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The theory of conformational analysis

The theory is introduced with reference to trying to explain why the eclipsed form of ethane is ~2.6 kcal/mol higher in energy than the staggered conformation (this barrier was inferred by Pitzer and Kemp from trying to reproduce the correct entropy for ethane: DOI: 10.1021/ja01281a014). There are three effects which are added together:

Od Hassel (in 1943) and Derek Barton (in 1948, DOI: 10.1039/JR9480000340) followed Bill Penney (1934) in recognising "The significance of van der Waals atomic radii for the stereochemical structure of molecules". Hassel and Barton won the Nobel prize in 1969. Remember also that whilst the interaction energy for any two atoms is relatively small (~1 kJ = 0.3 kcal/mol), the total number of such interactions goes up very rapidly for larger molecules (and it can be very large indeed for molecules the size of e.g. proteins). For ethane, the difference in van der Waals/dispersion attractions for the two orientations of ethane is tiny (~0.05 kcal/mol) but again the effect grows strongly with the size of the molecule.

For wonderfully detailed, but still readable explanations of what goes on at the surface of a gecko's foot, see 10.1002/adfm.201100493. or on the topic of bio-inspired adhesives: 10.1002/adfm.201100982. The other famous example of how attractive dispersion forces can be is the difficulty in pulling apart two telephone directories.

There is a crucial difference between the three effects. Effects 1 and 2 are highly directional (anisotropic) whereas effect 3 is isotropic, which means it depends only on the distance between two non-bonded atoms and not on their relative orientation.

Summary of important concepts

  1. Conformational processes are dynamic and the thermodynamic equilibrium population of conformers about the C-C bonds is determined by:

© Henry S. Rzepa, 2010-2014. Hide|show Toolbar.