Extreme NMR shielding in fluoro-nitrogen cations

Abstract

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, NF²⁺). 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. ¹⁵N and ¹⁹F NMR chemical shifts correlate well with available experimental data. Extreme ¹⁹F chemical shifts are predicted for HNF⁺ (1628.9 ppm) and NH₂F²⁺ (1298.0 ppm), which are by far the largest ¹⁹F chemical shifts ever reported and well outside the known range of +865 ppm (F₂O₂) to −448 ppm (ClF). The ¹⁵N chemical shift of −1283.07 ppm in HNF⁺ is similarly extreme, being well outside the known range of ¹⁵N chemical shifts of −730 to 260 ppm (CH₃NO₂ 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.