Computational studies of the cone and 1,2,3 alternate calix[6]arene bis-crown-4 isomers: structures, NMR shifts, atomic charges, and steric compression

摘要

The cone and 1,2,3 alternate isomers of calix[6]arene bis-crown-4 were investigated computationally. Structuraloptimizations, energies, bond distances, and Mulliken charges were calculated by the application of the B3LYP/6-31g(d) method/basis, followed by NMR calculations via both B3LYP/6-31g(d) and HF/6-31g(d). Calculations werecompleted at three different levels of imposed symmetry, and two calculations investigated the chloroform solventeffects. Better NMR results were obtained from HF/6-31g(d) calculations that did not impose molecular symmetryconstraints. Consideration of solvent effects improved ground state energies, but other improvements were minimaland not significant enough to justify the added computational expense of solvent calculations. Overall results areconsistent with known experimental assignments and were valuable for assigning previously unknown NMR peaks.Net charges, electrostatic forces, and local dipoles - but not bond lengths - are strongly correlated to spectroscopicmanifestations of steric compression