Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Attractive interactions between carbonyl groups have been studied extensively, primarily due to their prevalence in protein structure. However, prior investigations have pointed to conflicting origins; earlier investigations identified dominant electrostatic dipolar interactions, while others have implicated lone pair n→* orbital delocalisation. Here we reconcile these observations. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance-dependent observation of inductive polarisation manifested via n→* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short-range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context-dependency of the dominance of electrostatic interactions and the occurence of n→* orbital delocalisation in C=O···C=O interactions.