Solution equilibria of adducts

The use of lanthanide shift reagents as structural probes depends firstly on a knowledge of the stoichiometry and equilibria of interaction between the LSR and the substrate, and secondly on being able to separate the dipolar contribution from the total averaged isotropic shift. This latter aspect has already been considered. The accurate study of the binding modes of substrates to LSR is often not straightforward. Early 

Evans and Wyatt (481) have suggested an energy profile [Fig. 8(b)] for substrate exchange. The species RS2 is considered to be an unstable isomer of RS2 in which the configuration of the /1-diketone ligands is similar to that in the 7-coordinate RS. Other equilibrium studies involving Ln(dpm)3 have been reported. (482-485) 

Eu(N03)3 has been shown to form essentially 1 1 adducts with L-azetidine-2-carboxylic acid (480) and Eu(fod)3 forms similar adducts with dialkylnitrosamines. (487) 1 1 Stoichiometry between Eu(fod)3 and various cyclic 6-membered sulphites has been assumed. (488) Both 1 1 and 1 2 adducts have been reported for Eu(fod)3 bound to cyclohexanones and cyclohexanols, (489) n-hexylamine, (490) hexamethylphosphoramide, (491,492) and methyl dimethylcarbamate. (493) 

(a) Plot of log,0 1 against the 8-coordinate ionic radius of the lanthanide ion for Lnffod-dg) complexes. Ic, is the first-order rate constant for substrate exchange HMP = 1,2-hexamethylphosphoramide TMU = tetramethylurea. (b) Suggested energy profile for substrate exchange. (481) 

In some of these studies allowance has been made for the selfassociation of the LSR. Thus in addition to considering the two equilibria [equations (48)] other equilibria such as  

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