Fig. 3. (A) Single rotations are responsible for interconversion between isomers 3a–c as determined by a 2D ¹⁹F, ¹⁹F-EXSY experiments. The isomer ratio changed in more polar acetone with higher preference for the unsymmetric isomer E,Z-3a. The ¹H NMR (400 MHz, CDCl₃) inset shows the signal of the ethylene linkage for the different isomers: symmetric isomers exhibit singlet, while unsymmetric conformer two multiplets. (B) ¹H, ¹⁹F-HOESY NMR was instrumental in structural elucidation of the conformers. The calculated 3D structures of the most populated conformers are shown. The structures were simplified for the sake of the computations; N,N′-dimethyl derivatives were used instead of N,N′-dibenzyl derivatives. They have been shown to reproduce the actual populations accurately.³ The computed Gibbs free solvation energies as well as dipole moments in the gas phase are given. (C) Further indication of conformational preference and not conformational averaging of the difluoromethyl group rotation: unusually large ⁴J_CF coupling constants suggest strong W-type coupling. The connectivity was confirmed by a ¹⁹F, ¹³C-HMBC spectrum and two W-couplings are shown for 3a (right): fluorine to benzylic carbon (magenta) and fluorine to ethylene carbon (yellow). A large coupling was observed for 3b (blue; inset). The individual coupling constants were also computed using DFT.