Internal rotations in the difluorobutadiene and tetrafluorobutadiene molecules: a DFT B3LYP study

Abstract

Internal rotations around the central C–C bond in the ( t, t)-1,4-DFBD (difluorobutadiene), ( c, c)-1,4-DFBD, 2,3-DFBD, ( t, t)-1,2,3,4-TFBD (tetrafluorobutadiene), ( c, c)-1,2,3,4-TFBD, and 1,1,4,4-TFBD molecules were studied at the DFT B3LYP/6-311+G(d,p) level of theory. The calculated internal rotation potential energy curves (PECs) for the six molecules have the same skeleton: along the PEC there exist s-trans and s-gauche (at θ=37–55°) minima and two transition states (at θ=90–116° and at s-cis conformations). The relative energies and optimized geometries of the s-trans and s-gauche conformers and transition states are reported. The s-trans conformers are the lower-energy conformers for all the molecules except ( t, t)-1,2,3,4-TFBD, for which the s-gauche conformer is slightly more stable than the s-trans conformer. The barrier heights for the s-gauche→s-trans processes are predicted to be 1.8–2.9 kcal mol −1. Internal rotations in the cations of the six molecules were also studied at the same level. For each of the cations, two PECs were calculated: the 2B PEC starting at the s-trans conformation and the 2A PEC starting at the s-cis conformation. For each of the cations the s-trans conformer is lower in energy than the s-cis conformer, and, with the θ value, the energy monotonously decreases along the 2B PEC and monotonously increases along the 2A PEC. The s-trans conformers of ( c, c)-1,4-DFBD and ( c, c)-1,2,3,4-TFBD are predicted to be more stable than those of ( t, t)-1,4-DFBD and ( t, t)-1,2,3,4-TFBD, respectively, which supports the cis-effect for 1,4-DFBD suggested by the experimental workers. The s-trans conformer of 2,3-DFBD is slightly more stable than those of ( c, c)- and ( t, t)-1,4-DFBD, while the s-trans conformer of 2,3-DFBD + is much higher in energy than those of ( c, c)- and ( t, t)-1,4-DFBD +. The s-trans conformers of 1,1,4,4-TFBD and 1,1,4,4-TFBD + are much more stable than those of ( c, c)- and ( t, t)-1,2,3,4-TFBD and ( c, c)- and ( t, t)-1,2,3,4-TFBD +, respectively.