Carbon-13 NMR Spectra of Polybromoalkanes and Polychlorobromoalkanes. Structural Increments of Halogens in Polyhalogenated Groups

Article abstract of DOI:10.1002/mrc.1270160402

The 13C NMR spectra of 48 polychlorobromoalkanes have been studied.Unlike, the 13C signals of chlorine-containing groups (38-105 ppm), those of bromine-containing fragments, with the exception of CBr2 (60-70 ppm), appear in a rather narrow range (25-50 ppm) and are shifted to higher field in relation to similar chlorine-containing groups.The spin-spin coupling constants in similar bromine- and chlorine-containing groups practically coincide.Calculation of the chemical shifts for the polyhaloalkanes under study according to the additivity scheme, as previously observed for polychloroalkanes, renders values which are in considerable discord with experimental values (up to -32 ppm for CBr3).These discrepancies may be compensated for by corrections for the binary interaction of halogen atoms by grouping the halogen-containing fragments according to the geminal, vicinal, 1,3-, 1,3,5- and 1,2,3-arrangement of halogen atoms, and by introducing an increment for the position of the halogen at the secondary atom.It is established that as compared to 1-monohaloalkanes: (a) in the case of the geminal arrangement of halogen atoms the α- and γ-effects diminish (Δα from -3.2 to -8 ppm; Δγ=2.6 ppm), while the β-effect increases slightly (from 0 to 1.2 ppm); (b) in the case of a vicinal arrangement both the α- and β-effects diminish (by about -3.5 ppm) and the γ-effect remains constant, as if the vicinal system of the halogens was topologically insulated; (c) for the 1,3- and 1,3,5-arrangement of halogens their mutual influence is weak (about -0.5 ppm for each halogen atom in the α- and γ-positions); (d) the 1,2,3 system (serial arrangement of halogen atoms) is the sum of two vicinal fragments and hardly deviates from the additivity scheme; (e) the arrangement of a halogen at the secondary C atom enhances the α-effect (Δα=2.8 and 1.0 for methyl and methylene, respectively, in the case of Cl, and 3.5 and 3.7 ppm in the case of Br); the variation of the β-effect has a different sign in relation to CH3 and CH2 groups (+1.2 and -1.7 for Cl, and +2.5 and -1.0 for Br).More distant effects of halogens (δ and ε) were not considered.The determined increments (Δα, Δβ and Δγ) for the α-, β- and γ-effects of chlorine and bromine atoms allow the prediction of the 13C chemical shifts in polyhaloalkanes with an accuracy up to +/-1.5 ppm.Some deviations of up to +/-5 ppm may be connected with the influence of a three particle interaction of halogen atoms, which was taken into account only in the case of a geminal arrangement of halogen atoms.