RESUMO
The self-assembly into nanotubes in solution of an amphiphilic copolymer is characterized by small-angle neutron scattering (SANS). This study confirmed the shape and the size of the tubular association and the 3-D association of the tubes predicted by molecular orbital theory. Moreover, the characterization of the stability of the association has revealed that the addition of a very small amount of salt to the solution increases the size of the association. When more salt is added, the size of the association decreases, and the structure is altered. The association was found to be independent of temperature and therefore is very stable.
Assuntos
Nanotubos/química , Polímeros/química , Química/métodos , Ésteres/química , Concentração de Íons de Hidrogênio , Íons , Microscopia Eletrônica de Transmissão , Modelos Químicos , Modelos Teóricos , Peso Molecular , Nêutrons , Sais/farmacologia , Espalhamento de Radiação , Temperatura , TermodinâmicaRESUMO
A careful conformational analysis of poly(styrene-alt-maleic anhydride) at different pH values is presented. It is found that a strong internal hydrogen bond is formed at intermediate pH, which induces a change in the conformation of the polymer chain, which becomes linear. This linearity does not depend on the chirality of the polymer. The linearity of the chain occurs only at intermediate pH and can explain the association among the chains observed by dynamic light scattering at pH 7.
Assuntos
Anidridos/química , Físico-Química/métodos , Maleatos/química , Poliestirenos/química , Ar , Sítios de Ligação , Dimerização , Ligação de Hidrogênio , Concentração de Íons de Hidrogênio , Luz , Modelos Químicos , Modelos Moleculares , Conformação Molecular , Polímeros/química , Espalhamento de Radiação , Água/químicaRESUMO
The barriers to enantiomerization of a series of axially chiral biaryl analogues of 4-(dimethylamino)pyridine (DMAP) 1-10 were determined experimentally by means of dynamic HPLC measurements and racemization studies. The barriers to rotation in derivatives 1-6 (based on the bicyclic 5-azaindoline core) were lower than those in the corresponding derivatives 7-10 (based on the monocyclic DMAP core). Semiempirical (PM3), ab initio Hartree-Fock (HF/STO-3G), and density functional theory (DFT/B3LYP/6-31G*) calculations reveal that these differences in barriers to rotation are the result of differing degrees of hybridization of the non-pyridyl nitrogen in the enantiomerization transition states (TSs). The importance of heteroatom hybridization as a factor in determining nonsteric contributions to barriers to rotation in azabiaryls of this type is discussed.