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

The cone and 1,2,3 alternate isomers of calix[6]arene bis‐crown‐4 were investigated computationally. Structural optimizations, 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 were completed at three different levels of imposed symmetry, and two calculations investigated the chloroform solvent effects. Better NMR results were obtained from HF/6‐31g(d) calculations that did not impose molecular symmetry constraints. Consideration of solvent effects improved ground state energies, but other improvements were minimal and not significant enough to justify the added computational expense of solvent calculations. Overall results are consistent 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 spectroscopic manifestations of steric compression. Copyright © 2009 John Wiley & Sons, Ltd.