Illustrated Glossary of Organic Chemistry

An energy difference. In Chem 14C and Chem 14D refers to the energy difference between quantum mechanical energy states (rotational, vibrational, electronic, nuclear spin, etc.).

The HOMO-LUMO ΔE (the HOMO-LUMO gap) is an important property for conjugated molecules.

In 1H-NMR spectroscopy, the ground state-excited state ΔE controls the energy of radio wave photon required for nuclear spin flip, and hence the proton's chemical shift.