Excluding hyperconjugation from the Z conformational preference and investigating its origin: formic acid and beyond

Abstract

Carboxylic acids, esters, secondary amides, and related molecules share a thermodynamic preference for the Z arrangement of their XC–Y–R moiety. This conformational predisposition is known as the Z effect and its most common explanation invokes the hyperconjugation from a Y lone pair to the σ_CX* orbital. In this work, we present clear topological evidence that hyperconjugation is not responsible for the Z preference. Diverse tools defined within the quantum chemical topology framework (such as, for example, atomic and electron localization function populations or the interacting quantum atoms energy decomposition) were used to analyse the evolution of formic acid from the E conformer towards the Z conformation. The results highlight the important role of the π resonance in the barrier between conformers and they also indicate that the hyperconjugative interaction lacks a leading role. Concretely, in an XC–Y–R structure, the X⋯R interaction seems to be the key to understanding the preference for the Z arrangement of the moiety. Interestingly, our proposed explanation can be extended to a wide range of molecules presenting the same conformational preference, such as proteins or peptide nucleic acids.