3.15 NMR spectroscopy

NMR spectroscopy

• what it stands for

• what it tells us and how

• nuclear magnetic resonance spectroscopy

• gives information about the position of ¹³C or ¹H atoms in a molecule

• different bond environments within a molecule absorb different amounts of energy, so show different peaks on a spectra

axes on NMR spectroscopy spectra

• horizontal axis uses δ scale for recording chemical shift in parts per million (ppm)

• vertical axis represents the intensity of the absorption of radio waves (/energy)

how do ¹³C and ¹H behave as magnets?

how is this used in NMR spectroscopy?

• their nuclei have nuclear spin, which generates a small magnetic field

• this small magnetic field can be aligned with or against an external magnetic field

• when you put ¹³C and ¹H in a strong magnetic field and pass radio waves through them, their nuclei absorb the energy at specific frequencies which flips around their direction of spin

• as the nuclei relax back, they emit signals that are detected

chemical shift

• the difference in frequency / energy between the resonating nucleus and that of TMS

• how far the signal is away from the signal for TMS

• measured in ppm

how does the electron density around ¹³C or ¹H affect their chemical shift?

• higher electron density means greater shielding

• so smaller chemical shift

how does the bond environment affect chemical shift?

greater chemical shift if ¹³C or ¹H is closer to highly electronegative atoms or double bonds

• highly electronegative atoms and double bonds attract electrons towards themselves

• so decrease electron density around ¹³C or ¹H

• so less shielding

• so greater chemical shift

TMS

tetramethylsilane, Si(CH₃)₄

• used as a standard for bond environment peaks to be measured against in NMR

• seen as a peak at δ = 0 ppm on the x-axis

reasons for use of TMS

used as a standard because:

• it contains only one type of H environment and only one type of C environment

• it only gives one signal, which is further right than most signals from organic compounds

• it is inert, so won’t react with the sample or affect peaks produced

• it is non-toxic, so less risk of harm to analysts

• it is volatile (has low bpt) so easy to separate from the sample

deuterium

heavy hydrogen, ²H

• 1 neutron, 1 proton

• has no nuclear spin

¹³C NMR spectroscopy

• gives simpler spectra than ¹H NMR

• molecules with symmetry give fewer peaks than the number of C atoms in the molecule

• peaks have no integration value

• peaks have no splitting

how many peaks would benzene show in a ¹³C NMR spectra?

1, as all the C atoms are in the exact same environment

¹H NMR spectroscopy

• spectra obtained using samples dissolved in CCl₄ (nonpolar) or deuterated solvents like CDCl₃ (polar)

  • must be dissolved in a solvent that contains no ¹H so that it doesn’t produce any peaks on the spectrum

• integrated spectra / area under the graph gives the ratio of ¹H atoms in different environments in a molecule

• integration value of a peak = relative number of equivalent H atoms

• in high resolution NMR, peaks have splitting

  • n + 1 rule

integration value = 3

3 H’s in the same environment

CH₃ peak

give the integration trace on a ¹H NMR spectra for propane

6 : 2 = 3 : 1

give the integration trace on a ¹H NMR spectra for butane

6 : 4 = 3 : 2

why do peaks have splitting?

• splitting of peak is caused by H atoms on adjacent C atom (spin-spin coupling)

• only H atoms on C atoms cause splitting and are split

n + 1 rule

• n non-equivalent H’s on adjacent C atom(s) will split a peak into n + 1 smaller peaks

• there is no splitting when equivalent H atoms are on adjacent C atoms

  • e.g. in ethane CH₃-CH₃

splitting pattern: singlet

• no splitting

• no H atoms on adjacent C

  • H atom could be adjacent to O (—O-H)

  • H atom could be adjacent to N (—NH₃)

splitting pattern: doublet

• peak splits into 2

• 1 H atom on adjacent C

  • next to CH

splitting pattern: triplet

• peak splits into 3

• 2 H’s on adjacent C

  • next to CH₂

splitting pattern: quartet

• peak splits into 4

• 3 H’s on adjacent C

  • next to CH₃

splitting pattern: quintet

• peak splits into 5

• 4 H’s on adjacent C

  • one CH₂ on either side

splitting pattern: sextet

• peak splits into 6

• 5 H’s on adjacent C

  • one CH₃ and one CH₂ on either side

triplet next to a quartet

CH₂CH₃

integration trace: 2 : 3

two triplets

CH₂CH₂

integration trace: 2 : 2

explain how and why isomers X and Y can be distinguished by comparing each of their ¹³C NMR spectra

• compare number of peaks

• compare range of δ for each peak and the corresponding type of bond environment

  • X will show a peak at ___ - ___ ppm, due to _____ group

  • Y will show a peak at ___ - ___ ppm, due to _____ group