Special Topic 5.3 Chemically induced dynamic nuclear polarization

Special Topic 5.3 Chemically induced dynamic nuclear polarization 

The discovery of CIDNP was marked by the serendipitous observation of emission (negative peaks) in NMR spectra of reacting systems428 429 A personal view of the early investigations of the CIDNP phenomenon has appeared.430 

CIDNP is based on the following principle 431,432 Initially, the radical pair is born in a spin-correlated state. To form a product in the singlet ground state, the electronic spin state of the radical pair must be a singlet state. Importantly, the electron spins interact with the nuclear spin states. ISC from a triplet to a singlet radical pair is favoured, when 

Remarkable effects of nuclear spin on the photochemistry of dibenzyl ketone and its derivatives (see Special Topic 6.11) have been reported and substantial isotopic enrichment of 13C in specific products has been achieved under favourable conditions.434 The influence of magnetic fields and of microscopic reactors (micelles, zeolites, etc.) on these reactions was investigated in detail.435 

The radical cation of naphthalene has been generated in aqueous acetonitrile both by two-photon ionization and via reaction with S04 . Its lifetime in aqueous solution is 25 (is 439 

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The overall multiplicity of geminate radical pairs formed by bond fission is the same as that of the precursor excited state. Remarkably, the multiplicity of the precursor can often be established by NMR spectroscopy of the final products thanks to a phenomenon called chemically induced dynamic nuclear polarization (CIDNP, Special Topic 5.3). 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

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