Estrutura e estabilidade de complexos moleculares, na fase liquida, formados atraves de pontes de hidrogenio envolvendo alcoois fluorados

AUTOR(ES)
DATA DE PUBLICAÇÃO

1981

RESUMO

The study of molecular association through hidrogen bonding, using Infrared Spectroscopy, is usually done in the spectral region (4000 - 3000 cm ) of the (OH) ass. vibration. Our work consisted in a comparative study through Infrared spectra of the gaseous and liquid states of the following alcohols: 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP); 2,2,2-trifluoro ethanol (TFE), ethanol and isopropanol, we observed a narrowing in the bands assigned to (CC) and (CF3) of the fluorinated alcohols, when going from the gas state to the liquid state. This narrowing is due to the disappearance of branches P and R, followed by an intensive Q branch in the center of the bands. We suggest that the fluorinated alcohol molecules in the gas state have a free rotation movement and in the liquid phase the rotation around the intermolecular bond (C---C), is a sort of libration, owing to the formation of hidrogen bonds. We measured the dependence of the retractive index (n) with the percentage in volume(%Va/Vb) of a series of polar solutions (A - B): HFIP - acetone (I); HFIP - methanol (II); HFIP - water (III); TFE - acetone (IV); TFE - methanol (V); TFE - water (VI); methanol - acetone (VII); water - acetone (VIII) and methanol - water (IX). For the systems where the association is made through strong hidrogen bonds (I - VI), the relationship found has an anomalous form, it is neither a straight line nor a curve. For system (VII) where the association is done through weak hidrogen bonds, the dependence of (n) and (% Va/Vb) is additive. Finally, we have observed a great deviation from the additivity (parabolic function), for the systems (VIII and IX). The interpretation of these curves through the methods found in the literature was impossible, since these methods treat the refractive index curves as solely dependent on the molar density and molar volume of the system components. In this work interpret these correlations considering also the magnitude of the equilibrium constant for complex formation. We conclude that the anomalous curves found for the fluorinated alcohols are due to the formation of stable malecular complexes, associated strong hydrogen bonds but with uncertain compositions. We have also studied these systems with the aid of infrared spectroscopy, considering the relative width af the v(OH) ass. band. The results obtained agree with those derived from the refractive index experiments. Finally, we used the MINDO/3 program to minimeze the energy and obtain the most stable geometry for the TFE-water system (dimer). We found that in the most stable configuration the hydrogen bridge involves the fluoric atom from the CF3 grouping in the alcohol. There is also evidence for the possibility of association via the axygen atom of the alcohol. The results of our calculations are in agreement with ab initio theoretical studies found in the literature.

ASSUNTO(S)

hidrogenio alcoois compostos complexos

ACESSO AO ARTIGO

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