Comparison between diketones and diamides: Effects of carbonyl groups on the conformational preferences of small aliphatic segments

The conformational preferences of a set of diketones have been investigated by ab initio quantum mechanical calculations. In addition, SCRF calculations have been performed in order to ascertain the effect of aqueous and CCl4 solvents. Results indicate that diketones with two or three methylene units between the two carbonyl groups present a folding in the central aliphatic segment. More specifically, the central methylene units adopt a gauche conformation. Results have been compared with those previously obtained for related diamides. Finally, the conformational preferences of a compound with only one keto group have been studied. In this case, the lowest energy conformation corresponds to an extended structure, which indicates that the presence of the second carbonyl group plays a crucial role in the folding of methylene units.