Extreme NMR shielding in fluoro-nitrogen cations.

The structure and NMR shielding of a set of N-F containing cations is reported to near-quantitative accuracy from extensive ab initio calculations. Currently, the shortest experimentally confirmed N-F bond is 1.2461(10) Å in NNF + , however CCSD(T)-F12b/cc-pVQZ-F12 optimised geometries suggest that even shorter N-F bonds are possible for both monocations (1.236 Å, HNF + ) and dications (1.098 Å, NF 2+ ). NMR shielding constants have been calculated in a composite manner with individual components from coupled-cluster expansions up to CCSDTQP and basis sets up to aug-cc-pCV8Z, together with vibrational and relativistic corrections. 15 N and 19 F NMR chemical shifts correlate well with available experimental data. Extreme 19 F chemical shifts are predicted for HNF + (1628.9 ppm) and NH 2 F 2+ (1298.0 ppm), which are by far the largest 19 F chemical shifts ever reported and well outside the known range of +865 ppm (F 2 O 2 ) to -448 ppm (ClF). The 15 N chemical shift of -1283.07 ppm in HNF + is similarly extreme, being well outside the known range of 15 N chemical shifts of -730 to 260 ppm (CH 3 NO 2 reference). This work highlights the application of state-of-the-art theoretical techniques, and provides accurate NMR properties of both isolated and yet unknown N-F cations, which can serve to guide and supplement NMR experimentation.