The Pyramidalization of sp2 Centers in 3/2‐Systems Is a “Structural Breathing” Independent of Energy

In 3/2‐systems, such as the acetate anion H3CαC′OO−, an sp3 center and an sp2 center are connected by a covalent bond. The interaction of threefold and twofold symmetry results in the pyramidalization of the sp2 center during rotation about bond Esp3−Esp2. Rotation angles ψ=0°, ±60°, ±120°, and ±180° account for conformations with a symmetry plane containing the planar sp2 center CαC′OO. However, in all conformations with rotation angles ψ≠0°, ±60°, ±120°, and ±180° this symmetry plane is lost and pyramidalization must occur with maxima at rotation angles ψ=±30°, ±90°, and ±150°, because the two sides of the sp2 center CαC′OO are different. Inevitably, this leads to a pyramidalization/rotation profile θ/ψ with three maxima, three minima, and six zero‐crossings. Thus, myriads of 3/2‐compounds pyramidalize their sp2 centers each moment to the order of about 2° in a coupled pyramidalization/rotation molecular motion independent of energy.