A Molecular Model for the Helicity of Polytetrafluoro Ethylene (Tefloin®)

Ab initio molecular orbital theory has been used to model the structure of perfluoro-n-alkane chains. Ex tended basis set calculations on C 2 F 6 , C 3 F 8 , and n-C 4 F 10 show that C 2 F 6 and C 3 F 8 have staggered geometries with D 3d and C 2v symmetry, respectively. The butane n-C 4 F 10 has a helical C 2 structure that is 0.5 kcal/mol more stable than the C 2h structure. Smaller basis set calculations on the larger n-alkanes, n-C 6 F 14 and n-C 8 F 18 , also show he lical structures of C 2 symmetry. These results demon strate that the helicity observed in polytetrafluoroethy lene (Teflon®) chains is due to inherent electronic prop erties of the isolated chains and not to crystal packing forces. Structural, energetic, and vibration spectral infor mation on the perfluoro-n-alkanes is presented. This study demonstrates the power of modern theoretical chemistry methods, and how computational approaches can complement experimentation in predicting molecular structure and behavior.