Hyperfme coupling

Hence, a measurement of hyperfme coupling constants provides infonnation on spin densities at certain positions in the molecule and thus renders a map of tlie electronic wavefiinction. 

The simplest system exliibiting a nuclear hyperfme interaction is the hydrogen atom with a coupling constant of 1420 MHz. If different isotopes of the same element exhibit hyperfme couplings, their ratio is detemiined by the ratio of the nuclear g-values. Small deviations from this ratio may occur for the Femii contact interaction, since the electron spin probes the inner stmcture of the nucleus if it is in an s orbital. However, this so-called hyperfme anomaly is usually smaller than 1 %. 

The ESEEM methods are best suited for the measurement of small hyperfme couplings, e.g. for the case of... 

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Figure 1. ESR spectra at some temperatures of X-irradiated polycrystalline frozen solutions (c a 1 mol%) of C5H5 and CgH5D, respectively, a) at 15 K. b) simulated spectrum by using the parameters shown in Table 4. c) 05 150 " " at 30 K in CFCI3. d) simulation using hyperfme coupling data shown in Table 4 for a mixture of component 1 component 2 of C6H5D . The ratio 1 2 = 0.38 0.62 obtained from the component analysis of Figure 2 was employed.

Rao and Symons studied the formation of radicals in y-radiolysis of dilute solutions of dimethyl sulfoxide in fluorotrichloromethane. By ESR studies they found the radical cation (CH3)2SO- whose ESR spectrum show considerable g anisotropy and small methyl proton hyperfme coupling. 

The triaryl radicals of germanium exhibit lower g values than their trialkyl counterparts. In part, this arises from increased delocalization of the unpaired spin density onto the aryl rings (and the Ar3Ge radicals do show hyperfme coupling to the ring protons). For example, spin densities for the radicals PhmMe3 mGe, calculated by the Hiickel method (Table 2), reveal that there is a linear correlation between the g value of the radical and... 

Table 6. Comparison of the Isotropic Hyperfme Coupling Constants for the Flavosemiquinones...

Ring Position Hyperfme coupling constants in Gauss ... 

The Cu11 atoms are well separated and are not coupled magnetically, but absorb intensely (3800 1 mol-1 cm-1 at 635 nm) in the visible region. This charge-transfer absorption is reversibly lost on electrochemical reduction and regained on reoxidation by 02. These properties, together with A hyperfme coupling apparent in the ESR spectrum, are closely related to those of the distorted tetrahedral type-1 Cu site of blue Cu proteins,310 despite the different Cu coordination stereochemistry in the cluster. 

In the case of alkali metals, ion pairing can be visualized as hyperfme couplings from paramagnetic nuclei of the metal cations associated with organic anion radicals in ethereal solvents. In this respect, alkali metal cations associated with the anion radical of o-dimesi-toylbenzene in dimethoxyethane or tetrahydrofurane served as a paradigm (Herold and coworkers 1965). 

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