An energetic penalty from bond angles, torsions or nonbonded contacts forced away from favourable values by ring closure.
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Ring strain is a type of strain that occurs in cyclic molecules (or rings) when the bond angles deviate from their ideal values, leading to increased energy and instability of the molecule. This phenomenon is particularly relevant in smaller rings, where atoms are forced into bond angles that are significantly different from those found in typical tetrahedral (109.5°) or trigonal planar (120°) arrangements.