As interações entre ciclodextrinas e seus agregados com cadeias poliméricas, têm atraído a atenção de pesquisadores em diferentes subáreas da química supramolecular. Esses compostos, conhecidos como "necklaces", podem ser empregados na formulação de fios e nanotubos moleculares. No presente artigo a formação de dímeros de α-CD foi estudada teoricamente considerando as três possíveis orientações relativas, denominadas "head-to-head" (HH), "tail-to-tail" (TT) e "head-to-tail" (HT). A influência da cadeia polimérica na estabilidade relativa das diferentes associações foi avaliada através do estudo dos compostos de inclusão de ( α-CD)2 com oligo(etilenoglicol) (OEG). Os resultados mostram que a orientação relativa das CDs é definida, primeiramente, pelas interações intermoleculares entre as unidades de α-CD, tendo a cadeia polimιrica um papel secundário no processo de agregação.

Cyclodextrins (CDs) and polymeric chains have attracted considerable attention, being addressed in the literature as novel molecular assembly. The so-called "Molecular Necklace" synthesized by the inclusion of a polymeric chain inside CDs cavity has been employed in the formulation of molecular wires and nanotubes. In this paper we applied our previous reported mixed basis set approach in order to investigate theoretically the α-CD inclusion complexes formed by two CD units and an oligo(ethylene glycol) (OEG). In attempt to analyze the role played by the OEG in the formation of pseudo-rotaxane in gas-phase, DFT calculations were performed for six possible dimer associations of CDs, named head-to-head (HH), tail-to-tail (TT) and head-to-tail (HT) with or without an OEG threadlike molecule included in the cavity formed by the α-CD dimer. The comparison between relative energies of empty (HH, TT and HT) and filled associations (HH-OEG, TT-OEG and HT-OEG) shows that the OEG chain does not interfere significantly in the relative stabilization energies of the supramolecular systems, therefore the relative arrangements of CDs in the necklace structures should be primarily driven by interactions between cyclodextrin units.

In the present work, we analyzed the accuracy of distinct theoretical methods to reproduce the solid state structures of cyclodextrins. The a, b and g-cyclodextrins (CD) were considered and also their hydrates with included water molecules: a-CD.2H2O, b-CD.10H2O and g-CD.12H2O. The geometries were fully optimized using Molecular Mechanics (MM2), semiempirical (AM1 and PM3) and ab initio (HF/3-21G) methods and quantitatively compared with experimental data from X ray diffraction. The results obtained from the classical MM2 method were in best agreement with the experiment. The semiempirical and ab initio structures were also in satisfactory accordance with the experimental data. In general, the PM3 method was found to be more suitable than the AM1 to describe the CD geometries, mainly when the intramolecular hydrogen bonds are considered.