Energy as a function of internuclear distance for nonbonded atoms

The interplay between torsional strain and nonbonded interactions can be illustrated by examining conformational isomerism in n-butane. The diagram relating potential energy to torsion angle for rotation about the C(2)-C(3) bond is presented in Fig. 3.3. 

The rotational profile of n-butane can be understood as a superimposition of van der Waals forces on the ethane potential energy diagram. The two gauche 

This is the value most usually quoted (actually 0.77 kcal/mol) obtained by Raman spectroscopy G.J. Szasz, N. Sheppard, and D. H. Rank, J. Chem. Phys. 16,704 (1948) a value of 0.68 kcal/mol has been obtained by NMR P. B. Woller and E. W. Garbisch, Jr., J. Am. Chem. Soc. 94, 5310 (1972). 

The populations of the various conformers are related to the energy differences between them by the equation 

The van der Waals forces are apparently of the attractive type in some halogenated hydrocarbons. This is illustrated for the case of n-propyl chloride, where the gauche conformation is slightly preferred over the anti conformation at equilibrium. The gauche is favored not only on entropy grounds, but on enthalpy grounds as well. For the equilibrium  

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Fig. 3.2. Energy as a function of internuclear distance for nonbonded atoms.

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