PROTON NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY (1H NMR)
OVERVIEW:
- Another analytical tool used to help determine the structure/identity of organic compounds.
- Perhaps the most important structural technique available to an organic chemist.
- Gives information regarding different chemical environments of ___________________ atoms in a molecule.
- Protons in water molecules within human cells can be detected by magnetic resonance imaging (MRI), giving a threedimensional view of organs in the human body.
HOW IT WORKS (BASIC PRINCIPLES OF 1H NMR)
KHAN: https://www.youtube.com/watch?v=jjcHZuTGWXk (Intro to NMR)
-A spinning proton behaves like a very tiny bar_________________– it produces a _________________ field with a
corresponding ‘North’ and ‘South’ pole.
-When a proton (a tiny ‘bar magnet’) is placed in a stronger magnetic field it can exist in _______ possible spin states
(i.e., ________________ energy levels); it will either:
1) align ________ (parallel alignment configuration)this external magnetic
field (in much the same way a compass needle lines up with the Earth’s
magnetic field) – the lower energy ___________ spin state.
2) align __________(anti-parallel alignment configuration) this external
magnetic field – the higher energy _________spin state.
*Note that prior to exposure to an external magnetic field the spin states are
random and all have equal energy.
-The energy difference, ∆E, between the alpha and beta spin states is very small –
wavelengths in the ___________________region of the EMS. The stronger the applied magnetic field, the larger the ∆E.
The ∆E also depends on the chemical environments of the hydrogen atoms.
- In practice, a sample is placed in an ___________________. The field strength is varied until the radio waves have the
exact frequency needed to make the nuclei flip over and spin in the ________________direction. This is called
_________________ and can be detected electronically and recorded in the form of a spectrum.
1
H NMR SPECTRA ANALYSIS (OVERVIEW)
MAIN FEATURE
1. (a) *CHEMICAL SHIFT
(b) # OF PEAKS
INFORMATION PROVIDED BY FEATURE
(a) absorption __________ (hydrogens in different env’ts absorb different frequencies)
(b) # of peaks = # of different (hydrogen atom/proton) chemical environments in
molecule
2. INTEGRATION TRACE ______________ of hydrogens in each environment
3. SPLITTING PATTERNS The # of ___________________ hydrogens
*See Data booklet pp. 26-27 for common shift values.
1. CHEMICAL SHIFTS / # OF PEAKS
KHAN: https://www.youtube.com/watch?v=p9B4s0N5yk8 (CHEMICAL EQUIVALENCE)
KHAN: https://www.youtube.com/watch?v=y_sR3OyZUGo (CHEMICAL SHIFT)
Sample Shift Values
- Hydrogen nuclei in different chemical environments have different chemical
shifts (due to varying ΔE b/w  and  spin states).
- Therefore the number of signals on a 1H NMR spectrum shows the different
chemical environments in which the hydrogen atoms are found.
- In a 1H NMR spectrum, the position of the NMR signal relative to a standard
(tetramethylsilane, TMS) is termed the chemical ________________, δ.
- The chemical shift of the proton is expressed in parts per million, ppm.
- δ for TMS is ______ ppm.
Example: Ethanoic acid, H3CCOOH
Example 2: Ethanol, CH3CH2OH
Features (ethanoic acid spectrum):
-TMS at 0 ppm represents the
_________________.
-There are _______ signals which
shows the two different chemical
environments of the hydrogens. That
is, the three hydrogens bonded to
carbon 2 are in a different chemical
environment than the hydrogen bonded
to the oxygen in the _______________
group.
TETRAMETHYLSILANE (TMS) – Explanation of its use as the Reference Standard
- The position of the 1H NMR signal depends on the strength of the magnetic field, therefore
frequencies can be variable, as no two magnets will be identical. The TMS signal is used as the
universal reference standard and its use effectively overcomes this issue.
Why TMS?
1) All _____ hydrogen protons are in the _________ chemical environment, so there will be
just _______ single peak, which will be strong. The chemical shift of this signal for the TMS reference standard
is assigned δ = 0 ppm. (All other chemical shifts are measured relative to this.)
2) TMS is fairly ________________, so it will not interfere with the ______________ being analyzed.
3) It will absorb upfield (to the far ___________ at low frequency signal), well removed from most other protons
involved in organic compounds which typically absorb downfield (to the far left at higher frequency signal).
4) It can be easily ______________ from the sample after measurement due to its high _________________
(boiling point: 26-27 °C).
2. THE INTEGRATION TRACE
KHAN: https://www.youtube.com/watch?v=6dP-mNDnUA&list=PL0pJUwkI0YCZRYrgKqHXZ0XCIu11afbJ8&index=7 (INTEGRATION)
- contained within the 1H NMR spectrum
- shows the relative number of hydrogen atoms present in each chemical environment
- e.g., For methanoic acid, HCOOH, the ratio will be _______.
3. HIGH RESOLUTION 1H NMR  ANALYZING SPLITTING PATTERNS [Spin-spin Splitting (coupling)]
KHAN: https://www.youtube.com/watch?v=QlM21sl-TDg (SPLITTING)
*Features of High-Resolution 1H NMR:
-In practice, most 1H NMR spectra do not consist of sets of single peaks/signals –
though this may seem the case at low resolution. At high-resolution, ____________
patterns can be seen at certain absorptions.
-Splitting patterns result from spin-spin coupling.
*What is spin-spin coupling?
e.g., Consider the spectrum for 1,1,2-trichloroethane under high-resolution
Features:
- The compound has two different types of hydrogens (Ha and Hb) in two different chemical environments.
ANALYZING SPLITTING PATTERNS
*RULE 1 If a proton, Ha, has n protons as its nearest neighbours, that is n x Hb, then the peak of Ha will be split into
(n + 1) peaks.
*RULE 2
The ratio of the intensities of the lines of the split peak can be deduced using Pascal’s triangle.
SPLITTING PATTERNS (SUMMARY):
# of HYDROGEN # OF PEAKS IN SIGNAL
NEIGHBOURS (n)
(n + 1)
0
0+1=1
1
1+1=2
2
3
4
SPLITTING
CLASSIFICATION
Singlet
Doublet
RATIO OF
INTENSITIES
1:1
SAMPLE PROBLEMS
PROBLEM #1 (from Oxford pp. 465-466)
(a) Deduce the full structural formula of ethyl ethanoate.
(b) Using section 27 of the Data Booklet, predict the high-resolution 1H NMR spectrum of ethyl ethanoate.
*Your answer should refer:
i) to the integration trace of the spectrum,
ii) the approximate chemical shifts of the various protons, in ppm,
iii) any possible splitting patterns,
iv) and the relative intensities of the splitting patterns
ANSWER:
(a) ethyl ethanoate structure:
(b) *Recommendation – organize features in a table.
1
H NMR FEATURES
TYPE OF
PREDICTED
PROTON
CHEMICAL
(CHEM.
SHIFT
ENV’T)
(δ / ppm)
NUMBER OF
PROTONS
(INTEGRATION
TRACE)
NUMBER
OF
ADJACENT
PROTONS
SPLITTING
PATTERN
(RELATIVE
INTENSITIES)
SPLITTING
CLASS
ACTUAL
SHIFT
(δ / ppm)
2.038
4.119
1.260
PROBLEM #2
*An unknown compound, X, of molecular formula, C3H6O, with a characteristic fruity odour, has the IR and 1H NMR
spectra.
Figure 2: 1H NMR spectrum for X.
Figure 1: IR spectrum of X.
The MS of X showed peaks at m/z values = 58 and 29 (other peaks were also found).
Deduce the structure of X using the information given and any other additional information form the Data Booklet. For
each spectrum assign as much spectroscopic information as possible based on the structure of X.
RECOMMENDED STEPS
1. Deduce IHD.
2. Make educated guesses that focus on determining what functional group (class of compound) may be present in the
compound. (e.g. The compound is said have a fruity odour…ester? Why can’t it be an ester? What classes of organic
compounds contain one oxygen?)
3. Examine absorptions in the IR spectrum. (e.g. Is there a strong absorption (peak) in the wavenumber range 1700-1750
cm-1, indicating the presence of a C=O? *See section 26 in the data booklet for characteristic IR absorptions.
4. Compound X may be an aldehyde or ketone. Does 1H NMR spectrum confirm the presence of aldehyde proton?
5. Consider MS data.
MORE PRACTICE PROBLEMS
*From Pearson pp. 570-571:
1. (a) Draw the molecular structure of butanone.
(b) Use section 27 of the IB booklet to predict the high resolution 1H NMR spectrum of butanone. Your answer should
include the chemical shift, the number of hydrogen atoms, and the splitting pattern for the different environments of the
hydrogen atoms.
2. Compare the 1H NMR spectra of ethanal and propanone. Your answer should refer to the number of peaks, and the
areas and splitting pattern of each peak.
3. The key features of the 1H NMR spectrum of a compound with the molecular formula C3H6O2 are summarized below.
Chemical shift / ppm
1.3
4.3
8
What is the structure of the compound?
*Also: #1 & 2 on p. 470 of OXFORD.
Number of H atoms
3
2
1
Splitting pattern
3
4
1