Ethane torsion angles

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... 

Conformations m which the torsion angles between adjacent bonds are other than 60° are said to have torsional strain Eclipsed bonds produce the most torsional strain staggered bonds none Because three pairs of eclipsed bonds are responsible for 12 kJ/mol (2 9 kcal/mol) of torsional strain m ethane it is reasonable to assign an energy cost of 4 kJ/mol (1 kcal/mol) to each pair In this chapter we 11 learn of additional sources of strain m molecules which together with torsional strain comprise steric strain... 

Sketch the form of the potential energy as a function of torsional angle cj) for the torsional vibration in (a) ethane, (b) CH3NO2, (c) 2-fluorophenol, (d) CH2FOH, and... 

The torsional strain is a sinusoidal function of the torsion angle. Torsional strain results from the barrier to rotation about single bonds as described for ethane on p. 56. For molecules with a threefold barrier such as ethane, the form of the torsional barrier is... 

Fig. 3.1. Potential energy as a function of torsion angle for ethane.

Step through the sequence of structures depicting bond rotation in ethane. Plot energy (vertical axis) vs. HCCP torsion angle (horizontal axis). Do the minima correspond to staggered structures Do the maxima correspond tc eclipsed structures If not, to what do they correspond ... 

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... 

The lowest energy of ethane H3C—CH3 is the staggered form, with the equilibrium torsion angle of 60°. 

For example, where the reaction is rotation about the carbon-carbon bond in ethane, the reaction coordinate may be thought of as simply the HCCH torsion angle, and the structure may be thought of in terms of this angle alone. Thus, staggered ethane (both the reactant and the product) is a molecule for which this angle is 60° and eclipsed ethane is a molecule for which this angle is 0°. 

In open chain molecules the torsional angles are the obvious independent variables for the potential function. But even in the simplest case, that is ethane, the origin of the torsional barrier is not fully understood, though ab initio calculations represent the experimental barrier fairly well26 For larger molecules theoretical predictions for the potential functions are often based upon semiempirical molecular mechanics calculations27,2S ... 

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