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

Abstract

In this work, we present the first results of outer electronic shell visualization by using a He atom as a probe particle. As model objects we have chosen F, FH, and FH 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, ∇ED). We show that the Laplacian of helium chemical shift, ∇δ, 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 ∇δ 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).

Lone pairs mapping by Laplacian of 3He NMR chemical shift

“…Thus, it can be concluded, that for nitrogen-containing species, similarly to fluorine-containing ones, [11] there is a correlation between maximal values of |MESP| Analysis of dependencies presented in Figure 5 for oxygen containing species (5) and (6) mation for the studied set of electroneutral molecules. However, [42] it was shown that in case of anion F − and cation FH 2 + MESP extrema are missing, while r 2 δ He retains its features. We hope that this finding could be extrapolated to larger sets of molecules with localized LPs, at least to molecules similar to the ones studied here.…”

“…In other words, MESP and ∇ 2 δ He give qualitatively comparable information for the studied set of electroneutral molecules. However, it was shown that in case of anion F − and cation FH 2 + MESP extrema are missing, while ∇ 2 δ He retains its features. We hope that this finding could be extrapolated to larger sets of molecules with localized LPs, at least to molecules similar to the ones studied here.…”

“…At this level, all the essential characteristics of Laplacian isosurfaces are retained. This gives an opportunity to decrease the computational time significantly (in comparison with previous works), so that the total computational time is comparable with that required for methods of LPs visualization based on the electron density analysis (MESP, ELF). Isotropic nuclear magnetic shielding σ He was converted into chemical shift δ He by subtraction of shielding of a free helium atom from the shielding of the helium atom in a complex.…”

“…In our recent papers, we proposed and tested on some fluorine‐containing model objects a new quantum‐mechanical approach of outer electronic shell visualization of isolated molecules and their hydrogen‐bonded complexes. The proposed methodology is based on the idea that near LP regions the electronic shell of 3 He atom is slightly perturbed.…”

“…[38] Several experimental works demonstrate the possibility of using 3 He NMR methods to determine the structure of zeolites, [39] parameters of the pore size distribution [40] and the aromaticity of fullerenes. [41] In our recent papers, [11,42] Besides, a large basis set (aug-cc-pVDZ) was used for NMR calculation, therefore computational time was significant and incomparable with the time required for other methods.…”