A comparative study of density functional calculations with experimental molecular mechanics and analysis of complexation between the sodium ion and lariat crown ethers

Molecular mechanics and density functions were used in a theoretical study and research of determining the lowest energy, optimum geometric structure, and the relative geometric stabilities of the complex compounds of lariat crown ethers (LCE). Understanding of lariat 16-crown-5 ether and its complexation with the sodium ion, through the use of experimental data auxiliary to density function theoretical calculations to obtain a more optimal stereochemical molecular structure, can further improve the quality of calculation for the thermodynamic properties of the compounds. The objective of this study is to precisely handle large, electron-rich species, and provide a better understanding of the molecular behavior of a representative member that describes the complexation between this class of lariat crown ethers and the sodium ion. An experimental study and a quantum mechanical research, which used the density function theory (DFT) to conduct calculations at the B3LYP/6-31G∗ and SDD level in the Gaussian 03 package program, were conducted on these LCEs with sodium ion complex. The spatial coordinate positions of complex compounds 1–3 with sodium complex, as obtained from an X-ray structural analysis, were used as the initial coordinates for the theoretical calculations. Conformational isomers 4–6 are complexes of 2-methoxybenzyl sym-dibenzo-16-crown-5 ether and sodium ion. Their spatial coordinate positions were derived from the crystal structures of the complexes which were reported earlier [C.C. Su, L.H. Lu, J. Mol. Struct. 702 (2004) 23]. The conformational isomer 7 was based on the molecular model of complex compounds 1–3 obtained through geometric optimization calculations. The optimal geometric structures of these compounds were confirmed in this study. The atomic bond lengths, atomic torsion angles, binding energies, and binding enthalpies of these complex compounds were also calculated. The results of the DFT calculations can more accurately establish the molecular model of the complex compounds. This study also found that the effect of sidearm appeared to have strong influence on the binding ability of sodium ion.