NMR

Question 1

What does NMR stand for?

Answer 1

• nuclear magnetic resonance spectroscopy

Question 2

How does NMR work?

Answer 2

• magnetic field applied to a sample, which is surrounded by a source of radio waves and a radio receiver
• this generate an energy change in the nuclei of atoms in the sample that can be detected
• electromagnetic energy is emitted, which can then be interpreted by a computer

Question 3

What is special about carbon-13?

Answer 3

• 1% of carbon
• has spin
• behaves as a little magnet

Question 4

What is resonance condition>?

Answer 4

• when C-13 placed in magnetic field it aligns with the field (lowest energy state / most stable)
• radio waves of certain frequencies supply energy to flip to make the flip
  
  • possible to detect this at the right freq as it flips from one orientation to the other
  • this is the peak on the graph

Question 5

How does functional groups cause a difference in the resonance of carbon-13 atom?

Answer 5

• different functional gorups cause the atoms to resonate at different magnetic field strengths
• all nuclei are shielded from the external magnetic field by the electrons around them
• nuclei with more electrons around them are better shielded
  
  • greater the electron density, the smaller the magnetic field felt by the nucleu and the lower the frequency it resonates

Question 6

What does the NMR instrument produce?>

Answer 6

• graph of energy abosrbed (from the radio signal) vertically against a quantity called chemical shift (which is related to the resonant freq) horizontally

Question 7

What are the units of chemical shift and symbol>?

Answer 7

• δ
• ppm

Question 8

How is chemical shift defined as zero?

Answer 8

related to a compound called tetramethylsilane, TMS

Question 9

What does chemical shift depend on?

Answer 9

• related to the difference in freq between the resonating nucleus and that of TMS