Paramagnetic additives

C20-0057. An aqueous solution of Cr " (a g) is paramagnetic. Addition of sodium cyanide makes the... 

Effects of temperature and paramagnetic additives on the spectrum of poly(dichlorophenyleneoxide). The spectra of the polymers referred to in Figure 1 were all measured up to 120° in tetrachloroethylene. In no case did any significant change occur in the spectrum on heating. 

In the case of those complexes which are paramagnetic, additional information can be obtained from (a) the fine structure and occasionally (b) hyperfine structure of the observed electron-spin-resonance (ESR) spectra. 

The pseudocontact term Afipo,ar is due to the valence electrons of the shift reagent SR they cause an additional intramolecular field in the adduct S + SR, shielding or deshielding the nucleus i in the molecule S. The Fermi contact term Afon ac accounts for interaction between the nucleus i and the field of the unpaired electrons of the paramagnetic additive SR which may be delocalized within the adduct S + SR [103]. 

The above data show that the spin labelling technique indeed allows one to control the random character and the uniformity of the spatial distribution of the additives in vitreous matrices. The criterion of random character and uniformity of distribution is the linear dependence of the EPR line width on the concentration of paramagnetic additives and the coincidence of the experimentally measured coefficients A in eqn. (2) with the theoretical value of A for such distribution. 

In contrast to the absence of heavy-atom effects on both the formation and radiative decay of the triplet state, the ISC process can instead be assisted by molecular triplet oxygen [46,156,157]. The same mechanism is presumed to operate for other paramagnetic additives, e.g., the stable free radical TEMPO as quencher (see discussion of quenching rate constants below). 

There are some unwanted effects of paramagnetic additives. They may wash out spin-spin coupling, either by too fast relaxation (for small /), or by promoting decoupling by fast exchange. The magnitudes of coupling constants may be altered. 

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