The phenomenon where molecules with the same molecular formula have different three-dimensional arrangements that are not mirror images of each other.
From 'diastereoisomer' + '-ism,' the abstract noun suffix, creating a term for the general concept and category of this type of molecular variation.
Diastereoisomerism is why tartaric acid (the stuff in wine) comes in left-handed, right-handed, and mixed forms—Pasteur's work on this actually launched the entire field of stereochemistry!
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