A QTAIM-based energy partitioning for understanding the physical origin of conformational preferences: Application to the Z effect in O=C-X-R and related units

A quantum theory of atoms in molecules‐based energy partitioning was carried out for Z and E conformers of a series of O=C‐X‐R containing compounds. The results obtained for the simplest compound (formic acid) indicate that the attraction of the electron density within carbonyl oxygen by the nucleus of the acid hydrogen is the most important energy term for Z preference. This conclusion can be extended (mutatis mutandis) to larger carboxylic acids, esters, sulfur derivatives, secondary amides, and carbonyl isocyanates, and even explains the sequence of relative conformational energies in the HCXOH series (X = O, S, Se). In contrast, although the hyperconjugative model has been traditionally employed to explain this preference, we observe it is incompatible with: (i) relative values of diverse QTAIM atomic populations for the Z/E conformational equilibrium; (ii) conformational energies in the HCXOH series. © 2012 Wiley Periodicals, Inc. Energy partitioning based on QTAIM analysis indicates that the most important interaction in the stabilization of the Z conformer in formic acid is the attraction of the oxygen carbonyl electron density by the acid hydrogen nucleus. This method also reveals important shortcomings in the explanation provided by the hyperconjugative method.