Abstract
Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. 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 by n→π* 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 occurrence of n→π* orbital delocalisation in C=O⋅⋅⋅C=O interactions.
Highlights
Carbonyl groups are prevalent throughout chemistry and biology
The groups of Diederich and Raines previously employed the molecular balances 2[6,13] and 3-Y[2e] (Figure 1 B and C), respectively, to study C=O···C=O interactions, but came to different conclusions with regard to the major energetic contributions.[2d,e,3f,6,13] Diederich determined the carbonyl interaction in 2 to be driven by electrostatics, while Raines found n!p* orbital delocalisation between carbonyl groups to play an important role in the 3-Y series of balances
To investigate the apparent incongruity regarding the nature of carbonyl interactions we devised a new series of molecular balances, 1-X, to supplement the existing datasets (Figure 1 A)
Summary
The occurrence of C=O···C=O contacts (between the formamide oxygen and each X-substituted carbon, Figure 2) in the closed conformer of each balance was confirmed computationally (B3LYP/6-31G* and wB97X-D/ 6-31G*, Section S4.1), and by X-ray crystallography for 1-H and 1-Me These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.
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