The 3 kcal/mole energy barrier to rotation about the C–C bond in ethane is commonly ascribed to steric repulsion of the hydrogen atoms or electron repulsion of the C–H bonds. However, many theoreticians have proposed that the barrier is due to subtle molecular orbital interactions, such as hyperconjugation involving vicinal hydrogens. The upper equation in the following diagram shows one such relationship, which would be repeated for the remaining two anti-periplanar vicinal pairs. Rapid bonding relaxation during the rotation results in a small change in C–C bond length, from 1.535 Å (in the staggered conformer) to 1.545 Å (in the eclipsed), which is reflected by this hyperconjugation. The HOMO-LUMO interaction depicted below the equation provides another perspective. It is not clear, however, why an equivalent syn-periplanar hyperconjugation would not similarly stabilize the eclipsed conformer. Molecular orbital calculations have provided some clarification of this eclipsing strain, but arguments concerning the relative importance of various factors continue. A molecular orbital diagram of ethane is presented at the bottom left, and a jmol model of the staggered conformer of ethane at its right. Each of the bonding orbitals and the lowest antibonding orbital may be examined by clicking the appropriate button. The anti-periplanar relationships in πY', πZ' and σS* (LUMO) are evident.