Hyperfine couphng constants

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine couphng constant (ajj), can be observed in organic radicals, and CIDNP 3... 

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. 

The observation that the isomer shift varies between 0.39 mm s 1 at 4 K and 0.54 mm s-1 at 80 or 298 K only, irrespective of the charge n of the complexes (n = 0, 1,2), immediately implies that all complexes contain an octahedral, high-spin ferric ion (d5, S = ). Furthermore, from the applied-field Mossbauer spectra of [Fem(LBuMet)] (.S t = f), [Fem(LBuMet )]+ (5t = 2), and [Fem(LBuMet )]2+ (5t = ) intrinsic isotropic hyperfine couphng constants, AFe/gNpN, of -21.4, -21.0, -20.8 T, respectively, have been established. These values are also typical of high-spin iron(III). 

From CIDNP patterns it is possible to derive magnetic properties of free radicals as magnitudes and signs of hyperfine couphng constants, g factors and nuclear relaxation times. 

Similar to linear and branched alkanes, cycloalkanes also give rise to radical cations in zeolites, spontaneously or upon y-radiolysis. This brief discussion of selected examples is intended only to give a flavor of the work being done. Thus, a 13-line radical cation spectrum (a = 0.17 mT, g = 2.003) obtained upon incorporation of 1-methylcyclohexane, 43, into zeolites [71] was identified as 1,2-dimethylcyclopentene radical cation, 44 + (two sets of protons with hyperfine couphng constants in the ratio of ca 2 1 a = 1.67 mT, 2 CH3 a = 3.42 mT, 4H) [72]. The formation of 44 + was rationalized by protonation of the 3° carbon of 43, followed by loss of H2. Loss of a proton from a rearranged carbocation may generate 44, which is oxidized to 44 + by a Lewis site. 

Table 3.3 Partial parallel hyperfine couphng constants A (/) for various binding groups i, taken from refs 16-18.

Barone also introduces two new basis sets, EPR-II and EPR-III. These are optimized for the calculation of hyperfine couphng constants by density functional methods. EPR-II is a double zeta basis set with a single set of polarization functions and an enhanced s part. EPR-III is a triple zeta set including diffuse functions, double d polarization functions and a single set off functions. 

Type 2. This Cu2+ complex is characteristic of the multi-copper oxidases and is t5q)ified by its lack of a sufficient optical absorption to be observed above that of T37pe 1 Cu2+ and by its larger hyperfine couphng constant. An. It is often referred to as the more normal Cu2+ but this is quite misleading because instead of its physical properties being perturbed by its binding site, it has unusual chemical properties. 

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly... 

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