NMR Spectroscopy

What can spectroscopy tell us?

• Allows us to identify different substances that may look the same at surface level

• Allows us to identify impurities within a substance

• Tells us about the process of chemical synthesis of a substance

How does NMR work?

• finds the structures of complex molecules using a magnetic field.

• nuclei of an atom give off a signal when in the NMR machine

• the signal depends on what the atom is bonded to

Why is NMR a good technique?

• only a very small sample is needed.

• the sample is not destroyed or used up in NMR

NMR can only analyse…

Nuclei with odd mass numbers
 E.g. 1H, 13C

Why can NMR only analyse certain nuclei and not others?

Nuclei with odd mass numbers have a quantum mechanical property called ‘spin’

• » this gives the nuclei magnetic properties

  » so these nuclei are NMR visible

Which common nuclei are NMR invisible?

¹²C, ¹⁴N, ¹⁶O

What concept is the fundamental basis of NMR?

The Zeeman Effect

• In the presence of a magnetic field (e.g. inside an NMR machine), the spins of nuclei split into 2 energy levels

• Low energy = ‘spin up’ = aligned WITH the field

• High energy = ‘spin down’ = aligned AGAINST the field

What gives rise to NMR signals?

What 2 things does the transition frequency between the low and high energy states depend on?

• The nuclei transitioning between low and high energy levels

• Frequency of a transition = frequency of radiation required for the nucleus to move between the 2 energy states

Transition frequency depends on:

1. Strength of the magnetic field 

• Stronger magnetic field = greater difference in energy between high and low energy levels = higher transition frequency = greater NMR signal

2. Gyromagnetic ratio

• Tells us how magnetic a nucleus is

• Higher gyromagnetic ratio = nucleus has a stronger interaction with the magnetic field (e.g. NMR spectrum) = larger difference in energy between the low and high levels = higher transition frequency = stronger NMR signal

Chemical Shifts

How does chemical shift occur?

• Electrons around the nucleus can create opposing magnetic fields

• This can shield the magnetic nucleus from the external magnetic field ie. The NMR machine 

• So nuclei feel the magnetic field less

More e- around the nucleus = more shielding = weaker magnetic field felt by nucleus = lower transition frequency = position of signal is more to the right 

(vice-versa for less shielding)

Chemical Shift

What does chemical shift tell us?

• Tells us how shielded or deshielded a nucleus is compared to a reference compound

• The reference compound is TMS

Chemical Shift

What equation is used to calculate chemical shifts?

• v_obs​ = observed resonance frequency of sample nuclei

• v_ref ​ = resonance frequency of the reference compound (TMS)

• The factor of x10⁶ converts this ratio into parts per million (ppm) — which is the chemical shift scale

What is TMS?

• A chemical used as the reference point to calculate chemical shifts

• Given the value of 0 by definition

Why is TMS used to calibrate the spectrum?

• Electroposiitve silicon has a shielding effect - so It’s signal is away from all others and doesn’t overlap with peaks of interest

• Symmetric molecule - so only gives one signal so doesn't get in the way

• Inert so doesn't react with the compound

• Low boiling point so easy to remove, doesn't contaminate sample permanently

• Non-toxic

Give the structural formula for TMS

Si(CH₃)₄

Chemical Shift

Which symbol are chemical shifts represented by?

Delta - δ

Chemical Shifts

When there is less shielding, the resonance frequency (signal on the NMR) moves…

To the LHS

Chemical Shifts

When there is more shielding, the resonance frequency (signal on the NMR) moves…

To the RHS

Chemical Shifts

What can cause less shielding of the nucleus?

• Electron withdrawing groups e.g. F, Br, I

• Decrease the e- density around the nucleus

• Deshields the nucleus

• Resonance frequency more to the LHS

• So larger chemical shifts in ppm

Chemical Shifts

What causes a larger chemical shift?

Nearby electronegative atoms = e- withdrawing = less shielding = resonance frequency shifts to the LHS = larger chemical shift

Chemical Shifts

Aromatic molecules and chemical shifts 

• Aromatic rings have delocalised π-electrons

• In a magnetic field, π-electrons move in a circular motion → this is called the ring current

• The ring current induces a local magnetic field that:

  • Opposes the external magnetic field (B₀) ​inside the ring → so nuclei inside the ring are shielded

  • Reinforces the external magnetic force (B₀)​ outside the ring → so nuclei outside the ring are deshielded e.g. aromatic Hs

• Aromatic hydrogens are outside the ring = so are deshielded = feel stronger magnetic field

• So resonance frequency appears to the LHS on NMR spectrum = greater chemical shift

• Typical chemical shift: 6.5 – 8.5 ppm

• E.g. benzene shows one signal at ~7.3 ppm

Chemical Shifts

Hydrogen bonding and chemical shifts

• Labile protons form hydrogen bonds e.g. -OH or -NH

• Hydrogen bonding causes deshielding of these protons

• Because H-bonds constantly form and break, deshielding strength varies

• This means labile protons can be observed over a broad range of chemical shifts

• Solvent, acidity, concentration, and temperature also affect their chemical shifts

• This makes the chemical shifts of labile protons hard to predict

• -OH and -NH therefore appear as broad peaks in NMR (often labelled “br”)

Chemical Shifts

How do -OH and -NH proteins appear on the NMR soectrum

• As broad peaks

• Do not have integration, and usually marked as ‘br’

Chemical Shifts

How can -OH and -NH be identified experimentally?

• Identified by D₂O exchange experiment (“D₂O shake”)

• Labile proton replaced by deuterium = signal disappears

Chemical Shifts

Common ¹H chemical shifts to memorise (check these values idk if right)

-OH = 3.9 - 9.0ppm

-NH = 5.5 - 8.5ppm

O=C-H = 9.5ppm

Benzene-H = 6 - 8.5ppm

O-C-H and X-C-H = 2.8 - 4.9ppm

N-C-H, S-C-H, O-C-C-H, C=C-C-H, C6H6-CH2-H = 1.8 - 3.1ppm

-CH, -CH2, -CH3 = 0 - 1.9ppm

What is an environment?

• what the atoms are bonded to.

• number of environments = number of signals

If hydrogens are in the same environment they are…

Chemically equivalent

All hydrogens on the same carbon…

• are in the same environment

• so are chemically equivalent

How many environments are in propane?

2
Blue: each H is bonded to a C, which is bonded to H2, CH2, CH3
Green: each H is bonded to a C, which is bonded to H, CH3, CH3

Why are there 3 peaks in the NMR spectrum for ethanol?

There are 3 different environments:

• Blue: H is bonded to a C, which is bonded to a H, CH2, OH

• Green: H is bonded to a C, which is bonded to CH3, OH

• Red: H is bonded to O, which is bonded to CH2, CH3

How many signals would you see on the H-NMR spectra for this compound?

3

Symmetrical H…

Are in the same environment

How many environments in this molecule

• There is one line of symmetry

• 2 environments

Another way of looking at H in the same environment…

• same distance away from a particular atom

e.g. all the H in cyclohexane are the same number of H atoms away from the top C

How many environments in this molecule

• There are 2 lines of symmetry

• 2 environments

How many peaks would you see in the proton NMR of the following molecule: CH2ClCH2CH2CH2Cl

• 2 environments

• 2 peaks

Predict how many peaks there would be in an H-NMR spectrum of these molecules:

1. Cyclohexane

2. Methylcyclohexane

1. 1 peak

2. 5 peaks

How many peaks/environments in this molecule?

4 environments

How many peaks/molecules in this molecule

4 environments

Counting environments top top

• See if any lines of symmetry

• Draw in symmetry line

• All Hs on either side of symmetry line are in the same environment 

Which 2 rare isotopes are NMR-active?

Why do both have very low sensitivity?

• ¹³C and ¹⁵N

• Both have very low abundance (¹³C: 1.1%, ¹⁵C: 0.37%)

Why must samples be dissolved in solvents free of 1H atoms?

What is an exception to this?

• Solvents with 1H will produce a signal (residual peaks) in the NMR and interfere

• Deuterium (D) which is ²H — even mass number so does not produce a signal in the 1H NMR

• So deuterated solvents are NMR-invisible 

What is D?

Deuterium

• Written as D or 2H e.g. CDCl3

Why can samples be dissolved in solvents which contain D?

Deuterium (2H) has even mass number so does not produce a signal in the 1H NMR

Common solvents used in NMR

• CDCl₃ - deuterated chloroform

• D₂O

• Deuterated DMSO

How does NMR work?

1. Samples are dissolved in a deuterated solvent e.g. CDCl3 and placed in a strong magnetic field

2. A small amount of TMS is added to calibrate the machine

3. A radiofrequency pulse is applied to the sample, changing the nuclear spin of the nuclei

4. The energy associated with these transitions is recorded by the detector

5. Generates a spectrum for analysis

Summary of the different info in an NMR spectrum

1. Number of peaks

2. Peak integration (area of peaks)

3. Chemical shift (position of peaks)

4. Shape of peaks (multiplicity/splitting/J coupling)

What is the y-axis on an NMR spectrum?

Energy absorbed (used to work out integration)

What is the x-axis on an NMR spectrum?

Chemical shift 

Peak Integration

What is it?

The number of ¹Hs in each environment in the molecule

E.g. CH3-CH2-OH has an integration ratio of 3:2:1

Peak Integration

How is the peak integration shown on an NMR spectrum?

• As ratios under each peak

• As a whole number above each peak

• Shown graphically

Peak Integration

For each of the following molecules, state the number of peaks and their peak integrals

A. 2 peaks, 3:1

B. 4 peaks, 3:1:2:3

C. 5 peaks, 3:1:2:2:3

D. 4 peaks, 6:1:2:3

Peak Integration

How many peaks are present and what are the peak integrals?

• 2 peaks

• 3:2

Peak Integration

What is the integration ratio on this spectrum

0.6 : 3.6 : 1.2 : 1.8

=

1 : 6 : 2 : 3

(divide by smallest number 0.6)

Multiplicity / J Coupling

Why do signals on the NMR spectrum split?

Hydrogen atoms adjacent to each other interfere with the signal

Multiplicity / J Coupling

How to work out how much a peak splits

Number of Hs bonded to adjacent C + 1

Multiplicity / J Coupling

Explain why both H_A and H_B would split into a doublet:

H_A- C - C - H_B

• The signal for peak H_A is split into a doublet, depending on whether the spin of the adjacent H_B up or down

• The signal for peak H_B is split into a doublet, depending on whether the spin of the adjacent H_A up or down

Multiplicity / J Coupling

Explain what the multiplicity will be like for H_A in the following molecule

• 2 adjacent Hs = triplet

• Both H_Bs can spin up (1 way this can happen)

• 1 H_B can spin up and the other H_B can spin down (2 ways this can happen)

• Both H_Bs can spin down (1 way this can happen)

• So H_A forms a 1:2:1 triplet

Multiplicity / J Coupling

Explain what the multiplicity will be like for HA in the following molecule

1:3:3:1 quartet 

Multiplicity / J Coupling

Work out the degree of multiplicity for this molecule

• Blue: 3 peaks with ratio 1:2:1
• Yellow: 4 peaks with ratio 1:3:3:1
• Orange: 1 peak, no splitting

Multiplicity / J Coupling

Name all the types of splitting patterns

• singlet — no H on adjacent C
• doublet — 1 H on adjacent C
• triplet — 2 H on adjacent C
• quartet — 3 H on adjacent C

Multiplicity / J Coupling

How many H environments?

What is the splitting pattern?

2 environments

Green: triplet with ratio 1:2:1

Orange: quartet with ratio 1:3:3:1

Multiplicity / J Coupling

State the multiplicity of H_A in this molecule

• 3 environments 

• H_A is adjacent to both H_B and H_C which are in different environments

• H_B can either spin up or down - 2 options

• H_C can either spin up or down - 2 options

• 2 × 2 = 4

• So H_A forms a doublet of doublets (d) with the ratio 1:1:1:1

Multiplicity / J Coupling

When does a nucleus split into a doublet of doublets?

• When the H is adjacent to 2 Hs in different environments 

• Forms 4 peaks which are NOT evenly spaced (unlike a quartet)

Multiplicity / J Coupling

If a Q asks you to state the number of coupled spins

Just the number of adjacent Hs in different environments

CH3-NH2

Even though the chemical shift for CH3 says it is 0.7-1.2ppm on the data sheet, why may the peak for CH3 hydrogens be around 1.4ppm?

• If hydrogen atoms are close to electronegative atoms, their shift on the spectrum is higher than the data sheet predicts

• H atoms in CH3 are adjacent to N

Predict the chemical shift for the hydrogens in each environment. (2-bromo-2-methylbutane)

Red: 0.7–1.2 ppm
Blue: 2.1–2.6 ppm
Green: 0.7–1.2 ppm

2-bromo-2-methylbutane

Number of signals, Integration, J-coupling, Chemical shifts

Number of signals: 3
Integration: 3 : 2 : 6
J-coupling: t, q, s
Chemical shifts: 0.7–1.2 ppm, 1.2–1.4 ppm, 0.7–1.2 ppm

Methylpropene

Number of signals, Integration, J-coupling, Chemical shifts

Number of signals: 2
Integration: 3 : 1
J-coupling: s, s
Chemical shifts: 0.7–1.2 ppm, 4.5–6.0 ppm

Ethyl propanoate

Number of signals, Integration, J-coupling, Chemical shifts

Number of signals: 4
Integration: 3 : 2 : 2 : 3
J-coupling: t, q, q, t 
Chemical shifts: 0.7–1.2 ppm, 0.7–1.2 ppm, 2.1–2.6 ppm, 3.7–4.1 ppm

Predict the number of signals, Integration, J-coupling, Chemical shifts of: Cyclohexane

Number of signals: 1
Integration: n/a (as no other peaks so nothing to compare intensity to)
J-coupling: singlet
Chemical shift: 1.2–1.4 ppm

Predict the Number of signals, Integration, J-coupling, Chemical shifts of: Pentan-3-one

Number of signals: 2
Integration: 3 : 2
J-coupling: triplet and quartet
Chemical shifts: 0.7–1.2 ppm and 2.1–2.6 ppm

Steps for identifying compounds from their HNMR spectrum

1. Look at number of peaks/environments

2. Put chemical shifts in table in descending order.

3. Put relative intensities in table.

4. Put j-coupling in table with ratio

5. Draw structure fragments from table.

6. Join fragments using splitting pattern (fragments that overlap mean they are next to each other; find end pieces first).

7. Check molecular formula and splitting.

A singlet with an intensity of 1 is usually…

–OH

If you see an NMR with a peak that is a quartet, and a peak that is a triplet it is usually….

CH2–CH3 (but always double check)

How would you use NMR to distinguish ethyl acetate from methyl propionate

• Both have 3 environments

• Both have an integration of 3:2:3

• Both have j-coupling of s, q, t

However, they have different chemical shifts:

ethyl acetate:

• CH2 bonded to O = more shielding = quartet more towards RHS

• CH3 bonded to C=O = less shielding = singlet towards LHS

methyl propionate:

• CH3 bonded to O = more shielding = singlet towards RHS

• CH2 bonded to C=O = less shielding = quartet towards RHS

How do you work out the integration ratio if graphically shown on the spectra

• Use a ruler to measure the length of each curve from bottom to top

• That gives you the integration ratio 

• Use this to work out number of Hs in each environment

In this instance, there is a triplet overlapping with a quartet and 2 overlapping multipliers. How would you work out the integration for each. 

1. Split peak into triplet and quartet

2. Measure each with a ruler

3. That is the ratio for each

4. Divide all numbers by smallest number to get actual number of Ha in each environment

Assign this spectrum of paracetamol

1. Calculate integration, splitting and chemical shift for each environment in the molecule

2. Calculate integration, splitting and chemical shift for each signal on spectrum

3. Match them up

A molecule has the molecular formula C4H8O. Use the spectra to work out its structure.

• 3 peaks = 3 environments
• Peak integration 2 : 3 : 3
• Peak at 2.1 ppm is CH3–C=O. It is a singlet because adjacent C has no Hs
• Other peaks at 2.5 and 1 are CH3–CH2–C=O. They have a splitting pattern of triplet and quartet which means they are a CH3 and CH2 bonded together

The 1H NMR spectrum of C6H12O2 is shown. Deduce the structure of the compound.

• 5 peaks = 5 environments

• Peak integration is 2 : 3 : 2 : 2 : 3

• Peak at 3.2 ppm is –O–CH3. It is a singlet as no adjacent C with Hs

• Peak at 1 ppm is –CH3. It is a triplet so it is next to CH2

• Peaks at 2.6 ppm and 2.5 ppm are –CH2COCH2–

• Peak at 2.6 ppm triplet, so attached to C with 2 Hs

• Peak at 2.5 ppm is a quartet, so attached to C with 3 Hs

• Molecule is CH3CH2COCH2CH2OCH3

How are carbon environments commonly referred to?

1° = primary = 1 bond to heavy atoms = X-CH3

2° = secondary = 2 bonds to heavy atoms = X-CH2-X

3° = tertiary = 3 bond to heavy atoms = CX₃H

4° = quaternary = 4 bond to heavy atoms = CX₄

How many carbon environments are there?

6

How many carbon environments are there?

2

Rule for number of carbon environments

If Hs are in different environments, the Cs they are bonded to will be in different environments

C-NMR is done with…

Carbon-13 (13C)

Why do we not get peaks with carbon-12 atoms?

12C atoms don't have spin as they have an even mass number

How many peaks in the 13C NMR of butane?

2

How many carbon environments?

5

How many carbon environments?

5

How many carbon environments in but-2-ene?

2

The structure of N-phenylethanamide is shown. Determine the number of peaks in the 13C NMR spectrum

6

How many peaks in the 13C NMR spectrum?

5

In 13C NMR we only have to look at…

1. Number of peaks / environments

2. Chemical shift

» NO integration or scalar coupling considered

Scalar coupling in ¹³C NMR

• The chance of having 2 ¹³C atoms next to each other is very small » so very small chance of having ¹³C coupling to other ¹³C atoms

• Scalar coupling can occur between ¹³C and ¹H » however this is purposefully supressed via decoupling to make the spectra easier to interpret

Why do we experimentally suppress scalar coupling between ¹³C and ¹H in ¹³C NMR?

• ¹³C signals are naturally weak (only ~1.1% abundance).

• Coupling to multiple hydrogens would split signals and spread them out, reducing sensitivity.

• Removing coupling gives one sharp, strong peak per unique carbon — easy to assign

¹³C chemical shifts to memorise 

If an sp³ C is bonded to a more e- withdrawing group…

Chemical shift is larger and further to the left

Quaternary Cs in ¹³C NMR

quaternary carbons (C not bonded to any Hs) are usually less intense than other signals » have a shorter peak

If there is a very short peak on ¹³C NMR…

• We can assume it is a 4° C

• Can use this to work out which atoms have Cs or Hs attached e.g. in aromatic rings

E.g. the green Cs are quaternary so will have a less intense peak on the NMR

What is the Attached Proton Test (APS)?

• A variant of a ¹³C NMR experiment

• In APR spectra 1° and 3° Cs appear with the opposite sign to 2° and 4° Cs

E.g. in the example, C and CH2 are positive and CH and CH3 are negative

• We can work out which is positive and which is negative by locating the 4° C (peak with the lowest intensity)