Conformational characteristics of aliphatic diesters derived from fluorinated diacids

A comparative study of the polarity and conformational characteristics of diethyl hexafluoroglutarate (DEFG) and its hydrogenated diester counterpart, diethyl glutarate (DEG), is reported. The value at 30 degrees C of the mean-square dipole moment [mu(2)] of the latter compound, 5.26 D-2, is nearly 2 times larger the value of this quantity for the fluorinated diester, which amounts to 12.1 D-2 at the same temperature. Values of the conformational energies associated with the rotational stales about the skeletal bonds of the acid residue were calculated by combining molecular mechanics with the critical interpretation of the mean square dipole moment of these compounds, finding that the rotational populations about the different skeletal bonds are similar in both molecules. The evaluation of [mu(2)] as a function of the conformational energies indicates that the experimental results are reproduced assuming that the conformation in which the C*=O* group eclipses the C-alpha-C-beta bond in the (CCC)-C-beta-C-alpha*(O*)O residue is less stable than the alternative conformation in which the carbonyl group eclipses the C-alpha-F (or C-alpha-H) bond. The analysis suggests that the high polarity exhibited by DEFG, in comparison with that of DEG, arises from differences in the orientation and the modulus of the dipoles in both compounds rather than from differences in their conformational characteristics.