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

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 Moller-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 greater than or equal to 5). The attractive interaction is discussed here in relation to intramolecular (C-H)... O hydrogen bonding.