Rotation about a Covalent Bond and Pyramidalization of an Adjacent sp² Center are a Synchronized Molecular Motion

The correlation of the rotation about the C_α-C' bond and the pyramidalization of the sp²-hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron in the substructure C_β-C_αH-C'(═O)-OMe of substituted methyl acetates revealed that the two processes are not independent of each other but parts of a common molecular motion, as outlined in the preceding back-to-back paper. In the present study, we generalized the substructure to X_β-C_αH-C'(═Y)-Z with X, Y, and Z = O, N, C, and S, extending the analysis to several hundred thousand structures of the type carboxylates, carboxamides, ketones, imines, olefins, peptides, lactates, carbothioates, and phenyl derivatives, retrieved from the Cambridge Structural Database. ψ/θ Scatter plots of the individual structure points and their averaging in ψ/θ_av curves result in wavelike patterns with three maxima and minima and inversion symmetry at ψ = 0° and ±180 for a 360° rotation of C_β about the C_α-C' bond. The pyramidalization of the sp²-hybridized group C_αC_iC_oC_o', which is part of the aromatic system, even disturbs the planarity of phenyl rings. Density functional theory calculations confirm the results of the CSD search.