPR Parameters Theory and Applications, (eds M. Kaupp M. Biihl and V.G. Malkin), Wiley-VCH Verlag GmbH, Weinheim, pp. 279-291. 

Kaupp, M., Buhl, M. and Malkin, Y.G. (eds), (2004). Calculation of NMR and EPR parameters, theory and applications. Part A Introductory Chapters 1-5 Part B NMR Parameters, Methodological Aspects, Chapters 6-20. Wiley-VCH, Weinheim, Germany... 

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