Molecular Size, Shape and Self-aggregation

The Stokes radius of a molecule is related to its partial specific volume (v) and its molecular weight (M) by Eq. (6.13)  

Substitution of this equation into Eq. (6.2) and then into Eq. (6.1) when kb is replaced with R/N where R is the gas constant and N is Avogadro s number affords Eq. (6.14)  

This equation shows the reciprocal cubic root dependency of the diffusion coefficient on the molecular weight. This implies that, under identical conditions for two molecules diffusing with diffusion coefficients of Di and D2 and with molecular weights of Ml and M2 the following relation holds  

Single RoseCles Doahle Aoseites I etra RaseSes 

Larive and coworkers recently used NMR diffusion measurements to characterize a series of ligands (32-36) and rhenium complexes that were used as building units in the construction of molecular squares that could not be characterized by mass spectrometry (Fig. 6.17, [43]). There, good correlation was found between the diffusion coefficient and the reciprocal of the estimated Stokes radii (l/tg), as shown in Fig. 6.17c. The authors also concluded, based on the NMR diffusion measurements, that the complexity of some of the spectra are intrinsic characteristics of the supramolecular systems prepared, rather than contamination from low molecular species. The data extracted from these NMR diffusion experiments is presented in Table 6.4. 

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