Field dependence of CIDNP in photoreactions in plastic crystalline matrix

Photo-CIDNP (chemically induced dynamic nuclear polarization) of three ketones (cycloheptanone, cyclodecanone, and acetone) in cyclohexane-d₁₂ and its dependence on the external magnetic field (0-7 T) is measured in liquid, plastic crystalline and rigid crystalline phases by temperature variation between 185 and 300 K. Spin polarization is seen only in liquid and plastic crystalline phases, but not in the rigid crystal giving evidence that molecular mobility is a requisite for the formation of CIDNP. From the characteristics of the field dependence the mechanisms of polarization are identified and properties of paramagnetic reaction intermediates determined. The factors causing the observed differences in polarization between the liquid and the solid phase are discussed in detail. *** DIRECT SUPPORT *** A04RK026 00003