Nuclear hyperfine coupling with model spectra

Figure 5. Explanation of an S- T0-type CIDNP net effect with vector models (left), resulting schematic population diagram (center), and NMR spectrum (right). The example describes a radical pair with one proton in radical 1, triplet precursor, product of the singlet exit channel, gq > g2, and positive hyperfine coupling constant. For the vector models, a clockwise sense of precession has been chosen, the labels 1 and 2 designate the radical and a> and /i) the nuclear spin state, and the dotted vertical lines in the projections give the amount of singlet character. For further details, see the text.

E3.15 The direct field model introduced at an early stage [23] provides a pictorial explanation for the occurrence of forbidden (Ami =1) hyperfine lines. The nucleus is exposed to the hyperfine ( Ba) and the external (B) fields. The nuclear spin is oriented in the effective fields (B ) which are not parallel when the hyperfine coupling is anisotropic. A closer examination shows that Am/ = 1 lines then occur in the ESR spectrum, (a) How would the effective fields ( Ba) be oriented in the limits B Ba and B Ba (B Ba applies at X-band for nuclei with small gN, e.g. for N hyperfine structure... 

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