Expanding Lone Pair···π Interactions to Nonaromatic Systems and Nitrogen Bases: Complexes of C₂F₃X (X = F, Cl, Br, I) and TMA‑d ₉

The molecular electrostatic potential surface of unsaturated, locally electron-deficient molecules shows a positive region perpendicular to (a part of) the molecular framework. In recent years it has been shown both theoretically and experimentally that molecules are able to form noncovalent interactions with Lewis bases through this π-hole. When studying unsaturated perfluorohalogenated molecules containing a higher halogen atom, a second electropositive region is also observed near the halogen atom. This region, often denoted as a σ-hole, allows the molecules to interact with Lewis bases and form a halogen bond. To experimentally characterize the competition between both these noncovalent interactions, Fourier transform infrared and Raman spectra of liquefied noble gas solutions containing perfluorohalogenated ethylene derivatives (C₂F₃X; X = F, Cl, Br, or I) and trimethylamine­(-d ₉) were investigated. Analysis of the spectra shows that in mixed solutions of trimethylamine­(-d ₉) and C₂F₄ or C₂F₃Cl lone pair···π complex is present, while evidence for halogen-bonded complex is found in solutions containing trimethylamine­(-d ₉) and C₂F₃Cl, C₂F₃Br, or C₂F₃I. For all species observed, complexation enthalpies were determined, the values varying between −4.9(1) and −24.4 kJ mol^–1.