Signals, nuclear hyperfine

Table II. Nuclear Hyperfine Effects" on EPR Signals Elicited under Carbon Monoxide...

The ESR signal is highly dependent on the nature of the local environment about the absorbing electron that is, the position of the ESR signal and the overall ESR spectral pattern depend on the environment conditions in the vicinity of the electron. The most important types of interactions in the spin system that affect the position and pattern of the ESR spectrum are the electron Zeeman, nuclear hyperfine, and ligand superhyperfine interactions (Wertz and Bolton, 1972). 

When there is unpaired electron density produced at the nucleus being studied, a so-called scalar interaction between the electron and the nucleus can occur. This unpaired electron density is transmitted from the free radical to the nucleus by a similar mechanism to that giving rise to the nuclear hyperfine structure in normal e.s.r. spectra. Since the unpaired electron and the nucleus are usually in different molecules, the rapid molecular motion means that the scalar interaction, which of necessity can only occur when the unpaired electron and the nucleus are close to one another, will be varying rapidly. This rapid switching on and off of the scalar interaction means that although any intermolecular nuclear hyperfine structure in the e.s.r. signal is averaged to zero, the scalar interactions may now provide an efficient... 

Figure 5.12 shows the J= — 0 transition of the linear molecule cyanodiacetylene (H—C=C—C=C—C=N) observed in emission in Sagittarius B2 (Figure 5.4 shows part of the absorption spectrum in the laboratory). The three hyperfine components into which the transition is split are due to interaction between the rotational angular momentum and the nuclear spin of the nucleus for which 1= 1 (see Table 1.3). The vertical scale is a measure of the change of the temperature of the antenna due to the received signal. 

Mononuclear gold(II) complexes, consistent with a d9 configuration, must be paramagnetic (Meff=1.79 MB) and show a hyperfine four-line EPR signal, in accordance with the nuclear spin of 197Au (1 = 3/2). These two properties are evidence of a real gold(II) complex in addition to their stoichiometry. 

Electron paramagnetic resonance (EPR) yields the location of unpaired electron density from hyperfine splitting by metals or atoms with nuclear spin.21 The S = 0 Fe(III)—O 2 state of oxy-Mb or Hb would be indicated by the absence of an EPR signal, although other results such as the IR or resonance Raman absorption of the O2 moiety would be needed for positive confirmation. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

In contrast to the narrow signal of H(I), which exhibits a single Lorentzian profile, the feature observed for H(IV) and H(V) was more complex (Fig. 23 lb) with splitting into five lines indicating a superhyperfine structure (line denoted A in Fig. 23 2). This phenomenon is due to the hyperfine interaction between localized spin and two neighboring nitrogen nuclei (nuclear spin... 

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