A cryospectroscopic infrared and Raman study of the CX⋯π halogen bonding motif: Complexes of the CF3Cl, CF3Br, and CF3I with ethyne, propyne and 2-butyne

• IR and Raman spectra of solutions in liquid krypton allows the study of CX⋯π halogen bonding motif.
• Concentration and temperature studies lead to the stoichiometry and complexation enthalpy of the complexes.
• Experimental studies are supported by ab initio and MC-FEP calculations. Agreement between experiment and theory is observed.

Experimental information on the CX⋯π halogen bonding motif was obtained by studying the formation of molecular complexes of CF3Cl, CF3Br and CF3I with ethyne, propyne and 2-butyne in liquid krypton, using FTIR and Raman spectroscopy. For CF3Br, experimental evidence was found for the formation of 1:1 complexes with propyne and 2-butyne only, while for CF3I spectroscopic features confirming the existence of the halogen bonded complexes were observed for ethyne, propyne and 2-butyne. In addition, at higher concentrations of CF3I and 2-butyne, weak absorptions due to a 2:1 complex were also observed. The experimental complexation enthalpies, obtained by using spectra recorded at temperatures between 120 K and 140 K, are −5.9(3) kJ mol−1 for CF3I.ethyne, −5.6(3) kJ mol−1 for CF3Br.propyne, −8.1(2) kJ mol−1 for CF3I.propyne, −7.3(2) kJ mol−1 for CF3Br.2-butyne, −10.9(2) kJ mol−1 for CF3I.2-butyne and −20.9(7) kJ mol−1 for (CF3I)2.2-butyne. The experimental study is supported by theoretical data obtained from ab initio calculations at the MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) levels, and Monte Carlo Free Energy Perturbation (MC-FEP) simulations. The experimental and theoretical values on the CX⋯π halogen-bonding motifs studied are compared with previously reported data for the complexes with ethene and propene and with preliminary results obtained for benzene and toluene.

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