Probing the n → π* carbonyl-carbonyl interactions in the formaldehyde-trifluoroacetone dimer by rotational spectroscopy.

The non-covalent bonding features of carbonyl-carbonyl interactions have been investigated in the dimer of formaldehyde and trifluoroacetone using high resolution rotational spectroscopy combined with quantum chemical calculations. The observation of all possible isotopic substitutions for the heavy atoms in the complex enabled the determination of the accurate structure, characterized by the antiparallel arrangement of the two C=O bonds. The two moieties are connected through a dominant n → π* interaction enhanced by one weak C-H⋯O hydrogen bond, as revealed by supporting natural bond orbital analysis and symmetry-adapted perturbation theory analysis. Further computational investigations on 17 related adducts stabilized by carbonyl-carbonyl n → π* interactions show how the interaction strength is regulated by the incorporation of either electron-donating or withdrawing functional groups.