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Why can a molecule’s shape depend on the time scale used to observe it?

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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.)

A molecule's observed shape can depend on the time scale of observation because molecules are dynamic, mobile entities that constantly undergo various rotational and vibrational motions. These motions require energy to occur.1

If a molecule undergoes these motions rapidly, an observation made on a slow time scale may perceive the molecule as having an average shape or structure, effectively blurring the distinct positions of its atoms. Conversely, if the observation is made on a very fast time scale—or under conditions where the energy is insufficient to overcome the barriers to these motions—the molecule may be observed in a specific, discrete conformation.1

In the context of isomerism, this means that certain structures may appear identical on one time scale but distinct, or isomeric, on another. For an arrangement to be classified as a true isomer, it must represent an energy minimum, meaning it must reside within an energy well.1