Effects of fluorine-substitution on the molecular properties of dimethyl ethers: A theoretical investigation

Quantum chemical calculations using density functional theory with the hybrid B3LYP functional and the 6-311++G(d,p) basis set were performed to determine the conformation of fluorinated ethers (CF(nH3-n)2O) with nF = 1 to nF = 6 and to evaluate the proton affinities of the O atoms and the deprotonation enthalpies of the CH bonds. Symmetric as well as asymmetric substituted ethers are investigated. The calculated results include the optimized geometries, natural bond orbital (NBO) data along with relevant vibrational frequencies. In all the fluorinated derivatives, except for CHF2OCF3, the most stable conformation corresponds to a gauche orientation of the C-F bond relative to the C-H bond. The NBO analysis (occupation of σ∗ antibonding orbitals, atomic charges and hyperconjugative energies) shows that the conformation is mainly governed by the anomeric (hyperconjugative) effect taking place through an electron transfer from the lone pair of the O atom to the σ∗(C-F) orbital. Electrostatic interaction between the non-bonded atoms may also stabilize the conformation. Protonation of the O site where the lone pair is tied up in the OH+ bond results in marked changes of the geometry and occupation of antibonding orbitals. Large blue shifts of the CH stretching frequencies are also predicted in the protonated species. The results are in good agreement with the cancellation of the anomeric effect in the protonated species. Proton affinities of the most stable conformers range between 790 (CH3OCH3) and 580 (CF3OCF3) kJ mol−1. Deprotonation enthalpies (DPE) of the CH bonds comprise between 1717 (CH3OCH3) and 1504 (CHF2OCF3) kJ mol−1. The influence of fluoro-substitution on both C atoms is discussed. A rough correlation has been found between the DPE values and the % s-character of the C at the H atom involved in the deprotonation process.