H/D isotope effects on the low-temperature NMR parameters and hydrogen bond geometries of (FH)2F− and (FH)3F− dissolved in CDF3/CDF2Cl

Abstract

Using liquid state ¹H, ²H and ¹⁹F NMR spectroscopy in the temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)₂F⁻ and (FH)₃F⁻ and their partially and fully deuterated analogs dissolved in the low-freezing freon mixture CDF₃/CDF₂Cl, in the presence of (C₄H₉)₄N⁺ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)₂F⁻, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)₃F⁻ have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.