The extra 12 kJ/mol of energy present in the eclipsed conformer of ethane is called torsional strain. Its cause has been the subject of controversy, but the major factor is an interaction between C-H bonding orbitals on one carbon with antibonding orbitals on the adjacent carbon, which stabilizes the staggered conformer relative to the eclipsed conformer. Because the total strain of 12 kJ/mol arises from three equal hydrogen-hydrogen eclipsing interactions, we can assign a value of approximately 4.0 kJ/mol (1.0 kcal/mol) to each single interaction. The barrier to rotation that results can be represented on a graph of potential energy versus degree of rotation in which the angle between C-H bonds on front and back carbons as viewed end-on (the dihedral angle) goes full circle from 0 to 360 Energy minima occur at staggered conformations, and energy maxima occur at eclipsed conformations, as shown in Figure 3.7. (หน้า 94)
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