[¦dəb·əl ¦bänd ′shift] (organic chemistry) In an organic molecular structure, the occurrence when a pair of valence bonds that join a pair of carbons (or other atoms) shifts, via chemical reaction, to a new position, for example, H2C=C‒C‒CH2(butene-1) to H2C‒C=C‒CH2(butene-2).