Signals, nuclear hyperfine effects

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

Figure 5 Phase of a CIDNP net effect explained with vector models (projections). From left to right, starting state influence of theg-value difference influence of the nuclear spin state through the hyperfine coupling constant resulting population difference of the nuclear spin states in the product resulting CIDNP signal. The labels on the vector models denote the electron spin of radical 1 or 2. Further explanation, see text.

E.2.9 An ESEEM signal occurs in the presence of an anisotropic hyperfine interaction that is of a magnitude comparable to the nuclear Zeeman energy. The schematic ESR spectrum in the left part of the figure below contains allowed and forbidden lines due to a mixing of nuclear states. For an / = Vi nucleus the ESEEM amplitude is proportional to A = sin a/Z cos of/2 = 7o /, where a is the angle between the effective fields acting on the nucleus for ms = /2 and lo and li are the amplitudes of the outer and inner ESR lines. 

The pulse EPR methods discussed here for measuring nuclear transition frequencies can be classified into two categories. The first involves using electron nuclear double resonance (ENDOR) techniques where flie signal arises from the excitation of EPR and NMR transitions by microwave (m.w.) and radiofrequency (r.f) irradiation, respectively. In the second class of experiments, based on flic electron spin echo envelope modulation (ESEEM) effect, flic nuclear transition frequencies are indirectly measured by the creation and detection of electron or nuclear coherences using only m.w. pulses. No r.f irradiation is required. ENDOR and ESEEM spectra often give complementary information. ENDOR experiments are especially suited for measuring nuclear frequencies above approximately 5 MHz, and are often most sensitive when the hyperfine interaction in not very anisotropic. Conversely, anisotropic interactions are required for an ESEEM effect, and the technique can easily measure low nuclear frequencies. 

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