Basic Concepts of Nuclear Shieldings and Chemical Shifts

BASIC CONCEPTS OF NUCLEAR SHIELDINGS AND CHEMICAL SHIFTS 

Electrons shield or de-shield nuclei. For chemists the influence of core electrons is of no interest, hence they are disregarded. By referring only to the difference with respect to an appropriate shielding reference, Uref — n, their contribution is eliminated. 

Let us consider the current density j r) of bond electrons. It is zero, when the molecular structure is isolated in free space 

The bond electrons at position r are described by a real wave function x) with momentum p. The coordinate vector r is the distance between the nucleus position and the electron location r = ro — Rn- If the system is located in a homogenous magnetic field Sz along the z-direction with Bz = rot Az and. (/ I 2/L X r, then the current density becomes non-zero 

the wave function I/ ) is not real anymore and acquires an imaginary contribution. According to Biot-Savart s law the external magnetic field Bz, in z-direction induces a local field B at the position R due to the electron current 7(r) 

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