Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran-Formaldehyde Complex

The 1:1 benzofuran-formaldehyde complex has been chosen as model system for analyzing pi ->pi* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. A joint rotational spectroscopy-quantum chemistry strategy unveiled the dominant role of pi ->pi* interactions in tuning the intermolecular interactions of such adduct. The exploration of the intermolecular potential energy surface led to the identification of 14 low-energy minima, with 4 stacked isomers being more stable than those linked by hydrogen bond or lone-pair ->pi interactions. All energy minima are separated by loose transition states, thus suggesting an effective relaxation to the global minimum under the experimental conditions. This expectation has been confirmed by the experimental detection of only one species, which was unambiguously assigned owing to the computation of accurate spectroscopic parameters and the characterization of 11 isotopologues. The large number of isotopic species opened the way to the determination of the first semi-experimental equilibrium structure for a molecular complex of such a dimension.

Automated summary

The 1:1 benzofuran-formaldehyde complex was chosen as a model system to analyze pi ->pi* interactions in supramolecular organizations involving heteroaromatic rings and carbonyl groups. The study revealed the dominant role of pi ->pi* interactions in tuning intermolecular interactions, with stacked isomers being more stable than those linked by hydrogen bond or lone-pair ->pi interactions. The experimental detection of only one species confirmed the expectation of effective relaxation to the global minimum under the given conditions.