Chapter 16, Problem 16.4YT

Interpretation Introduction

Interpretation:

Out of the two given protons, which proton is shielded to a greater extent and which proton requires higher frequency radio waves to cause a spin flip is to be explained.

Concept introduction:

When H atoms (surrounded by electrons) are subjected to ${\text{B}}_{\text{ext}}$, those electrons move in a concerted fashion and create a local magnetic field (${\text{B}}_{\text{loc}}$) that opposes ${\text{B}}_{\text{ext}}$. The magnetic field that is “felt” by the nucleus, called the effective magnetic field (${\text{B}}_{\text{eff}}$), is somewhat less than ${\text{B}}_{\text{ext}}$, so protons in a molecule are said to be shielded.

Chemically distinct protons are surrounded by different electron distributions and therefore have different values of ${\text{B}}_{\text{loc}}$. Consequently, chemically distinct protons are shielded from ${\text{B}}_{\text{ext}}$ to a different extent and will absorb photons at different frequencies – they will have different chemical shifts.