FIG. 2.

A small two-dimensional ring molecule in the shape of a quadrilateral. The shaded regions schematically show the allowed geometrical variation for fixed topology indicative of the degree of flexibility. Configuration (a) is topologically distinct from (b) and (c). For identical atoms at each corner, configurations (b) and (c) represent the same topology of constraints, but are distinct otherwise. The framework in (a) is referred to as state H; where it has greater energy and conformational entropy than (b)/(c). The [(b) and/or (c)] framework is referred to as state L; where the bond along the diagonal leads to a lower energy and conformational entropy than (a).