Meso configuration, definition

The equilibrium ensemble of the configurations of meso-tartaric acid 10 a—c has umin = 0. From the definition of umm follows immediately that for all achiral molecules min = 0 holds. This applies also to those cases where asymmetric C-atoms are present but cancel each other because of overall molecular symmetry. 

The Lewis method of formulation therefore shows more isomers than actually exist. The minimum in curve C is an energy minimum with respect to both electron distribution and atomic configuration, and hence represents a definite compound but one that cannot be represented by a single Lewis formula. This compound is a meso-meric one. We have represented its energy and atomic configura-... 

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