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

Using liquid state H-1, H-2 and F-19 NMR spectroscopy in the temperature range 110-130 K we have studied the hydrogen-bonded anions (FH)(2)F- and (FH)(3)F- and their partially and fully deuterated analogs dissolved in the low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)(4)N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)(2)F-, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)(3)F- 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.