Competition of C(sp2) X•••O Halogen Bonding and Lone Pair•••π Interactions: Cryospectroscopic Study of the Complexes of C2F3X (X = F, Cl, Br, and I) and Dimethyl Ether

Inspection of the electrostatic potential of C2F3X (X = F, Cl, Br, and I) revealed a second electropositive region in the immediate vicinity of the C=C double bond apart from the sigma hole of chlorine, bromine, and iodine, leading to C(sp(2))-X center dot center dot center dot Y halogen bonding, through which complexes stabilized by so-called lone pair center dot center dot center dot pi interactions can be formed. Consequently, the experimental studies for the complexes of dimethyl ether with C2F3X (X = F, Cl, Br, and I) not only allowed one to experimentally characterize and rationalize the effects of hybridization on halogen bonding but, for the first time, also allowed the competition of C-X center dot center dot center dot Y halogen bonding and lone pair center dot center dot center dot pi interactions to be studied at thermodynamic equilibrium. Analysis of the infrared and Raman spectra reveals that in the cryosolutions of dimethyl ether and C2F3I, solely the halogen-bonded complex is present, whereas C2F3Br and C2F3Cl give rise to a lone pair center dot center dot center dot pi bonded complex as well as a halogen-bonded complex. Mixtures of dimethyl ether with C2F4 solely yield a lone pair center dot center dot center dot pi bonded complex. The experimentally derived complexation enthalpies for the halogen bonded complexes are found to be -14.2(5) kJ mol(-1) for C2F3I center dot DME and -9.3(5) kJ mol(-1) for C2F3Br center dot DME. For the complexes of C2F3Cl with dimethyl ether, no experimental complexation enthalpy could be obtained, whereas the C2F4 center dot DME complex has a complexation enthalpy of -5.5(3) kJ mol(-1). The observed trends have been rationalized with the aid of an interaction energy decomposition analysis (EDA) coupled to a Natural Orbital for Chemical Valence (NOCV) analysis and also using the noncovalent interaction index method.