Development of the CIDNP-detected Electron Spin Resonance Apparatus for the Aid of Investigation of Radical Pairs

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In most of chemical reactions in solution, the whole fate of the reaction path is governed by the small interaction that is operative within a short lifetime of radical pairs. In this respect one whishes to observe the spectrum of radical pairs and analyze it to fully understand chemical reactions. However, in any conventional spectroscopy, the spectrum due to radical pairs and that due to free radicals cannot be separately observed.The CIDNP-detected ESR spectroscopy has been developed in order to separately obseve the ESR spectrum of radical pairs and that of free radicals. We have further tried to extend this spectroscopy to time-resolved spectroscopy.By this new spectroscopy, a lot of chemical reactions have been investigated. As an example, the photo-induced hydrogen abstraction reaction of dichlorobenzoquinone has been investigated. It has found that the hydroxybenzoquinone radical that exists as a radical pair and that exists as a free radical are different. This amazing conclusion can never be obtained by any other spectroscopic means. Further, the lifetime of radical pairs was determined as 20 ns.