Heinz D. Roth

Chemically Induced Nuclear Spin Polarization in Photo-Initiated Radical Ion Reactions

Chemically induced nuclear spin polarization was discovered in thermal reactions 1, 2 but its value as a tool in mechanistic photochemistry was soon recognized. The first observation of CIDNP in a photoreaction was reported by Cocivera in 1968;3 since then, more than one hundred publications dealing with a wide range of photochemical systems have appeared. The first five years of photoinitiated CIDNP were reviewed in 1972.4

The electron transfer induced interconversion of cyclobutanes and diolefins

Abstract The photoreactions of quinones with diolefins ( 1 ) or cage compounds ( 2 ) give rise to nuclear spin polarization predominantly for the dilefins. These findings indicate that the cage cations ( 4 ) have shallow energy minima and suffer fast ring opening to the diolefin cations ( 3 ).

Dependence of nuclear spin polarization intensities on the plane of polarization of the exciting light. A caveat

The dependence of nuclear spin polarization effects on the plane of polarization of the exciting light has been evaluated for a wide range of radical (ion) pair reactions. The observed variations are of the same order of magnitude and in the same direction as an artifact expected on the basis of optical principles.

Magnetic field dependence of nuclear spin polarization as a criterion for the triplet‐Overhauser mechanism

The magnetic field dependence of nuclear spin polarization effects is evaluated as a mechanistic criterion for the triplet‐Overhauser mechanism. The relation between CIDNP intensity and magnetic field strength is generally complex since at least three of the four steps of this mechanism are magnetic field dependent. The effects observed during the photoreaction of trifluoroacetophenone with dimethoxybenzene in acetonitrile show a high field maximum, which is compatible with the triplet‐Overhauser mechanism, and two low field maxima, which can be assigned to S–T± mixing in a radical ion pair. These assignments were confirmed by the quencher concentration dependence at the fields that give rise to maximum effects.