Precursor multiplicity, CIDNP

The CIDNP technique has led to similar conclusions about the precursor multiplicity for methyl benzyl ketone (Blank et al., 1971). With dibenzyl ketone and phenyl a-phenylethyl ketone (Closs, 1971a Muller... 

Flip-flop transitions are bidirectional, so the precursor multiplicity is a key prerequisite, in the same way as for S-To-type CIDNP. 

N-CIDNP was used to investigate photoinduced nitrations of aromatic compoimds ArH with tetranitromethane. The polarizations invariably arise from radical pairs ArH NO, but the experiments reveal different pathways of formation of these pairs. With a substrate such as 1,2-dimethoxybenzene, the precursor multiplicity is triplet and the pairs are produced by a dissociative electron transfer from the aromatic compound to tetranitromethane, which then cleaves into an NO2 radical and C(N02)3. In contrast, the very similar substrate anisole (methoxybenzene) exhibits polarizations indicating a singlet precursor, and the radical pair is thought to be formed by decomposition of a preceding unstable diamagnetic intermediate, most probably a nitro-trinitromethyl adduct. ... 

Photoionization also occurs in the already mentioned photoreactions between frans-anthole and fumarodinitrile. In this case, the electron is immediately scavenged by the solvent to give a monomeric or dimeric anion of acetonitrile, which constitutes the second radical of the pair. The precursor multiplicity is again singlet, and CIDNP experiments with varying laser intensity show the ionization to be a two-photon process. 

Precursor multiplicity and exit channel of the intermediates can be determined from the overall polarization phases. The possibility of obtaining the precursor multiplicity is probably one of the most valuable assets of CIDNP, since no other kind of spectroscopy provides access to this information in such a direct way. Section V.C.2 shows examples of this. 

CIDNP spectroscopy has been employed to unravel the reactions following photoexcitation of PtMe4(bipyridyl) [129], From the polarization, it was inferred that the precursor multiplicity is triplet and that the primary photochemical step is cleavage of a Pt—Me-bond. Escape products are formed by deuterium abstraction from the solvent and geminate products by disproportionation. 

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

The effects are also influenced by the modes of pair formation. Geminate radical pair products and transfer products of radicals escaping the pairs show different CIDNP patterns when pairs are formed from reactions of singlet or triplet state precursors, so that CIDNP can be used to determine the spin multiplicities of pair precursors. 

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). 

As follows from Section 3.1.2, only the To) state is relevant, and molecules bom in one of the other two triplet states can be ignored because they do not lead to any CIDNP effects of the S-Tg-type as this is tacitly assumed, only the multiplicity of the pair is specified in the scheme.) That state is the same as that of the radical-pair precursor, in other words its multiplicity is that of the excited state from which the photoreaction starts. 

One has to bear in mind, however, that the CIDNP effect responds to the concentration difference between singlet and triplet precursors, so the "logical box" captures the prevailing multiplicity. If a variation of the experimental conditions affects the balance between these two entry channels to the radical-pair... 

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