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Why must a structure be an energy minimum to qualify as an isomer?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — isomerism Tetrahedral geometry of methane: (A) stick-and-ball model and (B) diagram showing bond angles and distances. (Plain bonds represent bonds in the plane of the image; wedge and dashed bonds represent those directed toward and away from the viewer, respectively.)

In chemistry, an isomer must be an energy minimum—meaning it lies in an energy well—because molecules are mobile entities that constantly undergo rotational motions and changes in shape.1

Because these motions are governed by energy conditions, a structure that is not at an energy minimum would be unstable and prone to spontaneous transition into a more stable state. By residing in an energy well, an isomer achieves a stable state that allows it to exist as a distinct chemical entity with identifiable physical and chemical properties. If the energy barriers between different conformations are sufficiently high, these stable states can be isolated as distinct stereoisomers. Conversely, if a structure does not occupy a stable energy minimum, it is generally considered a transient or unstable intermediate rather than a distinct isomer.12