nonsuperimposable

Nonsuperimposable describes two objects that cannot be moved in space to occupy exactly the same position. No amount of rotation or translation can make one coincide with the other. The concept is fundamental in geometry and stereochemistry and is closely tied to chirality. An object is chiral if it is not nonsuperimposable on its mirror image; the two mirror images are nonsuperimposable and are called enantiomers. The classic example is the pair of human hands, which are mirror images but cannot be superimposed.

In chemistry, enantiomers are nonsuperimposable mirror images that arise from stereogenic centers or other forms of stereochemistry. They have the same connectivity of atoms but differ in the three-dimensional arrangement of that connectivity. The mirror image of a chiral molecule cannot be superimposed onto the original, even after rotation. Molecules that lack chirality are achiral and can be superimposed on their mirror images because they possess an internal plane of symmetry. Diastereomers are other stereoisomers that are nonsuperimposable but not mirror images of each other, often with distinct physical properties.

Determining nonsuperposability relies on three-dimensional models, visualization, or symmetry analysis. This concept has practical implications in fields such as pharmacology, where enantiomeric purity can affect biological activity and safety, and in materials science, where chiral molecules and crystals exhibit unique optical and functional properties.