engleski

Estimation of stability of copper(II) chelates with N-alkyl-1, 2-diaminoethanes by the overlapping spheres method

The stability constants for mono-complexes of 1, 2-diaminoethane (ehylenediamine) and its five N-alkylated derivatives (methyl-, ethyl-, propyl-, iso-propyl-, and butyl-) were extracted from literature [1] and were correlated on overlapping volumes of central sphere and the van der Waals volumes of neighbouring atoms [2-4]. The central sphere was situated at central (Cu), bite (N1, N2) or « dummy» atoms (X1, X2), apically placed at distance 250 pm from the central atom. The radius of the central sphere was fixed at 300 and 400 pm. The full conformational analysis of all chelates were performed by using molecular-mechanics method. The conformer with the lowest overlapping spheres (OS) volume was selected for every complex. Linear regression of stabilitiy constants (log β 1) on the OS volumes yielded regression coefficients r = 0.855 – 0.943 and residuals in the range 0.01 – 0.45 log β 1 units (the OS volume for the two apically placed atoms was calculated as the mean of X1 and X2 OS volumes). Better results were obtained by averaging the stability constants calculated from the all 8 regressions. The residuals were obtained in the range 0.02 – 0.27 with the mean value 0.16 log β 1 units. Binomial regression on the OS volumes for the central sphere centered at N1 and N2 atoms and the geometrical mean of the OS volumes for the two atoms (altogether four regressions) gave correlation coefficients R = 0.988 – 0.996 and residuals in the range 0.01 – 0.22 log β 1 units. Substantially better results were yielded by averaging the values calculated from all the four regression functions. In that way log β 1 was reproduced in the range 0.01 – 0.1 (mean = 0.04) log β 1 units.

overlapping spheres method

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