No abstract

An ab initio study of methylenediamine and several methylated derivatives in the gas phase and aqueous solution was performed. The conformational preferences can be considered adequately described at the HF/6-31G * * //HF/6-31G * * level, because these results agree with those obtained using larger basis sets and including ZPE and electron correlation. The energy ordering is clearly dependent on the number and position of the methyl groups present in a molecule. For a first set of the compounds, the energies obtained were interpreted in terms of the anomeric effect because the favored conformers show two or one anti orientation between the nitrogen lone pair and the C-N polar bond. Reverse anomeric effects were found for a second set of compounds. The NBO analysis was used to interpret these energetic tendencies and the rotational barrier around the N-C bonds. Thus, the preference for the anti-Lp-N-C-N orientations is mainly due to charge delocalization, always stronger than the electrostatic and steric contributions included in the Lewis term. However, the origin for the reverse anomeric effect may be related to the steric hindrance associated with the methylation. The influence of water on the conformational preferences was evaluated by means of the PCM method. Contrary to expectation, the anomeric effect is not reduced in water, and the most stable conformers in the gas phase are maintained in solution. The electrostatic term of the free solvation energy is the main responsible of the energetic changes, and depends strongly on local solute-solvent interactions.