Time-resolved CIDNP investigation of the cross-relaxation mechanism of H-1 nuclear polarization

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) studies are reported for radical reactions following laser pulse excitation of 2,2,4,4-tetramethylpentan-3-one and 2,4-dihydroxy-2,4-(d)imethylpentan-3-one in various solvents and of acetone in propan-2-ol-d(8). The results show that cross-relaxation does not lead to polarization of the protons of 2-hydroxy-2-propyl and tert-butyl radicals at the 4.7 T field of the CIDNP experiment. Effects previously ascribed to cross-relaxation for the acetone/propan-2-ol system are due to side reactions and a fast solvent dependent nuclear relaxation. In radicals containing hydroxy groups, the CIDNP is strongly influenced by hydrogen bonding, which also affects their OH to CH disproportionation ratios. Auxiliary CIDEP experiments with 2,4-dihydroxy-2,4-dimethylpentan-3-one clearly indicate that the triplet mechanism polarization for this ketone is in absorption.