Density functional calculations and experimental studies of the sidearm remote controlling effect in eight lariat crown ethers

A series of eight lariat 16-crown-5 ethers (LCEs), which have the same parent macrocycle but with a different sidearm, have been synthesized. An experimental study and a quantum mechanical research, which uses the density functional theory (DFT) to calculate at the B3LYP/6-31G∗ level in the Gaussian 98 and Gaussian 03 package program, were conducted on these eight LCEs. The space coordinate positions of compounds 1–4, obtained from X-ray structural analysis, were used as initial coordinates for these theoretical calculations. The observed X-ray crystal structures of these LCEs 1–4 were compared with the optimized geometries obtained from DFT calculations. As a result, an excellent agreement between the X-ray crystallography and the four calculated compounds of conformational analysis has been found. However, the LCEs 5–8 were based on the LCEs 1–4 molecular model, which is the starting molecular structure that processes the geometric optimization calculations. Molecular mechanics and density functions were used in the theoretical study as well to determine the relative geometric stabilities of the eight compounds. The optimal geometric structures of these compounds were confirmed as well. The ionization potentials, highest occupied molecular orbital energy, lowest unoccupied molecular orbital energy, energy gaps, heat of formation, atomization energies, and vibrational frequencies of these compounds were also calculated. The results of the DFT calculations show that eight different LCEs are stable molecules with various thermodynamic properties. Thus, sidearm effect appears to cause strong influence on the metal ion binding capability.