THEORETICAL-STUDY OF THE SUBSTITUENT EFFECTS OF 4-SUBSTITUTED MONOBENZO CROWN-ETHERS AND THE EFFECTS OF RING SIZE OF 3N-CROWN-N (N=4-7) ETHERS ON THE CATION SELECTIVITY

Ab initio studies of the binding of Na+ with a series of 4-substituted monobenzo 15-crown-5 ligands are reported and compared with experimen tal data. The substituent effect on the cation binding is small and al most negligible for methyl and amino groups but a difference of ca. 12 % in the binding energy is found between the 4-methyl and 4-nitro deri vatives. The substituent effect is related to the basicity of the arom atic ether oxygens as demonstrated by the correlation found by plottin g binding energy (Delta E) vs. average charge of aromatic ether oxygen s. A Hammett correlation is found by plotting Delta E vs. sigma(p)(-)sigma(m). Ab initio studies of the binding of Na+ and K+ with 3n-crown -n (n=4-7) ligands are reported and the effect of a change in the ring size on the cation selectivity is evaluated. Both Na+ and K+ bind to 18-crown-6 ether more strongly than any other 3n-crown-n ether studied here. The ring-size effect on the cation selectivity is found to depe nd not only on the hole-size concept, but also on the number of availa ble donor atoms that can effectively participate.