Theoretical study on the low-energy and high-energy conformers of the three isomers of 1,4-difluorobutadiene

The low-energy and high-energy conformers of the three isomers of 1,4-diflorobutadiene (DFBD) have been investigated with the Gaussian-3 (G3) and G3//B3LYP (G3B3) methods. The geometrical structures of the conformers have been gradient optimized by the HF and B3LYP methods with the 6-31G(d,p) basis set. Natural bond orbital (NBO) analysis has also been performed at the same levels of theory. The computational results show that the high-energy and low-energy conformers of a given isomer is essentially the same in geometry, except that their torsional angles about the C-C bonds are different, and among them only the high-energy conformer of the cis-trans isomer is coplanar. Harmonic vibrational frequency analysis indicates that the high-energy conformers are characterized by their smaller separations of the two C=C stretching modes and the reversed order of the v(C=C)_sym and v(C=C)_asym peaks compared to that of their low-energy conformer partners. NBO analysis indicates that the π-π* conjugative interaction in a high-energy conformer is smaller than that in its low-energy conformer partner and that the significant n-π* interactions in the high-energy conformers contribute to their extra stabilities. Through the G3B3 calculations, the conformational energies of the cis-cis, cis-trans, and trans-trans isomers are estimated to be 17.1, 7.9, and 9.8 kJ mol^-1, respectively. The stability sequence of the high-energy conformers in different isomers is (cis-trans) > (trans-trans) > (cis-cis), while that of the low-energy conformers is (cis-cis) > (cis-trans) > (trans-trans). The distinctive energy relationships between the conformers of different energy groups are attributed to the intramolecular hydrogen bonds in the high-energy conformers.