Electron spin vectors radical pairs

Figure 2. Precession of electron spin vectors S/ and S of a radical pair viewed in...

FIGURE 9.1 Vector model for intersystem crossing of radical pairs in high magnetic fields. Left, singlet state centre, superposition state right, state ITq). The labels 1 and 2 denote the electron spins of the two radicals. Further explanation, see text. 

FIGURE 9.2 Vector models (projections) illustrating pair substitution. The labels 1 and 2 denote the electron spin of the first and the second radical of the pairs. The observed proton is contained in the first radical. Its spin state, la) or IP), is displayed at the respective leftmost projection. The radical pairs are bom in the triplet state, and the product is formed from the singlet state c gives the singlet character. First radical pair RPi, positive g-value difference, zero hyperfine coupling constant second radical pair RP2, equal g values, positive hyperfine coupling constant. For the situations without pair substitution, the spin evolutions under the influence of the Zeeman and the hyperfine interaction have been separated for clarity. Further explanation, see text. 

Vector Model. Now the question still remains how nuclei manage to change the spin-multiplicity of a radical pair and why this would depend on their spin state. This is most easily explained in terms of a vector representation for the unpaired electron spins. Let us consider a one proton radical... 

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