The Gauche, Ortho, and Anti Conformers of Perfluoro-n-butane: Matrix-Isolation IR Spectra

摘要

We report the IR spectra of three distinct (racemic) conform-ers of perfluoro-n-butane, recorded under conditions of matrix isolation, and assigned by comparison with HF/6-31G~* calculations. We propose to call them gauche (g_±), ortho (o_±), and anti (a_±). Textbook notions of saturated chain conformational analysis assume the general existence of two and only two conformers in a saturated X_4Y_(10) four-atom chain: gauche at about ±60° dihedral angle (racemic), and anti, either exactly at 180° (achiral), as in n-butane, or merely close to 180° (racemic), as in helical polymers. Over the years, some of the many semiempirical calculations performed for saturated chains predicted the existence of a larger number of conformational minima for rotation around a single bond, not only for permethylated and perfluorinated chains but also for parent n-alkanes and n-oligosilanes. In general, these results have not been taken very seriously. Very recent ab initio results for Si_4Me_(10), C_4F_(10), at least one other type of X_4Y_(10) chain, and poly(tetrafluoroethylene), which predict the existence of three racemic conformers at about 55°, 90°, and 165°, cannot be dismissed as easily. Also, it has been pointed out that the splitting of the "ordinary" gauche minimum at 60° into two at 55° and 90°, presumably due primarily to 1,4 substituent interactions, would be analogous to the splitting of the "ordinary" anti minimum at 180° into two at ±165°, presumably due to 1,3 substituent interactions.