We report a microcalorimetry study of the association of various inorganic and organic cations with p-sulfonatocalix[4]arene at 298.15 K. First, we have studied the electrostatic attraction between host 1 and seven rare-earth-metal cations representative of the whole lanthanide series (La 3+ , Nd 3+ , Sm 3+ , Eu 3+ , Gd 3+ , Dy 3+ and Yb 3+ ) in an acidic solution (pH 2). In order to compare the charge effects in the electrostatic interactions we have also studied the attraction between host 1 and two alkaline-earth-metal cations (Ca 2+ and Mg 2+ ). Next, we investigated the binding of a series of quaternary ammonium cations in an acidic solution (pH 2). For each system, both the apparent association constant and enthalpy of complexation have been extracted from the calorimetric data. In all cases, our results are consistent with the formation of 1 : 1 complexes. Whereas for the inorganic cations (alkaline-earthmetal and lanthanide cations) the association process is enthalpically unfavoured (∆ r H Њ>0) and entropically favoured (∆ r SЊ>0), the complexation is driven by a favourable enthalpy change for the organic cations. These thermodynamic properties show that the organic and inorganic cations bind in very different modes.
We report results of simulations of association between p-sulfonatocalix [4]arene and inorganic (rare-earth metal cations) and organic cations (a series of quaternary ammonium cations) in aqueous solution. Our main goals were to obtain structural features for these complexes in aqueous solution and to study the role of water on the cation binding by the p-sulfonatocalix [4]arene. The MD calculations show that the organic and inorganic cations bind in very different modes. The lanthanide cations are located outside the cavity of the calixarene forming an outer-sphere complex, while for the organic cations, the quaternary ammonium cation is included into the cavity of the calixarene. In fact, the Me 4 N + cation penetrates deeply into the cavity. As concerns the Et 4 N + cation, one of the alkyl chain is close to the center of mass of the calixarene, whereas two other alkyl chains are located near the border of the cavity. In the case of Pr 4 N + cation, only one propyl chain is inside the cavity, while the others are outside the cavity of the calixarene. Additional simulations have been carried out using two different free energy perturbation formalisms to calculate differences in Gibbs free energies of complexation of lanthanide complexes. These simulations are consistent with the thermodynamic properties of association obtained recently by microcalorimetry, and the calculated differences in Gibbs free energies of complexation are in excellent agreement with the experimental ones.
We report free energy calculations of FcC(6)S-/C(4)S-Au and FcC(6)S-/C(12)S-Au binary self-assembled monolayers (SAMs) formed by one ferrocenylhexanethiolate chain and alkylthiolate chains. We demonstrate that the free energy perturbation methods are able to reproduce the positive shift of the redox potential when the coadsorbed butanethiolate C(4)S chains are replaced by dodecanethiolate C(12)S chains. The different contributions to the Ewald summation involved in the perturbation process are thoroughly described. We complete the study by a microscopic description of the binary SAMs before and after oxidation. The molecular dynamics (MD) simulations evidence that the formation of the ion-pair between the ferricinium and a single perchlorate anion of the supporting electrolyte is more favored in FcC(6)S-/C(12)S-Au SAM.
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