Effect of Fluorination on the Competition of Halogen Bonding and Hydrogen Bonding: Complexes of Fluoroiodomethane with Dimethyl Ether and Trimethylamine

To further rationalize the competition between halogen and hydrogen bonding, a combined experimental and theoretical study on the weakly bound molecular complexes formed between the combined halogen bond / hydrogen bond donor fluoroiodomethane and the Lewis bases dimethyl ether and trimethylamine (in standard and fully deuterated form) is presented. The experimental data are obtained by recording infrared and Raman spectra of mixtures of the compounds in liquid krypton, at temperatures between 120 K and 156 K. The experiments are supported by ab initio calculations at the MP2/aug-cc-pVDZ-PP level, statistical thermodynamics and Monte Carlo Free Energy Perturbation calculations. For the mixtures containing fluoroiodomethane and dimethyl ether a hydrogen bonded complex with an experimental complexation enthalpy of -7.0(2) kJ mol-1 is firmly identified. In contrast, only a single weak spectral feature is observed that can be tentatively assigned to the halogen bonded complex. For the mixtures involving trimethylamine, both halogen and hydrogen bonded complexes are observed, the experimental complexation enthalpies being -12.5(1) and -9.6(2) kJ mol-1 respectively. To evaluate the influence of fluorination on the competition between halogen and hydrogen bonding, the results obtained for fluoroiodomethane are compared with those of a previous study involving difluoroiodomethane.