A perturbation method is applied to the problem of low-field CIDNP. It is shown that in the intermediate magnetic field region the most important deviation from the high-field formulae is due to a perturbation of the S—T 0 mixing by interaction with the T ± levels of the radical pair. In the case of a symmetrical radical pair this contribution decreases to zero and higher order contributions become dominant. An experimental example is the reversal of low-field CIDNP of ethane formed during the thermal decomposition of acetyl peroxide compared with that of ethane- d 3 during the decomposition of acetylperoxide- d 3. A qualitative rule is given for net CIDNP effects in the intermediate field region.
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