摘要
The enhancement factors of the 1H polarization induced in radical pairs generated by α-cleavage of 12C[DOUBLE BOND]O- and C[DOUBLE BOND]O-labelled ketones deviate from the ratios expected on the basis of the relative abundances of the respective carbon isotopomers. For dibenzyl ketone, the ratio of absolute CIDNP intensities of the 1H(13C) doublet (90% 13C[DOUBLE BOND]O) to the 1H(12C) singlet (10% 12C[DOUBLE BOND]O) was found to be >9:1 at 58·3 kG and <9:1 at 21 kG. Similar deviations were observed for other 13C labelled ketones, viz. deoxybenzoin (13C[DOUBLE BOND]O) and acetophenone (13C[DOUBLE BOND]O or 13CH3). This novel enhancement or suppression effect on the CIDNP enhancement can be ascribed to the presence of a second hyperfine coupled nucleus in the intermediate radical pair. The individual contributions of the g factor difference (Δg), the external magnetic field strength (Ho), and the hfc constants of the observed (aA) and the interacting nucleus (ax) in the enhancement-suppression are exemplified. The experimental results are consistent with theoretical calculations of the mutual effect of Δg, H0, aA and ax on the CIDNP enhancement factor of the observed nucleus.