The anomeric effect in anionic systems XHm-CH2-NH− and XHm-CH2-CH2−, with XHm = F, OH, NH2 and CH3

Ab initio geometry optimizations were performed at the RHF/6-31 + G* level on anionic acetal-like systems of the form XHm-CH2-NH− and XHm-CH2-CH2−, with XHm = F, OH, NH2, and CH3. Using Fourier analysis combined with energy decomposition methods, the electronic part of the anomeric stabilization, e, has been evaluated. For the NH− systems, [Formula: see text]was calculated to be −16.5 for F, −5.2 for OH, −2.7 for NH2, and −1.0 for CH3. For the CH2− systems the [Formula: see text] values are −8.1 for F, −5.3 for OH, −3.0 for NH2 and −1.2 for CH3. The units are kcal/mol. Comparing the trends in calculated energies, bond distances, and charges with those obtained for the model compounds CH2-NH and CH2-CH2, the anomeric effect can be understood as the tendency of the anion to form a π-bond in the CH2-NH− orCH2-CH2− portion of the molecule, caused by charge transfer from this portion to XHm. The CH3-CH2-NH− and CH3-CH2-CH2− systems form an exception.