Potential energy as a function of torsion angle for ethane

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

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... 

Not all molecular mechanics force fields use torsional potentials it may be possible to rely upon non-bonded interactions between the atoms at the end of each torsion angle (the 1,4 atoms) to achieve the desired energy profile. However, most force fields for orgamc molecules do use explicit torsional potentials with a contribution from each bonded quartet of atoms A-B—C-D in the system. Thus there would be nine individual torsional terms for ethane and 24 for benzene (6 x C—C—C—C, 12 x C—C—C—H and 6 x H—C—C—H). Torsional potentials are almost always expressed as a cosine series expansion. One functional form is ... 

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