Selective Anion Complexation by a Calix[4]pyrrole Investigated by Monte Carlo Simulations

Abstract

The complexation of anions by octamethylcalix [4]pyrrole 1 and 2,5-dimethylpyrrole has been investigated by energy minimizations in the gas phase and by Monte Carlo (MC) simulations in dichloromethane using the OPLS force field. In agreement with experiment, the 1,3-alternate conformation of 1, in which adjacent pyrrole rings are pointing in opposite directions, was shown to be the most stable conformation in the absence of a halide anion. The cone conformer of 1, having all pyrrole units in a parallel orientation, is not stable in the absence of a halide anion, but it is the most stable conformation upon anion binding due to the formation of four NH-halide hydrogen bonds. The relative free energies of binding of chloride, bromide, and iodide with the cone of 1 in dichloromethane are calculated with free energy perturbation (FEP) simulations to be in excellent agreement with experiment. However, the calculations predict a far greater affinity for fluoride ion than was measured. This can be explained by the presence of trace amounts of water. For reference, MC/FEP calculations were also carried out for 2,5-dimethylpyrrole, which was predicted to bind only fluoride ion, consistent with NMR experiments. On the technical side, the MC sampling efficiency of alternative internal-coordinate representations of the complexes was also considered.

Semifield testing of a bioremediation tool for atrazine‐contaminated soils: Evaluating the efficacy on soil and aquatic compartments

“…Compared to the wealth of experimental studies of calix[4]arenes, their computational study lagged behind due to the large system size of calixaromatics that limited the methods that could be used in their study. Since 1990, computational studies have focused on structural features34–36 and properties37, 38 initially using molecular mechanics and later semi‐empirical methods such as PM3 and AM1 39–45. However, most of these studies were not able to accurately describe the structures and the conformational equilibria and therefore only yielded a very qualitative understanding of the experimental results.…”

Missing-Link Macrocycles: Hybrid Heterocalixarene Analogues Formed from Several Different Building Blocks

“…On the other hand, detailed analyses of the 1 H NMR spectra of compounds 1 a, 2 a, 3 , and 7 carried out in CD 2 Cl 2 , revealed that these prototypic species, while best described as being in conformations analogous to those seen in the solid sate, are nonetheless subject to dynamic distortions. As in the case of calix[4]pyrrole,43–45 these motions are rapid at room temperature and begin to slow on the NMR timescale only at 188 K. While detailed analyses of the conformations present at room temperature are ongoing, they are believed to involve principally breathing motions such as those illustrated in Scheme , although rapid interconversions between the dominant 1,3‐alternate and flattened partial‐cone conformations highlighted in these schemes (and seen in the solid state) and more minor species (e.g., cone) cannot be rigorously ruled out.…”

Multiple hydrogen-bonding interactions between macrocyclic triurea and F−, Cl−, Br−, I− and NO3−: a theoretical investigation

“…DFT can also be used in the investigation of hydrogen-bonding interactions, provided that the accuracy limitations are understood. 36 Although some theoretical studies have been performed on the hydrogen-bonding interaction of halide anions with water, [37][38][39][40][41] cyclopeptide 42,43 and aromatic systems, [44][45][46][47][48][49] theoretical quantum-chemical calculations concerning oligourea macrocycles and their anion binding behavior are almost absent in the literature except for some scattered computational studies. 27,29,30 We present here results of DFT studies on a series of halide anions (F À , Cl À , Br À , and I À ) and nitrate ion bound to the oligourea 1 by multiple hydrogen bonds.…”