6.3.2 - NMR Spectroscopy

Question 1

What are nucleons?

Answer 1

• Word used for number of protons + neutrons
• Nuclei with odd number of nucleons have spin
• Protons and carbon-13 are relevant for NMR as they have an odd number of nucleons

Question 2

What is resonance?

Answer 2

• The combination of a stong magnetic field and radio frequency radiation will be absorbed by different nuclei based on their environments
  
  • Once energy is absorbed the nucleus flips rapidly between the stated
  • This is detected by an NMR spectrometer

Question 3

What is chemical shift?

What are chemical shift values relative to?

Answer 3

Chemical Shift: Variation in frequency absorbed for NMR, dependant on the chemical environment surround the nucleus, measured in ppm, denoted lowercase delta

Chemical shift is relative to TMS - tetramethylsilane (CH₃)₄Si which has a chemical shift of 0ppm

Question 4

What is a deuterated solvent?

Answer 4

• Most common solvents have carbon and hydrogen which will show up on carbon-13 and proton NMR spectra
• A deuterated solvent is used where all ordinary hydrogrens are replaced by deuterium (²H or D), e.g. CDCl₃
  
  • A peak will show up for deuterium and the computer will filter it out

Question 5

What does ¹³C NMR tell you?

Answer 5

• The number of different carbon environments (from the number of peaks) and the types of carbon environment (from the chemical shift)

4 Main Types of Carbon Env. on Data Sheet

• Carbon to carbon
• Carbon in carbonyl
• C=C or aromatic
• Carbon bonded to electronegative atom (O, N, Cl, Br…)

Question 6

What is a carbon chemical environment?

Answer 6

• The different atoms or groups a carbon is bonded to determines if it is an environment

Question 7

What does proton NMR tell you?

Answer 7

• Number of proton environments - from the number of differernt peaks
• Types of proton environment - from chemical shift
• Relative numbers of each proton type - from integration traces or ratio number of relative peak areas
• Number of non-equivalent protons adjacent to a given proton from spin-spin splitting patter

Question 8

What is the difference between an equivalent and non-equivalent proton?

Answer 8

• Chemically equivalent protons exist within the same environment
  
  • ​If 2 or more protons are equivalent, they absorb the same chemical shift, increasing peak size
• Non-equivalent protons absorb different chemical shifts

Question 9

How many proton environments are there in ethanol?

Answer 9

Question 10

What does the relative area under a peak in proton NMR tell you?

Answer 10

• The ratio of number of protons per peak
• An integration trace - shows the relative area under each peak

Question 11

What is spin-spin coupling?

Answer 11

On an NMR spectrum , the interaction between spin states of non-equivalent nuclei results in the splitting of peaks

n+1 rule

• For a proton with n protons attached to an adjacent carbon, the number of sub-peaks is n+1
  
  • The number of sub peaks is 1 greater than the number of adjacent protons (that cause the splitting)

Question 12

What are the spin-spin splitting patterns?

Answer 12

Singlet: No adjacent H atoms

Doublet: Adjacent CH

Triplet: Adjacent CH₂

Quartet: Adjacent CH₃

Question 13

How can you spot hydroxyl or amino protons on proton NMR?

Answer 13

• Hydroxyl and amino protons do not undergo spin-spin coupling

Proton Exchange

1. Run normal proton NMR
2. Add small amount of D₂O and shake
   
   • Run a second spectrum

The deuterium replaces protons in OH and NH with D atoms, making the 2 peaks disappear

Compare the 2 spectra, the missing peak is OH/NH