Solvent effect on the role of methyl groups in formation of O•••HO hydrogen bond in dimethyl ether-methanol complex

The role of the methyl groups in the formation of the O center dot center dot center dot HO hydrogen bond between dimethyl ether (DME) and methanol molecules in different solvents has been studied with quantum chemical calculations at the MP2/6-31+G(d,p) and B3LYP/6-311++G(d,p) levels. As indicated in the changes of the O center dot center dot center dot H binding distance, O center dot center dot center dot O distance, OH bond length, OH stretching frequency, and complexation energy, the strength of the O center dot center dot center dot HO hydrogen bond in the complex increases as solvent polarity increases. The enhancing effect of solvent on the interaction is analyzed with the natural bond orbital (NBO) method. The charge changes of the methyl groups show that the methyl groups of DME are electron-donating and the methyl group of methanol is electron-withdrawing, both making positive contributions to the formation of the O center dot center dot center dot HO hydrogen bond. However, these contributions decrease with increasing solvent polarity as indicated in the decrease of the methyl charge transfer. The electron-donating ability of the methyl group in DME is greater than the electron-withdrawing ability of the methyl group in methanol, and such difference increases with increasing solvent polarity. The methyl role and weakening effect of solvent on the methyl role in the formation of hydrogen bond are investigated in views of the orbital interactions. (C) 2008 Elsevier B.V. All rights reserved.