NMR Chemical Shift of a Helium Atom as a Probe for Electronic Structure of FH, F^-, (FHF)^-, and FH2⁺

In this work, we present the first results of outer electronic shell visualization by using a 3He atom as a probe particle. As model objects we have chosen F-, FH, and FH2+ species, as well as the hydrogen-bonded complex FH···F- at various H···F- distances (3.0, 2.5, 2.0, and 1.5 Å and equilibrium at ca. 1.14 Å). The interaction energy of investigated objects with helium atom (CCSD/aug-cc-pVTZ) and helium atom chemical shift (B3LYP/pcS-2) surfaces were calculated, and their topological analysis was performed. For comparison, the results of standard quantum mechanical approaches to electronic shell visualization were presented (ESP, ELF, ED, ∇2ED). We show that the Laplacian of helium chemical shift, ∇2δHe, is sensitive to fluorine atom lone pair localization regions, and it can be used for the visualization of the outer electronic shell, which could be used to evaluate the proton accepting ability. The sensitivity of ∇2δHe to lone pairs is preserved at distances as large as 2.0-2.5 Å from the fluorine nucleus (in comparison with the distance to ESP minima, located at 1.0-1.5 Å or maxima of ELF, which are as close as 0.6 Å to the fluorine nucleus).