Hydrogen bonding in alcohol–ethylene oxide and alcohol–ethylene sulfide complexes

The hydrogen bonds involving sulfur are generally regarded as weak hydrogen bonds in comparison with conventional ones. The O–H⋯O and O–H⋯S hydrogen bonds in the alcohol–ethylene oxide (EO) and alcohol–ethylene sulfide (ES) complexes in the gas phase have been investigated by FTIR spectroscopy. Three alcohols, methanol (MeOH), ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) were used as hydrogen bond donors, and comparable OH-stretching red shifts were observed for the complexes with EO and ES as hydrogen bond acceptors. DFT calculations were used to determine the stable structures and interaction energies of the complexes. The equilibrium constant for the complex formation was determined from the experimental integrated absorbance and the computational IR intensity of the OH-stretching transition band of the complex. The effect of CF3 substitution on the hydrogen bond strength in alcohol–EO/ES molecular complexes was investigated and the TFE complexes form much stronger hydrogen bonds than the MeOH and EtOH complexes. Atoms-in-molecules (AIM) analysis revealed that several hydrogen bond interactions were present in the complexes. In addition, the localized molecular orbital-energy decomposition analysis (LMO-EDA) was implemented to analyze the intermolecular interactions. The O–H⋯O and O–H⋯S hydrogen bonds were found to be of similar strength, on the basis of the geometric parameters, binding energies and AIM analysis.