| Abstract
| - The conformational preference of calix[4]pyrrole and its fluoride and chloride anion-bindingproperties have been investigated by density functional theory calculations. Geometries wereoptimized by the BLYP/3-21G and BLYP/6-31G* methods, and energies were evaluated with theBLYP/6-31+G** method. To model the effect of medium, the SCIPCM solvent model was alsoemployed. Four typical conformations of the parent substituent-free calix[4]pyrrole were studied.Both in the gas phase and in CH2Cl2 solution, the stability sequence is predicted to be 1,3-alternate> partial cone > 1,2-alternate > cone. The cone conformation is predicted to be about 16.0 and11.4 kcal/mol less stable in the gas phase and CH2Cl2 solution, respectively. This is mainly due toelectrostatic repulsions arising from the all-syn pyrrole/pyrrole/pyrrole/pyrrole arrangement presentin this conformer. The existence of possible 1:1 and 1:2 anion-binding modes were explored in thecase of fluoride anion, and the factors favoring the 1:1 binding mode are discussed. The calculatedbinding energy for fluoride anion is about 15 kcal/mol larger than that for chloride anion. Thecalculated binding energy for chloride anion agrees with the experimental value very well. Thepresence of meso-alkyl substituents destabilizes the cone conformer with respect to the 1,3-alternateconformer and, therefore, reduces the anion-binding affinity by 3−4 kcal/mol. The strength ofN−H- - -anion hydrogen bonds in the various structures subject to study were estimated on thebasis of the calculated anion-binding energies and the predicted structural deformation energiesof substituent-free calix[4]pyrrole.
|
