Influence of polyether chain on the non-covalent interactions and stability of the conformers of calix[4]crown ethers

Conformational preferences of the calix[4] crown-(4,5,6,7)-ethers were studied utilizing HF/6-31G(d), B3LYP/6-31G(d), B3LYP/6-311G(d), MPWB95/6-31G(d), MPWIPW91/6-31G(d) and M062X/6-31G(d) methods. DFT-D3 (Becke-Johnson) dispersion correction method was also employed. The results acquired from the analysis of the computational data indicated that the stability of conformers of calix[4] crown-(4,5,6,7)-ethers follows the order; cone conformer > partial cone conformer > 1,3-alternate conformer. It was initially assessed, in general, that both hydrogen bond and OH center dot center dot center dot pi interactions play a crucial role in determining the stability of the different conformers. It was further observed that the strength of both H-bond and OH center dot center dot center dot pi interactions, are dependent on the length of the polyether bridge. Hence, in order to comprehend them better, the effect of these interactions was studied systematically on the conformational stability of different isomers. It is found that both intramolecular hydrogen bonds and OH center dot center dot center dot pi interactions are mainly responsible for relative stabilities of these conformers, which in turn is also dependent on the length of the polyether bridges.