Ethane, bond angles rotational barrier

If this carbon holds in different atoms, the bond angles are somewhat (a little) changed and the tetrahedron ceases to be regular. But the real foundation for conformational study was laid in 1935 when it was observed that there was discrepancy between the entropy of ethane as found from the heat capacity measurements and as calculated from spectral data. From this the physical chemists concluded that there must be hindrance to rotation about the carbon bond in ethane. Later it was found that there was tortional barrier to free rotation to the extent of about 2.8 K cals per mole. 

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 barrier 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... 

The molecules with E = N are planar. The molecules with E = P or As are nonplanar, with significantly acute angles. For E = P or As, the planar form is the transition state for inversion, which interconverts the nonplanar minima. The twisting about the M-E bond when E = P or As has rotational barriers on the order of ethane, particularly if the M is not boron. 

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... 

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