Intermediates diamagnetic

Figures 17, 18, and 19 show the different catalytic cycles derived from the foregoing approach. The chain-carrying species of the cycles in Figs. 17 and 18 is an alkyl radical, whereas in the cycles of Fig. 19 it is a Ni(I) or an allylNi(I) complex. We may note that, for this example where both paramagnetic and diamagnetic intermediates were considered, TAMREAC did not find more solutions than Hegedus and Thompson did (127b).

The achievements of mechanistic studies employing spin chemistry methods convincingly demonstrate the potential of these techniques in revealing the detailed reaction mechanisms of a number of homolytic processes involving organic Ge, Sn and Pb compounds. The role of short-lived paramagnetic (and sometimes diamagnetic) intermediates, such as free... 

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. ... 

Paddlewheel tetra(carboxylato)di-iron(II) complexes [Fe2(//-02Cdxl)4L2] (22a)-(24a) react with dioxygen at —80°C to yield deep red-brown diamagnetic intermediates (Amax = 500-550 nm e = 1,000-1,200 M cm ). 33 structural information obtained by resonance Raman and Mossbauer spectroscopy of (23b) (Table 2), derived from (23a), suggested an unusual Fe —X—Fe (r/ -02) coordination mode as shown in Scheme 15. 

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