I'm reading Organic Chemistry by David Klein (2nd Ed), and when discussing the causes of torsional strain, he writes this
Based on recent quantum mechanical calculations, it is now believed that the staggered conformation possesses a favorable interaction between an occupied, bonding MO and an unoccupied, antibonding MO. This interaction lowers the energy of the staggered conformation.
This favorable interaction is only present in the staggered conformation. When the C—C bond is rotated (going from a staggered to an eclipsed conformation), the favorable overlap above is temporarily disrupted, causing an increase in energy.
I've searched online and most sources provide a similar explanation and diagram. My issue is mainly with the diagram: why are the MOs illustrated as though they span only the C—H bonds? I thought that MOs were supposed to span the entire molecule, rather than being localized along/parallel to the internuclear axes of two nuclei. Are these MOs correct? Do MOs cover the entire molecule or do they each exist between two bonding atoms?
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