Theoretical study of the cooperativity in substituted dimethyl ethers complexed with two water molecules. Red or blue shifts of the ν(CH) vibrations?

Abstract

The cooperative interactions in CH3OCH3, CH3OCH2F, CH3OCHF2, CH2FOCHF2 and CHF2OCHF2 complexed with two H2O molecules are investigated using the B3LYP method with the 6-311++G(d,p) level. The calculations include the optimized geometries, the cooperative energies along with a natural bond orbital (NBO analysis). Cyclic complexes characterized by OwHw…O and CH…Ow hydrogen bonds are formed. The pairwise or two body interaction energies are computed. The results show that the OwHw…O interaction energies increase with the proton affinity of the O atom of the ethers and that the CH…Ow interaction energies increase with increasing acidity of the CH bond. This is in agreement with the intermolecular distances. The cooperative energy represents 15–20% of the total energy. When nF=2–4, blue shifts are predicted for the ν(CH) vibration for the 1–1 complexes but red shifts are predicted for the 1–2 complexes. The variation of the CH distances is explained by a competition between the variation of the intramolecular hyperconjugation energies and the intermolecular hyperconjugation energies. The charge transfer between the ether and water molecules is analyzed. When the negative charge on one H2O molecule decreases, the positive charge on the other H2O molecule increases, indicating a nice reciprocal effect.