Competition between the hydrated fluoride anion and hexanoic acid for inclusion in beta-cyclodextrin: An H-1-NMR study in aqueous solution


beta-cyclodextrin (bCD) and two guests ( hexanoic acid and the hydrated fluoride ion) are considered as a system in the NMR chemical shift fast exchange regime. The measured chemical shift variations for the beta CD H5 protons are expressed as functions of the mole fractions for the free and complexed states, and their slopes, Delta delta(i)(0) + (theta Delta delta/theta x(i))(xj(j not equal i)) are evaluated. When the NaF concentration is varied ([NaF](0)/mM = {50,100, 150, 200, 250}) for a defined value of the hexanoic acid concentration ([Hex](0)/mM = {5, 7.5, 10, 12.5, 15}), the chemical shift variations of the beta CD H5 protons for 2.5 mM beta CD solutions in D2O present a second order polynomial variation with a minimum, suggesting two opposing trends. The first trend corresponding to the decrease of Delta delta values ( relative shielding) is traced to increasingly negative values of Delta delta(beta CD.hEX-F)(0)(-). Corresponding to the increase of Dd values ( relative deshielding), the second trend is mainly associated with the increase of the mole fraction for the inclusion complex bCD F(H2O)(6)-, as the concentration of NaF increases. The kosmotropic, stabilizing features of the fluoride ion are consistent with the results of HF/6-31G* single point energy calculations showing that beta (CDF)-F-.(H2O)(6)- is energetically favourable, in contrast with beta (CDCl)-Cl-.(H2O)(6)- and bCD Br(H2O)(8)-. Due to its smaller size, F(H2O)(6)- is not appreciably distorted inside the bCD cavity as compared with Cl(H2O)(6)- and Br(H2O)(8)-.






Lima, S; Goodfellow, BJ; Teixeira-dias, JJC

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