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

Abstract

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 σ hole of chlorine, bromine, and iodine, leading to C(sp(2))-X···Y halogen bonding, through which complexes stabilized by so-called lone pair···π 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···Y halogen bonding and lone pair···π 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···π bonded complex as well as a halogen-bonded complex. Mixtures of dimethyl ether with C2F4 solely yield a lone pair···π bonded complex. The experimentally derived complexation enthalpies for the halogen bonded complexes are found to be -14.2(5) kJ mol(-1) for C2F3I·DME and -9.3(5) kJ mol(-1) for C2F3Br·DME. For the complexes of C2F3Cl with dimethyl ether, no experimental complexation enthalpy could be obtained, whereas the C2F4·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.

Competition between hydrogen and halogen bonding in halogenated 1‐methyluracil: Water systems

“…Since the pioneering work by Brinck et al, many studies have used molecular electrostatic potential maps to demonstrate the σ‐hole in possible halogen‐bond donors, see for example references . Figure shows the electrostatic potential surfaces of the halogenated methyluracil molecules studied in this work.…”

Lone pair···π interactions involving an aromatic π-system: Complexes of hexafluorobenzene with dimethyl ether and trimethylamine

“…For DME-d 6 the vibrational modes 13 , 20 and 19 have a large overlap, so that the spectra had to be decomposed in order to determine frequencies. For TMA the discrepancy for the 4 mode can be explained by the overlap with 16 mode, which has a red shift, as calculated. By constructing Van 't Hoff plots from measurements between 120 K and 156 K, 7 experimental complexation enthalpies between −5.0 and −6.8 kJ mol −1 have been determined for the lp··· complex of C 6 F 6 with DME(-d 6 ), the average being of −6.0(6) kJ mol −1 .…”

“…The existence of lp··· bonded complex at thermodynamic equilibrium between the donor molecules C 2 F 4 and C 2 F 3 Cl and Lewis bases dimethyl ether [16] and trimethylamine [17] has already been demonstrated. Furthermore, lp··· complex was also found experimentally using pulsed-jet Fourier transform microwave spectroscopy for the complexes of C 2 F 3 Cl with water [18], dimethyl ether and ammonia [19].…”

Halogen bonding with the halogenabenzene bird structure, halobenzene, and halocyclopentadiene

“…We observed this before, and changing the nature of the halogen atom (from Cl to Br) was listed as one of the measures to make X‐bonds stronger than hydrogen bonds in H 2 C=S• • •HOX . The increasing strength of halogen bonds involving molecules with the heavier halogens has also previously been linked with the increased σ–hole for these systems, as visualized using molecular electrostatic potential maps . We will use these features to evaluate the existence of X‐bonds in the complexes studied in this work.…”