[29.2] nuclear magnetic resonance (NMR) spectroscopy

Question 1

what does NMR involve?

Answer 1

interaction of nuclei with an external magnetic field and radiofrequency radiation

Question 2

nuclear spin

Answer 2

• protons and neutrons can be regarded as spinning about their axis
• in many atoms, these spins are paired against each other so the nucleus has no overall spin (eg. 12 C)
• in atoms with an odd number of p + n (eg. 13 C) the nucleus has an overall spin
• a nucleus that spins generates a magnetic field

Question 3

what does the direction of the magnetic field of a nucleus depend on?

Answer 3

which way the nucleus spins

Question 4

what is resonance?

Answer 4

when the nuclei of atoms absorb energy and flip their spin state to a higher energy level at specific radio frequencies

Question 5

what does the frequency of resonance depend on?

Answer 5

the environment of each atom

Question 6

what does an NMR spectrum tell us?

Answer 6

• for a given radio frequency, each atom will need a slightly different magnetic field applied to it to bring it into the resonance condition depending on what exactly it is bonded to
• the magnetic field needed is a useful guide to the carbon atom’s environment in the molecule

Question 7

what is the significance of C and H bonding to other atoms in organic molecules?

Answer 7

• surrounding electrons / bonds affect the energy and radio frequency for resonance to take place
• therefore, atoms in different environments respond differently to the field
• each different environment produces a signal in a different position

Question 8

what is chemical shift?

Answer 8

• scale which measures frequency shift
• units = ppm, signified by δ
• amount of chemical shift is determined by chemical environment

Question 9

which atoms’ environments are most commonly studied in NMR spectroscopy for organic compounds?

Answer 9

¹³C and ¹H

Question 10

what is the significance of tetramethylsilane (TMS)?

Answer 10

• peaks are measured relative to the standard reference compound, TMS, in parts per million (ppm)
• TMS is given a chemical shift value of 0

Question 11

solvents and calibration in NMR

Answer 11

• samples are dissolved in solvents free of ¹H atoms eg. CCl₄, CDCl₃
• TMS is added to calibrate the spectrum

Question 12

why is CDCl₃ commonly used as a solvent in NMR spectroscopy? (5)

Answer 12

• signal is away from all the others
• only gives one signal
• non-toxic
• intert
• low bp so easy to remove