Quantum-chemical study of CnF2n+2 conformers.: Structure and IR spectra

Quantum-chemical calculations of the geometrical structure and vibrational spectra of CnF2n+2 oligomers (n = 5-8) in the chain and branched conformations are reported. The lengthening of the chain of CnF2n+2 does not substantially affect the geometrical parameters of the oligomers. In all cases under study, the most optimal structure of the molecule is a zigzag chain with bond lengths R(C-C) = 1.53 angstrom - 1.54 angstrom and R(C-F) = 1.36 angstrom - 1.34 angstrom; the chain is rolled into a helix, which makes an angle of 17 degrees with the plane. The IR spectra are sensitive to the structural deficiency of oligomers CnF2n+2 associated with the lateral trifluoromethyl groups formed in the chain; the spectra can be used for revealing defects of this type in the structure of polytetrafluoroethylene (PTFE). The possibility of defects associated with the lateral CF3 groups in the structure of PTFE and its low-temperature modifications is explained based on the calculated total energies of CnF2n+2.