is steric hindrance, but, with a C-C bond length of 154 pm and a Van der Waals radius for hydrogen of 120 pm, the hydrogen atoms in ethane are never in each other's way. The question of whether steric hindrance is responsible for the eclipsed energy maximum is a topic of debate to this day. One alternative to the steric hindrance explanation is based on hyperconjugation as analyzed within the Natural Bond Orbital framework. In the staggered conformation, one C-H sigma bonding orbital donates electron density to the antibonding orbital of the other C-H bond. The energetic stabilization of this effect is maximized when the two orbitals have maximal overlap, occurring in the staggered conformation. There is no overlap in the eclipsed conformation, leading to a disfavored energy maximum. On the other hand, an analysis within quantitative molecular orbital theory shows that 2-orbital-4-electron (steric) repulsions are dominant over hyperconjugation. A valence bond theory study also emphasizes the importance of steric effects. Nomenclature Naming alkanes per standards listed in the IUPAC Gold Book is done according to the Klyne–Prelog system for specifying angles (called either torsional or dihedral angles) between substituents around a single bond: a torsion angle between 0° and ± 90° is called syn (s) a torsion angle between ± 90° and 180° is called anti (a) a torsion angle between 30° and 150° or between –30° and –150° is called clinal (c) a torsion angle between 0° and ± 30° or ± 150° and 180° is called periplanar (p) a torsion angle between 0° and ± 30° is called synperiplanar (sp), also called syn- or cis- conformation a torsion angle between 30° to 90° and –30° to –90° is called synclinal (sc), also called gauche or skew a torsion angle between 90° and 150° or –90° and –150° is called anticlinal (ac) a torsion angle between ± 150° and 180° is called antiperiplanar (ap), also called anti- or trans- conformation Torsional strain or "Pitzer strain" refers to resistance to twisting about a bond. Special cases In n-pentane, the terminal methyl groups experience additional pentane interference. Replacing hydrogen by fluorine in polytetrafluoroethylene changes the stereochemistry from the zigzag geometry to that of a helix due to electrostatic repulsion of the fluorine atoms in the 1,3 positions. Evidence for the helix structure in the crystalline state is derived from X-ray crystallography and from NMR spectroscopy and circular dichroism in solution.