Author Question: Instructions: For each of the compounds below tell how many signals you would expect the molecule to ... (Read 75 times)

tuffie · **on:** Aug 23, 2018

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.

Number of signals:

Question 2

Explain why all protons in a molecule do not absorb rf energy at the same frequency.

itsakadoozi · **Reply #1 on:** Aug 23, 2018

Answer to Question 1

three

Answer to Question 2

All nuclei in molecules are surrounded by electron clouds. When a uniform external magnetic field is applied to a molecule, the circulating electron clouds set up tiny local magnetic fields of their own. These local magnetic fields act in opposition to the applied field, so that the effective field actually felt by a nucleus is a bit smaller than the applied field.
B_effective = B_applied B_local
This effect is termed shielding. Each nucleus is shielded to a slightly different extent, so each unique kind of proton in a molecule resonates at a slightly different frequency and gives rise to a unique NMR signal.