Cation Complexation and Stereochemistry of Macrocycles Studied by 13C NMR Relaxation-Time Measurements

Abstract

Cation binding by macrocycles was studied using 13C NMR dipole-dipole relaxation measurements in solution. The association constants of [3n]Crown-n and its cyclohexyl, benzo and dioxo derivatives with Li+, Na+, K+ and Ca2+ cations were studied in polar solvents. The relaxation times, T1 obs and NOE factors (η) of 13C atoms in the backbone of free and complex macrocycles were estimated in the extreme narrowing NMR conditions and gave T0 DD and T10 DD dipolar relaxation times. The internal motions coupled with the dipolar relaxation times are influenced by cation binding characterized by the association constants, Ka. The experimental results showed the effect of ionic radii and macrocycle size on ion selectivity as well as on the selectivity between non equivalent binding sites. The energy barriers of the preferential a, ±g,a conformational sequences present in the bound oxyethylene backbone were examined by molecular dynamics simulations using the MM+ force field in commercial software.