Skew-staggered

There are no uniformly accepted names for the various conformers. Other names used in the literature include tnins-staggered trans, antiparallel, antiperiplanar (lUPAC), anti gflufhe-staggered gauche, skew-staggered, skew, synclinal flUPAC) cis-eclipsed di, ecliptic, synperiplanar (lUPAC) gflut-Ae-eclipsed anticlinal (lUPAC), The lUPAC terms, antiperiplanar (ap). synperiplanar (sp), anticlinal (ac), and synclinal (sc), have not become widely accepted in macromolecular chemistry. 

Certain physical properties show that rotation about the single bond is not quite free. For ethane there is an energy barrier of about 3 kcal mol-1 (12 kJ mol-1). The potential energy of the molecule is at a minimum for the staggered conformation, increases with rotation, and reaches a maximum at the eclipsed conformation. The energy required to rotate the atoms or groups about the carbon-carbon bond is called torsional energy. Torsional strain is the cause of the relative instability of the eclipsed conformation or any intermediate skew conformations. 

In addition to this extended conformation there are two gauche (skewed or synclinal) conformations which are only slightly less stable than the staggered conformation and in which A and B interfere only if they are very bulky. In one of the two gauche conformations B lies to the right of A and in the other to the left of A when viewed down the axis. 

Any conformation can be specified by its dihedral angle (0), the angle between the C—H bonds on the front carbon atom and the C—H bonds on the back carbon in the Newman projection. Two of the conformations have special names. The conformation with 6 = 0° is called the eclipsed conformation because the Newman projection shows the hydrogen atoms on the back carbon to be hidden (eclipsed) by those on the front carbon. The staggered conformation, with 6 = 60°, has the hydrogen atoms on the back carbon staggered halfway between the hydrogens on the front carbon. Any other intermediate conformation is called a skew conformation. 

Ethane conformations. The eclipsed conformation has a dihedral angle 0 = 0° and the staggered conformation has 0 = 60°. Any other conformation is called a skew conformation. 

Staggered rings Eclipsed rings Skew rings... 

R = 3.5 A for ( 2 4)2 found two equally deep minima at AE. = 5.CTkJ/mol, one for a staggered parallel structure with R — 3.93 A the other for a skew structure with R = 3.87 A (see table 4). The potential surfaces are rather flat around these minima, however, and many other possible structures exist with a binding energy hi ier than 80% of AE. Therefore, we expect the Nj and molecules in the dimers to make rather wide angular oscillations (librations) in some directions. For instaiKe, the barr for a complete rotation over < ) in the N2 dimer with 0 = 0g = 90° is about 0.2 kJ/mol (13 % of AE ) with practically no variation of the equilibrium distance (see fig. 10) (the rotational constant of free N2 is 2.0 cm" = 0.024 kJ/moI). At somewhat hi er temperatures where the dimer is still stable, (in some cases maybe at zero temperatures already) these oscillations will go over into (weakly) hindered rotations. In other directions, rotations of the molecules are strongly hindered the dimer must almost dissociate before such a rotation becomes possiUe. (e.g. the rotations throu the orientation 0 = 0 =... 

The eclipsed conformation of ethane is 3 kcal/mol less stable than the staggered conformation ( 1 kcal/mol for each eclipsed H/H pair)." Any conformation between staggered and eclipsed is referred to as a skew conformation. 

The barrier to rotation around the C-C bond in ethane has three fold periodicity. Three conformations, in order of decreasing energy, are eclipsed, skewed and staggered. 

At 60° away from this eclipsed conformation comes the most stable conformation, called staggered intermediate conformations are called skewed. 

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