The geometry of calix[3]pyrrole and the formation of the calix[3]pyrrole·F− complex in solution.

The accurate determination of the spectral data for strained molecules is challenging, as their geometries in solid state and in solution may differ because the molecules undergo continuous dynamical changes between different conformations. Calix[3]pyrrole, the recently synthesized smallest calixpyrrole, has the C_s point group of symmetry in its crystal structure, despite showing a single chemical shift value for the N–H protons in solution, which suggests that the geometry of calix[3]pyrrole could take a time-averaged, bowl-shaped geometry having a C_3v or C₃ point group of symmetry in solution. Density functional theory calculations and molecular dynamics simulations showed the most stable geometry of calix[3]pyrrole in the C_s point group of symmetry, and continuous transformation among the equivalent geometries of calix[3]pyrrole. The rapid dynamical change among the conformations of calix[3]pyrrole in the C_s point group of symmetry is the reason behind the observed experimental chemical shift value. The presence of halide (X⁻ = F⁻, Cl⁻, Br⁻, I⁻) anions can form stable, bowl-shaped calix[3]pyrrole·X⁻ complexes, and the study on calix[3]pyrrole·F⁻ showed that three NH···F hydrogen bonds are responsible for their stability. The chemical shift value for the N–H protons in calix[3]pyrrole·F⁻ is highly deshielded, as the F⁻ anion lowers electron density in the vicinity of these H-atoms. The consideration of dynamics is helpful in understanding the structure of strained molecules and associated spectral data. [ABSTRACT FROM AUTHOR]