Outer electronic shell visualization by NMR chemical shift laplacian of a helium probe

Abstract

A new method of visualization of lone pairs is proposed using fluorine atoms in fluoroacetylene, trifluoroethylene, and fluoroform (sp, sp² , and sp³ hybridized carbons) as an example. The space around fluorines was probed by a helium atom and at each point ³ He NMR chemical shift δ_He and its Laplacian ∇² δ_He were calculated. The ∇² δ_He isosurfaces have symmetrical toroidal shapes along CF axes; the maximum values of ∇² δ_He are reached at about 1.5 Å from fluorine atom center in the direction roughly coinciding with the lone pair localization. The absolute values of the ∇² δ_He could be used to quantify the electron-rich regions of lone pairs. The results are compared with more common visualization methods-3D maps of the molecular electrostatic potential and the electron localization function. We show that a helium atom could be used for evaluation of fine features of molecular electronic shell, since ³ He NMR spectral characteristics remain sensitive at relatively large distances. © 2018 Wiley Periodicals, Inc.

Keywords: 3He NMR; electronic structure; lone pairs.