Theoretical investigation of the interaction between fluorinated dimethyl ethers (nF = 1-5) and water: role of the acidity and basicity on the competition between OH...O and CH...O hydrogen bonds.

Theoretical calculations have been carried out using ab initio MP2 and B3LYP density functional methods to investigate the interaction between fluorinated dimethyl ethers (nF = 1-5) and water. Depending on the number of F atoms implanted on the dimethyl ethers, linear structures stabilized by intermolecular O(w)H(w)...O or CH...O(w) hydrogen bonds or closed structures involving both hydrogen bonds are formed. Binding energies of the hydrogen-bonded complexes range between 4 and 12 kJ mol(-1). Blue shifts of the CH stretching vibrations are predicted even in the absence of a direct CH...O interaction. The red shifts of the OH stretching vibrations of water in the open and closed structures are analyzed as well. The natural bond orbital analysis includes the sigma*(O(w)H(w)) and sigma*(CH) occupation, the hybridization of the C atom, the atomic charges, and the intra- and intermolecular hyperconjugation energies. These parameters are discussed as a function of the proton affinity (PA) of the O atom and the deprotonation enthalpy (DPE) of the CH bonds of the fluorinated ethers calculated in a previous work. (16) Our results show that the effective PA in determining the intermolecular O --> sigma*(O(w)H(w)) hyperconjugation energies decreases with increasing acidity of the CH bond. In turn, the effective acidity of the CH bond in determining the intermolecular O(w) --> sigma*(CH) hyperconjugation energies decreases with increasing basicity of the O atom.