C–X⋯π halogen and C–H⋯π hydrogen bonding: interactions of CF₃X (X = Cl, Br, I or H) with ethene and propene

Abstract

Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I with ethene and propene dissolved in liquid argon has been investigated. For CF(3)Br and CF(3)I, evidence was found for the formation of C-X···π halogen bonded 1:1 complexes. At a higher ratio of CF(3)I/propene, weak absorptions due to a 2:1 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -5.3(2) kJ mol(-1) for CF(3)Br·ethene, -7.5(2) kJ mol(-1) for CF(3)I·ethene, -5.6(1) kJ mol(-1) for CF(3)Br·propene, -8.8(1) kJ mol(-1) for CF(3)I·propene and -16.5(6) kJ mol(-1) for (CF(3)I·)(2)propene. The complexation enthalpies of the hydrogen bonded counterparts, with CF(3)H as the Lewis acid, were determined to be -4.6(4) kJ mol(-1) for CF(3)H·ethene and -5.1(2) kJ mol(-1) for CF(3)H·propene. For both hydrogen bonded complexes, a blue shift, by +4.8 and +4.0 cm(-1), respectively, was observed for the C-H stretching mode. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.

Exploring the C-X…π Halogen Bonding Motif: An Infrared and Raman Study of the Complexes of CF3X (X = Cl, Br and I) with the Aromatic Model Compounds Benzene and Toluene

“…The general trends showing a significant increase from CF 3 Cl towards CF 3 I are in line with the generally accepted ideas suggesting a significant increase in size and depth of the respective σ-holes [18]. The observed increase in stability when passing from benzene to toluene is in line with the electron donating character of the additional methyl group [13]. The X … C π interatomic distances are obtained by calculating the distance between the interacting halogen atom and nearest carbon atom of the toluene ring; The values for the van der Waals radii used to calculate sum vdW are 1.70 Å for C, 1.80 Å for Cl, 1.95 Å for Br and 2.15 Å for I.…”

“…This suggests that, analogous to the 2:1 halogen bonded complexes observed in previous studies [10,12,13], the binding of the second CF 3 I unit weakens the first halogen bond and vice versa. The occurrence of an anti-cooperative effect is also illustrated by the characteristic frequencies for the 1:1 and 2:1 complexes and those of the monomers, summarized in Table 7, showing that the shift for modes in the benzene moiety for the 2:1 complex is larger than that for the 1:1 complex, while for the modes in the competing CF 3 I moieties the opposite trend is observed.…”

“…The vibrational spectra of the trifluorohalomethanes CF 3 Cl, CF 3 Br and CF 3 I and of benzene dissolved in liquid rare gases have been studied in detail before [10,13,20,21] and are not commented here. The observed frequencies for toluene dissolved in LKr and their assignments, based on literature data for the vapor and liquid phases [19] and on the harmonic vibrational frequencies derived at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels, are summarized in Table S7.…”

“…The Lewis bases examined so far are dimethyl ether [10], trimethylamine [11] and dimethyl sulfide [12]. In addition, weak C-X···π halogen bonded complexes involving the isolated π system in the alkenes ethene and propene were reported [13]. The cryosolutions used create a weakly interacting environment that, combined with low temperatures, leads to small band widths in the spectra and that facilitates the detection of bands due to complexes that are only slightly shifted from the corresponding monomer modes, as is often the case for weak complexes.…”

Preferred Configurations of Peptide–Peptide Interactions

“…All geometries were optimized at the ab initio MP2/aug-cc-pVDZ level of theory which has been shown to be of high accuracy especially for those systems with intermolecular interactions of the type of interest here [43][44][45][46][47][48][49] where the data are in close agreement with CCSD(T) with larger basis sets [50][51][52] . Optimizations were carried out both with and without inclusion of counterpoise 53 in the algorithm.…”