Ethane dihedral angle

The structural feature that Figures 3 2 and 3 3 illustrate is the spatial relationship between atoms on adjacent carbons Each H—C—C—H unit m ethane is characterized by a torsion angle or dihedral angle which is the angle between the H—C—C plane... 

Next come the dihedral angles (or torsions), and the contribution that each makes to the total intramolecular potential energy depends on the local symmetry. We distinguish between torsion where full internal rotation is chemically possible, and torsion where we would not normally expect full rotation. Full rotation about the C-C bond in ethane is normal behaviour at room temperature (although 1 have yet to tell you why), and the two CH3 groups would clearly need a threefold potential, such as... 

Peptides built from y-amino acids with L-amino acid-derived chirahty centers form a right-handed (P)-2.6i4 hehx of ca. 5 A pitch with both ethane bonds in a -y)-synclinal conformation (mean values for dihedral angles 9 and 82 of central residues 2-5 in compounds 141 are 72.5 and 64.3°, respectively Fig. 2.36 A and B Tab. 2.8). 

Reduction of Ni11 chloride complexes [NiCl2(L)] (L = various diphosphinomethanes, -ethanes, and -propanes) with, for example, potassium naphthalenide in THF gives the corresponding Ni1 chlorides [NiCl(L)].2368 By treatment of (963) with LiNHAr, a terminal amido complex of Ni1 (964) was prepared (Scheme 13).2369 It contains planar three-coordinate nickel and a planar amido ligand with d(Ni—N) = 1.881(2) A. The P,Ni,P and C,N,H planes are orthogonal with a 91° dihedral angle. 

The bond angles are also satisfactorily reproduced in most of the cases. This is also true for dihedral angles which are sometimes more accurately predicted than by the use of the larger, double-zeta sets. For instance, experimental evidence favors the gauche structure for 1,2 difluoro ethane. STO-3G calculations lead to a gauche structure, while 4-31G calculations predict a trans structure. 

For a four-atom molecule A—B—C—D, the plane formed by A—B—C may or may not lie in the plane formed by B—C—D, and the angle between these two planes is called the torsion or dihedral angle r. For instance, in ethane, the sequence H—C—C—H has an equilibrium torsion angle of 60° or 180°. When the torsion angle is not at the equilibrium value, the energy of the molecule is increased by... 

Fig. 3.4 Variation of the energy of ethane with dihedral angle. The curve can be represented as a cosine function...

Parameterizing the Torsional Term For the ethane case (Fig. 3.4), the equation for energy as a function of dihedral angle can be deduced fairly simply by adjusting the basic equation E = cos 6 to give E = HEmiLX[ + cos3(0 + 60)]. 

Plot of energy versus dihedral angle for conformations of ethane. 

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. 

Let US first examine current practice in nomenclature of the phenom-menon of internal rotation. Standard textbooks on conformational analysis on the one hand and those on spectroscopy on the other hand deal differently with the basic definitions. Internal rotation is measured by one or more torsion angles (dihedral angles, azimuthal angles). In the simple cases of e.g. a 1,2-disubstituted ethane the potential energy associated with the internal rotation may be written in a formal sense as a truncated Fourier expansion ... 

Rotating the atoms on one carbon by 60° converts an eclipsed conformation into a staggered conformation, and vice versa. These conformations are often viewed end-on— that is, looking directly down the carbon-carbon bond. The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle. For ethane in the staggered conformation, the dihedral angle for the C-H bonds is 60°. For eclipsed ethane, it is 0°. 

---