Abstract :
Geometrical parameters, ultraviolet and vibrational frequencies, relative stabilities, and heat of association of the six crown ethers (12O4, 12N4, 12S4, 12O2N2, 12O2S2, 12N2S2) were calculated by HF/STO-6G and HF/6-31G level of theory. The energies were calculated by single-point MP2 method using polarized 6-31G basis functions. The potential energy surface and HOMO-LUMO of these ligands indicate a remarkable change in electron density of 12O4 when nitrogen and sulfur atoms replaced the oxygen atoms which result in a major change in their ion selectivity toward transition (Co2+ , Ni2+ , Cu2+ , Rh2+ , and pd2+ ) and lanthanide (Nd2+ , Sm2+ , Eu2+ , Tm2+ , Yb2+ ) ions . The free ligands 12N4 and I2N2S2 were unstable in gas phase through geometry optimization but they are stable through complexation calculations. The heats of association with A- type and B-type ions were calculated by MMplus. In aqueous solution the 12N4 and 12S4 showed a high selectivity for Pd2+, Cu2+ions while the ligand 12O4 preferred to select Pd2+ and Rh2+ ions. The six ligands showed the same tendency for the lanthanide metal ions in the gas phase but they behave in a different manner in the aqueous phase. The theoretical IR vibrational spectra showed that Pd-N stretching, Pd-O stretching, and Pd-S stretching have a similar values within the range 603-616 cm-1.
