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-diflo robutadiene (DFBD) have been investigated with the Gaussian-3,(G3) and G3// B3LYP (G3B3) methods. The geometrical structures of the conformers have bee n gradient optimized by the HF and B3LYP methods with the 6-31G(dp) basis s et. 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 geometr y, except that their torsional angles about the C-C bonds are different, an d among them only the high-energy conformer of the cis-trans isomer is copl anar. Harmonic vibrational frequency analysis indicates that the high-energ y conformers are characterized by their smaller separations of the two C do uble bondC stretching modes and the reversed order of the v(C double bondC) (sym) and v(C double bondC)(asym) peaks compared to that of their low-energ y conformer partners. NBO analysis indicates that the pi-pi* conjugative in teraction in a high-energy conformer is smaller than that in its low-energy conformer partner and that the significant n-pi* interactions in the high- energy conformers contribute to their extra stabilities. Through the G3B3 c alculations, the conformational energies of the cis-cis, cis-trans, and tra ns-trans isomers are estimated to be 17.1, 7.9, and 9.8 kJ mol(-1), respect ively. The stability sequence of the high-energy conformers in different is omers is (cis-trans) > (trans-trans) > (cis-cis), while that of the low-ene rgy 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 conforme rs.