Torsional energy ethane

It is customary to shift the zero point of the potential by adding a factor of 1 to each term. Most rotational profiles resemble either the ethane or ethylene examples above, and a popular expression for the torsional energy is... 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational banier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have (slightly) different balances of these terms, and while one force field may contribute a conformational difference primarily to steric interactions, another may have the... 

Werpetinski, K. S., Cook, M., 1997, A New Grid-Free Density Functional Technique Application to hie Torsional Energy Surfaces of Ethane, Hydrazine, and Hydrogen Peroxide , J. Chem. Phys., 106, 7124. 

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. 

Torsional energy and torsional strain Torsional energy is the energy required for rotating about the C-C a bond. In ethane, this is very low (only 3 kcal). Torsional strain is the strain observed when a conformer rotates away from the most stable conformation (i.e. the staggered form). Torsional strain is due to the slight repulsion between electron clouds in the C-H bonds as they pass close by each other in the echpsed conformer. In ethane, this is also low. 

The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 12.6 kJ/mol (3.0 kcal/mol) higher in energy. At room temperature, this barrier is easily overcome and the molecules rotate constantly. 

A C—H bond eclipsed with another C—H bond contributes 4.2 kJ/mol (1.0 kcal/mol) torsional energy (one- third of eclipsed ethane). A C—H bond eclipsed with a C—CH3 bond contributes 5.4 kJ/mol (1.3 kcal/mol). 

Torsional energy of propane. When a C—C bond of propane rotates, the torsional energy varies much like it does in ethane, but with 13.8 kJ/mol (3.3 kcal/mol) torsional energy in the eclipsed conformation. 

The energy required to rotate the ethane molecule about the carbon-carbon bond is called torsional energy. We speak of the relative instability of the eclipsed conformation—or any of the intermediate skew conformations—as being due to torsional strain. 

The energy required to rotate the ethane molecule about the C-C bond is called its torsional energy. Torsional strain is the repulsion between neighboring bonds (electron clouds) that are in an eclipsed relationship. 

Figure 3-9 shows a graph of the torsional energy of propane as one of the carbon-carbon bonds rotates. The torsional energy of the eclipsed conformation is about 13.8 kJ/mol (3.3 kcal/mol), only 1.2 kJ (0.3 kcal) more than that required for ethane. 

A very different explanation was put forward by Pophristic and Goodman. They proposed that the rotational barrier in ethane results not from steric destabilization of the eclipsed conformation but, instead, from stabilization of the staggered conformation arising from delocalization of the cr bonding electrons. We will discuss the mechanism of this proposed stabilization in Chapter 4, but there is one point to be made here. When observable physical properties such as torsional energy are attributed to nonobservables—that is, to concepts such as steric effects that are inherently associated with other nonobservables such as molecular orbitals—it is difficult to establish the origin of the physical property... 

We can explore bond torsion in ethane to understand the barrier to internal rotation of one bond relative to another in saturated carbon chains, such as those found in lipids. The potential energy of... 

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