absent. The linear nature of this relationship predicts that Gd-(NOTA) should be less stable than Gd(EDTA)".DOTA is a octadentate chelate like DTPA. Although DOTA has one less oxygen donor atom than DTPA, the Ln(DOTA)" complexes are all more stable than the corresponding Ln(DTPA)2" complexes. This is precisely what one would predict from the £pAia values for the two chelates. The log K value predicted for Gd(DOTA)" from the correlation shown in Figure 2 is within 5% of our measured value. Loncin et al.22 have recently reported log K values for Eu(DOTA)" and Tb(DOTA)" (28.2 and 28.6, respectively) as determined by a gravimetric technique. Our measurements indicate that these values are about 3 orders of magnitude too high.The solid-state structure of Eu(DOTA)" indicates that the chelate is strained, resulting in variable Eu-N bond lengths.23 This suggests that the Eu3+ cation is too large to fit into the lowest energy [3333] square conformation of the tetraazadodecane ring, and hence the complex is slightly distorted. A NMR study13 has shown that La(DOTA)", Pr(DOTA)", and Eu(DOTA)" have similar free energies of activation for ethylenediamine proton interconversion between two staggered conformations. Our results indicate that these three ions are too large to satisfy the stereochemical requirements of the tetraazadodecane ring and, although these complexes are quite rigid on the NMR time scale at room temperature, they are considerably more flexible than the complexes formed by the heavier cations,13 i.e., Yb(DOTA)" and Lu(DOTA)". The sharp increase in log K we observe between Eu(DOTA)" and Gd(DOTA)" may reflect a more precise cation size requirement of the chelate. The nearly constant log K values found for Gd3+-Tm3+ complexes with DOTA indicate that ion size is not a critical determinant of stability once a cation of favorable size, i.e., Gd3+, has been presented to the chelate. It is not until the charge denstiy on the cation greatly increases does the stability of the complex begin to increase again, as in the case of Yb(DOTA)" and Lu(DOTA)".
A comparative kinetical study of flames burning in mixtures of nitrousoxide and different hydrocarbons has been made. It has been found that the mean molecular weight of the chain carriers changes with the nature of the fuel. A probable reaction mechanism requests the formation of alkoxy radicals which decompose more or less easily according to their structure. Concerning the apparent activation energy derived from burning velocities at different degrees of dilution with nitrogen, no systematic change with the nature of the fuel has been observed.
UNIVERSIT~ DE LOUVAINLaboratoire de Chimie Inorganique Communiqut a la Soci& Chimique de Belgique le 31 octobre 1963.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.