Abstract
The formation of halogen bonded complexes formed between the trifluorohalomethanes CF3Cl, CF3Br and CF3I and the Lewis bases benzene and toluene at temperatures below 150K was investigated using FTIR and Raman spectroscopy. Experiments using liquid krypton as solvent show that for both CF3Br and CF3I substantial fractions of the monomers can be involved in 1:1 complexes. In addition, weak absorptions illustrating the formation of 2:1 complexes between CF3I and benzene are observed. Using spectra recorded at temperatures between 120 and 140 K, observed information on the relative stability was obtained for all complexes by determining the complexation enthalpies in solution. The resulting values for CF3Br·benzene, CF3I·benzene and (CF3I)2·benzene are −6.5(3), −7.6(2) and −14.5(9) kJ mol−1. The values for CF3Br·toluene and CF3I·toluene are −6.2(5) and −7.4(5) kJ mol−1. The experimental complexation enthalpies are compared with theoretical data obtained by combining results from MP2/aug-cc-pVDZ(-PP) and MP2/aug-cc-pVTZ(-PP) ab initio calculations, from statistical thermodynamical calculations and from Monte Carlo Free Energy Perturbation simulations. The data are also compared with results derived for other C-X···π halogen bonded complexes involving unsaturated Lewis bases such as ethene and ethyne.
Highlights
Because of their role in biomolecular systems, protein-ligand binding and in supramolecular chemistry in general, the nature of C-X···Y halogen bonded interactions with X = Cl, Br or I has attracted widespread interest of chemists and biochemists [1,2,3]
Because the cryosolutions are in thermodynamical equilibrium, the solutions allow the determination of experimental data on the relative stability of the complexes observed
The average complexation enthalpies for the 1:1 complexes, obtained by analyzing and averaging out the data for a series of solutions and by correcting the slopes of the different regression lines for density variations in the temperature intervals used, are −6.5(3) kJ mol−1 for CF3Br.benzene, −7.6(2) kJ mol−1 for CF3I.benzene and (CF3I).benzene, −6.2(5) kJ mol−1 for CF3Br.toluene and −7.4(5) kJ mol−1 for CF3I.toluene
Summary
To deliver systematic data allowing the study of the nature of the weak halogen bonding and their role in various applications, we have recently initiated an infrared and Raman spectroscopic study of halogen bonded complexes formed in cryogenic solutions. Weak C-X···π halogen bonded complexes involving the isolated π system in the alkenes ethene and propene were reported [13]. Triggered by the recent experimental observation of the complexes of the trifluorohalomethanes with ethene and propene, and by the aim at expanding further our knowledge of C-X···Y halogen complexes, we have initiated an infrared and Raman spectroscopic study of the complexes formed between the aromatic model compounds benzene and toluene and the trifluorohalomethanes CF3Cl, CF3Br and CF3I. The experimentally obtained data and the results of the ab initio and Monte Carlo calculations, supporting the experimental study, are reported below
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