Shapes of dendrimers from rotational-echo double-resonance NMR
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The solid-state shape, size, and intermolecular packing of a fifth-generation dendritic macromolecule were determined by a combination of site-specific stable-isotope-labeling, rotational-echo double-resonance (REDOR) NMR and distance-constrained molecular dynamics simulations. REDOR experiments measured dipolar couplings between 13C atoms located near the chain ends and an 19F label placed at the core of benzyl ether dendrimers (generations 1-5) based on 3,5-dihydroxybenzyl alcohol as the monomeric repeat unit. Intramolecular 13C-19F coupling was distinguished from intermolecular coupling by dilution with nonlabeled dendrimer. The average intramolecular 13C-19F distances for generations 3-5 were each approximately 12 , which indicates inward-folding of chain ends with increasing generation number. The average intermolecular 13C-19F dipolar coupling decreased with increasing generation number, consistent with decreased interpenetration for larger dendrimers. The measured intra- and intermolecular distances for the fifth-generation dendrimer were used as constraints on energy minimizations and molecular dynamics simulations, which resulted in visualizations of the dendrimer packing and an estimate of density in the solid state.
