structure conformation analysis of three new lariat crown ethers based on theory and experiment

Three new lariat crown ethers (LCEs) were synthesized. Their structures were determined by NMR spectroscopy, mass spectroscopy, and X-ray crystallography. The methoxybenzyl sidearms of c-pivot LCEs of methoxy group are located at ortho, meta, and para position, respectively. The three LCEs are found to be structural isomers. The observed X-ray crystal structures of the 2-methoxybenzyl sym-dibenzo- 16-crown-5 ether showed three important conformational isomers that are stable conformations not commonly seen in X-ray crystallography. An experimental study and a theoretical analysis using the B3LYP/6-31G* method in the GAUSSIAN 98 package program were conducted on the three new 16-crown-5 ethers. Molecular mechanics and density functions were used in the theoretical study to determine the relative geometric stabilities of the three isomers. Optimal geometric structures of these compounds were also determined. The calculated molecular mechanics compared quite well with those obtained from X-ray crystallography. The ionization potentials, highest occupied molecular orbital energy, lowest unoccupied molecular orbital energy, energy gaps, heat of formation, atomization energies, and vibration frequencies of the compounds were also calculated. The results of the calculations show that these new ethers are stable molecules with high reactivity and various other physical properties. The study of these 2-, 3-, 4-methoxybenzyl sym-dibenzo-16-crown-5 ethers also provided a better understanding of the conformational properties of macrocyclic ligands. q 2004 Elsevier B.V. All rights reserved.