An ab initio description of the low-lying electronic states of NF2 and its ions

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, 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.

Positive ion chemistry of SiH4/NF3 gaseous mixtures studied by ion trap mass spectrometry

The positive ion chemistry occurring in silane/nitrogen trifluoride gaseous mixtures has been investigated by ion trap mass spectrometry. Reaction sequences and rate constants have been determined for the processes involving the primary ions SiH(n)(+) (n = 0-3) and NF(x)(+) (x = 1-3) and the secondary ions obtained from their reactions with SiH(4) and NF(3). The SiH(n)(+) efficiently react with NF(3) and undergo cascades of abstraction and scrambling reactions which form the fluorinated and perfluorinated cations SiHF(m)(+) (m = 1, 2), SiH(2)F(+) and SiF(x)(+) (x = 0-3). Fluorinated Si(2)- clusters such as Si(2)H(2)F(+), Si(2)H(3)F(+) and Si(2)H(5)F(+) were also observed. The reaction of both SiH(3)(+) and SiH(2)F(+) with NF(3) produces the elusive fluoronitrenium ion NHF(+). Any NF(x)(+) reacts with SiH(4) mainly by charge transfer. Additional ionic products are, however, observed which suggest intimate reaction complexes. Worth mentioning is the formation of SiNH(2)(+) from the reaction of both NF(+) and NHF(+) with SiH(4). The primary ions NF(2)(+) and SiH(2)(+) are also "sink" species in our observed chemistry.