engleski

Estimation of stability of diaminoethane complexes by the overlapping spheres method with a variable central sphere radius

The method of overlapping spheres (OS) for the estimation of the stability constants of coordination compounds is based on the calculation of the overlapping volume, V*, of the central sphere (situated at the central or a bite atom) and the van der Waals spheres of surrounding atoms (Figure 1).1-3 The most critical parameter in the model is the radius of the central sphere, Rv, which is usually taken to be 3 or 4 &#197; . Here we propose the model with variable value of the Rv parameter, making it to correspond to its value at the maximum of the volume density, V*(Rv)/ (4/3 Rv3 &#960; ), for each molecule. For the training set (N = 14) the range of maximum volume denisities and Rv are 0.556 &#8211; 0.722 and 2.7 &#8211; 3.0 &#197; , respectively. Linear regressions of stability constants (log K1) for mono-complexes of diamines with copper(II) and nickel(II) measured at 0 and 25 oC on overlapping spheres volume, V*, yielded slightly, but significantly better results than calculations done at Rv = 3 &#197; (Figure 2). The four regression yielded <r> = 0.786 (r = 0.677 &#8211; 0.858) and <r> = 0.796 (r = 0.681 &#8211; 0.878) for the constant and varibable Rv value, respectively. Also, test on five copper(II) mono-complexes (25 oC), not included in the training set, gave residuals in the range 0.01 &#8211; 0.71 (r.m.s. = 0.33) log K units (Table 2), which is slightly better than the results for Rv = 3 &#197; (range = 0.02 &#8211; 0.75, r.m.s. = 0.34 log K units), but substantially better than results obtained with Rv = 4 &#197; (range 0.15 &#8211; 0.91, r.m.s. = 0.48 log K units). In spite of the slight diferences, which should be mostly attributed to Rvvar &#8776; 3&#197; , the proposed approach seems to be sound for the further improvement of the model.

molecular volumes; overlapping spheres

nije evidentirano