Nature of the non-bonded (C—H)⋯O interaction of ethers CH3O—(CH2)n—OCH3(n= 4–8)

Abstract

Conformational free energies of dimethoxy ethers CH₃O—(CH₂)_n—OCH₃(n= 4–8) were calculated by ab initio molecular orbital calculations at the Hartree–Fock level of theory using the 6-31G* basis set with the electronic correlation effects evaluated using second-order Møller–Plesset theory on the 6-31 + G* basis set. The dimethoxy ethers are model compounds of polyethers [—(CH₂)_y—O—]_x and have been investigated in terms of the non-bonded intramolecular (C—H)⋯O interaction that occurs in the g^±g^∓ conformations for the C—O/C—C pair. This interaction was found to be attractive (–0.43 kcal mol^–1, 1 cal = 4.184 J) only for 1,4-dimethoxybutane (n= 4) and repulsive (0.24–0.77 kcal mol^–1) for the other longer ether chains (n 5). The attractive interaction is discussed here in relation to intramolecular (C—H)⋯O hydrogen bonding.