Photochemical Magnetic-Field Effects of Isoquinoline N-oxide in Various Alcohols

Article abstract of DOI:10.1246/bcsj.59.2723

A pronounced solvent dependence of the magnetic-field effect due to the HFI-J mechanism was found for the first time in the case of a photochemical isomerization of isoquinoline N-oxide into lactam (1-isoquinolone).The chemical yield of the lactam showed a minimum at a higher magnetic field as one changes the reaction medium from t-butyl alcohol to 2-propanol (or ethanol) and then to methanol, which was the same order as the pK_a value (t-butyl alcohol>2-propanol ca. ethanol>methanol).Such a solvent dependence provides corroborating evidence that the photoisomerization proceeds via the S₁-born singlet radical-ionpair intermediate, which is linked by a hydrogen bond between the N->O group in the cation radical of N-oxide and the OH group in the anion radical of alcohol.Another interesting finding was that, in the case of the photochemical isomerization in t-butyl alcohol, the chemical yield of the lactam showed distinct maxima at approximately 0.9T and 1.35T in addition to a minimum at about 0.72T.The anomalous magnetic-field effect in a high field region (>0.8T) could reasonably be explained in terms of an HFI-J mechanism by assuming the T₁-born triplet hydrogen-bonded radical-ion pair to have an electron-exchange interaction larger than that in the S₁-born singlet hydrogen-bonded radical-ion pair.