Zusammenfassung
The correlation of the rotation about the bond C'-C-alpha and the pyramidalization of the sp(2) hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron was investigated for the substructure C-beta-C alpha H-C'(=O)-OMe. A search in the Cambridge Structural Database (CSD) gave 15,295 structures with a substituted methyl acetate group at the end of the molecules. The ...
Zusammenfassung
The correlation of the rotation about the bond C'-C-alpha and the pyramidalization of the sp(2) hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron was investigated for the substructure C-beta-C alpha H-C'(=O)-OMe. A search in the Cambridge Structural Database (CSD) gave 15,295 structures with a substituted methyl acetate group at the end of the molecules. The scatter plot of the rotation angle psi = O=C'-C-alpha-C-beta versus the pyramidalization angle theta = O(MeO)C'C-alpha and the psi/theta(av) curve show an unusual undulating pattern with three maxima and minima for a 360 degrees rotation about the bond C'-C-alpha. There is no net chiral induction from the (M-psi)/(P-psi) conformations of the bond C'-C-alpha to the (R-theta)/(S-theta) configurations of the flattened tetrahedron because the contributions of the three maxima and minima cancel each other. The wavelike psi/theta(av) curve demonstrates that the rotation about the bond C'-C-alpha and the pyramidalization of the group C'(=O)(OMe)C-alpha are not independent processes. They are coupled with each other in one common molecular motion. The psi/theta(av) curve appears as the third harmonic of a sinusoidal fundamental frequency. DFT calculations of the propanoate anion, methyl propanoate, and methyl 2-methylpropanoate confirm the results of the CSD search.
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