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Advanced Level Organic Chemistry: 15.3 Theory and practice of 1H NMR spectroscopy

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Doc Brown's Advanced Chemistry: 15.3.1 1H (proton) NMR Spectroscopy Theory

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses Spectroscopic methods of analysis and molecular structure determination

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15.3.2 The uses and applications of 1H (proton) NMR spectroscopy

15.3.3 Index of H-1 (proton) NMR spectra of organic compounds  (below on this page)

15.3.4 Some simple NMR-IR problem solving questions


15.3.1 The theory of 1H (proton) NMR spectroscopy (pre-university level)

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(a) The basis of 1H (proton) NMR molecular spectroscopy

(b) How an NMR spectrometer is designed and functions

(c) Experimental conditions - techniques, data and problems

(d) Interpreting 1H NMR spectra (at low resolution)

(e) Interpreting 1H NMR spectra and field splitting effects (high resolution spectra)


(a) The basis of 1H (proton) NMR molecular spectroscopy

Nuclear magnetic moment and quantum levels

(c) doc b

The above diagram represents the basic picture of an atom, i.e. the nucleus of positive protons and neutrons (except 1H) surrounded by electrons in their appropriate quantum levels (shells) - a typical picture of the Bohr model of an atom.

But there is one property that this atomic model can represent, and this called a magnetic moment.

It is found that if an atomic nucleus has an odd number of protons or an odd number of neutrons (or both), the nucleus, apart from its positive charge, also has magnetic moment - meaning the nucleus can behave as a tiny atomic bar magnet producing a magnetic field with a north and south pole.

This phenomenon, in terms of attraction or repulsion from the two possible alignments of the poles of the atomic magnet interacting with another 'external' magnet field can be demonstrated.

The magnetic moment of the nucleus can only be aligned either parallel or opposed to the direction of an applied magnetic field (which I refer to as the 'external' magnetic field).

This results in two different quantum levels, a higher and lower energy state of the nucleus in terms of its magnetic moment.

The difference in energy corresponds with the radio wave region of the electromagnetic spectrum.

The difference between the higher state E2 and the lower state E1, can therefore be expressed in terms of Planck's equation:  ∆E = E2 - E1 = h

E = quantum level energy change

h = Planck's constant

= radio wave frequency

Therefore, via the magnetic moment of the atomic nucleus, radio frequency photons can be absorbed or emitted if the nuclear atomic poles can be 'flipped' when interacting with an 'external' magnetic field - illustrated in the diagram below.

The interaction is called nuclear magnetic resonance and NMR spectroscopy is now considered one of the most powerful tools for investigating molecular structure, particularly complex organic molecules.

diagram explaining the theory of magnetic resonance spectroscopy magnetic moment of a proton alignment of poles radio frequency quantum levels absorption and emission

Slightly more than 50% of the nuclei are in the lower energy level than those in the higher level.

If the correct radio frequency of EM radiation is applied, i.e. governed by ∆E = h , some of the 1H nuclei will move up in energy to the higher quantum level.

The energy needed to move the nuclear 'magnet' to the higher quantum level depends on the strength of the magnetic field it experiences.

This is not the same as the 'external' magnetic field applied in an NMR spectrometer because the electrons associated with the neighbouring atoms and groups (to the proton) give rise to tiny magnetic fields of their own.

These 'local' magnetic fields are usually opposed to the external field applied in the NMR spectrometer.

The overall field experienced by a proton (or other nucleus) is therefore slightly smaller than the external magnetic field, depending on the local field from the atoms/groups surrounding that proton part of the molecule.

This means for every type of molecular arrangement (molecular/chemical environment), the proton experiences a slightly different magnetic field.

Therefore the 1H proton nuclei in these different chemical environments will resonate with slightly different slightly different frequencies () because the ∆E resonance energy will differ, this enables 1H NMR spectroscopy to provide valuable information on molecular structure and the number of protons in each different chemical environment.

Examples of nuclei which may or may not exhibit nuclear magnetic resonance

Reminder: The atomic nucleus must have an odd number of protons or an odd number of neutrons (or both).

  and (c) doc b will both have a magnetic moment, 1 proton, and is the basis hydrogen-1 NMR spectroscopy.

will not have a magnetic moment, even number of protons and even number of neutrons, no NMR resonances will be detected.

isotope nuclide symbol protons neutrons nuclear magnetic moment % abundance
carbon–12 126C 6 6 NO ~98.9%, stable
carbon–13 136C 6 7 YES ~1.1%, stable
carbon–14 146C 6 8 NO trace, unstable radioactive

Only one of the isotopes of carbon has a nucleus with a magnetic moment, all the proton or neutron numbers are even, except the 13C isotope has an odd number of neutrons and is the basis of carbon-13 NMR spectroscopy.

19F and 31P nuclei also exhibit NMR spectra, as they both have an odd number of protons.


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(b) How an NMR spectrometer is designed and functions

Below is a schematic diagram of one type of infrared spectrometer

diagram explaining how a 13C 1H NMR spectrometer works permanent external magnetic field radio frequency generator detector recorder

The above diagram explains how an NMR spectrometer works using

  1. a permanent external strong magnetic field from a permanent magnet
  2. a radio frequency generator - the source transmitter - pulsed band of radio waves into the sample
  3. a rotated sample tube - spun at high speed to give the sample the most homogenous magnetic field
  4. a radio frequency receiver and amplifier - signal strength increased and amplified from pulse to pulse
  5. a signal recorder to modify the signal for display and data storage - processed to calculate chemical shift (δ)
  6. a computer, displays the signal strength for each proton environment versus chemical shift (δ in ppm).
  7. The chemical shift is a relative value of the frequencies based on a reference standard.

Other experimental points are discussed in section (c) below including the explanation of what is a chemical shift.

Left part of diagram from https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Spectroscopy/Nuclear_Magnetic_Resonance_Spectroscopy adapted by doc b


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(c) Experimental conditions - techniques, data and problems (with reference to the (b) diagram above)

(i) The sample under investigation

The sample tube is rotated to ensure the sample experiences a homogeneous magnetic field.

The substance under investigation can be the pure liquid or dissolved in a deuterated solvent.

The solvent is based on common organic solvents, but the 1H atoms must be replaced by the 2H isotope (deuterium, symbol D used).

This is to avoid a 1H NMR solvent spectrum overlapping and interfering with the 1H NMR spectrum of the substance under investigation. The NMR frequencies of 2H are different to those of 1H.

Typical solvents include deuterated dichloromethane CD2Cl2, deuterated trichloromethane CDCl3 and deuterated propanone CD3COCD3.

You can also use non-proton solvents like CCl4, tetrachloromethane.

(ii) Obtaining the NMR spectrum (with reference to the diagram in (b) above)

To obtain the NMR spectrum, the external magnetic field is kept constant and the sample is irradiated with a pulsed band of radio frequencies.

The energy is absorbed to raise the magnetic moment quantum levels of all the hydrogen nuclei.

Immediately after the pulse, the precise frequencies absorbed are re-emitted as the 1H nuclei return to their lower energy ground state.

The pulse time is short, so the pulse is rapidly repeated many times to build up a more accurate stronger signal and the data stored for an electronic analysis by a computer.

(iii) NMR data standardisation

In mass spectrometry the base-line standard is the carbon-12 isotope arbitrarily given the value of 12.00000 amu.

You also need a base-line standard in NMR which acts as a reference point for all the NMR resonance frequencies.

The difference between the NMR reference frequency and the frequencies emitted by the protons in their different chemical environments is called the chemical shift (δ).

This is quoted as parts per million (ppm) which is just a mathematical manipulation of the frequency compared to a reference standard.

Chemical shifts are a set of relative values, usually based on the reference standard tetramethylsilane.

Si(CH4)4, which has 16 protons in an identical molecular/chemical environment. - so minimises complications.

TMS gives a single sharp reference signal significantly different in frequency from those emitted from most 1H nuclei chemical environments of interest to chemical structure investigations.

Chemical shifts are referenced to TMS which is arbitrarily assigned a δ value of 0.00

(iv) Data table of chemical shifts for the different chemical environments of 1H (proton) nuclei (shown in blue).

Type of Proton (R = alkyl) and comments Structure 1H Chemical Shift δ, ppm
alkane - primary H, R = alkyl R-CH3 0.7 - 1.6
alkane - secondary H, R = alkyl R2-CH2 1.4 - 2.3
alkane - tertiary H, R = alkyl R3-C-H 1.4 - 2.3
alkene - H directly attached to C of double bond C=C-H 4.5 - 6.0
alkyne Ctriple bondC-H 2 - 3
aromatic (arene) e.g C6H5CH3, often in a close cluster of δ Ar-H 6 - 9
aromatic - aliphatic/alkyl group on ring e.g. C6H5CH3 Ar-C-H 2.2 - 3.0
alkene - methyl group on the next carbon beyond C=C C=C-CH3 1.6 - 1.7
alkene - next carbon beyond C=C in longer chain C=C-CH2-R ~2.3
fluorides, electronegative elements like halogens, oxygen and nitrogen, attached to the same carbon atom as the proton, tend to increase the chemical shift of that particular proton. H-C-F 4 - 4.5
chlorides (see fluorides comment) H-C-Cl 3 - 4
bromides (see fluorides comment) H-C-Br 2.5 - 4
iodides (see fluorides comment) H-C-I 2 - 4
alcohols (alkyl H, NOT the H of the -OH group) H-C-OH 3.4 - 4.8
ethers H-C-OR 3.3 - 4 (- 4.8 ?)
esters - H on 1st C of ...alkyl/aryl group RCOO-C-H 3.7 - 4.1 (- 4.8 ?)
esters - H on 2nd C of ...oate group H-C-COOR 2.0 - 2.2
carbonyl Compounds CH next to C=O in carbonyl compounds - aldehydes, ketones, carboxylic acids, esters, amides H-C-C=O 2.0 - 2.7
aldehyde - the proton of the aldehyde group -CHO R-(H-)C=O 9 - 10
hydroxyl group in alcohols, can be very varied from 0.5 to 4.5 R-C-OH 3.3 - 3.6 *
phenols (hydroxy-arenes), -OH attached to benzene ring Ar-OH 4 - 12 *
carboxylic acid, proton of hydroxyl group RCOOH 9 - 15 *
amino/amines -C-NH- RNH2 and R2NH 1 - 6 *
amides -CO-NH 5 - 12 *

 * The signal (chemical shift) from the protons in the -OH group in alcohols, phenols, carboxylic acids and the and the -NH- group in amines and amides are very variable due to sensitivity to temperature, concentration in the deuterated solvent and hydrogen bonding complications - the stronger the hydrogen bonding, the greater the chemical shift.


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(d) Interpreting 1H (proton) NMR spectra (at low resolution)

In section (a) the equation for the magnetic nuclei quantum level change ∆E = E2 - E1 = h was introduced.

∆E varies because hydrogen nuclei behave differently in different molecular environments - referred to in NMR spectroscopy as different chemical environments.

NMR spectroscopy can tell as what the chemical environment of a proton (1H nucleus) is in a molecule and the ratio of protons in the different chemical environments. e.g. see the data table in section (c).

This means for every type of molecular arrangement (molecular/chemical environment), the proton experiences a slightly different magnetic field and data can be accumulated corresponding to different parts of a molecule's structure.

Therefore the 1H proton nuclei in these different chemical environments will resonate with slightly different slightly different frequencies () because the ∆E resonance energy will differ, this enables 1H NMR spectroscopy to provide valuable information on molecular structure and the number of protons in each different chemical environment.

From the NMR spectrometer you can now obtain the intensity of the NMR emissions versus the chemical shift and referenced to the TMS standard where δ = 0.00 ppm.

The chemical shift results from a mathematical manipulation of ∆E and calibrated against the TMS reference (δ = 0 ppm).

 

Five examples interpreted with explanation in terms of a low resolution 1H NMR spectra

1.  

Example 1. Low resolution 1H NMR spectrum of ethanal

The hydrogen atoms (protons) of ethanal occupy 2 different chemical environments so that the low resolution NMR spectra should show 2 peaks of different H-1 NMR chemical shifts (diagram above for ethanal).

CH3CHO 

Note the 3:1 ratio of the 2 colours of the protons in the 2 chemically different environments

Although there are 4 hydrogen atoms in the molecule, there only 2 possible chemical environments for the hydrogen atoms in ethanal molecule, so you only see two peaks at low resolution.

The proton ratio 3 : 1 observed, corresponds with the structural formula of ethanal.

Note the relatively large chemical shift for the proton of the aldehyde group.

 

2.

Example 2. Low resolution 1H NMR spectrum of ethanol

The hydrogen atoms (protons) of ethanol occupy 3 different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 peaks (diagram above).

CH3CH2OH

Note the ratio of the 3 colours for the 3 proton chemical environments in ethanol in the ratio 3:2:1 for the 6 protons.

 

3.

Example 3. Low resolution 1H NMR spectrum of ethyl ethanoate

The hydrogen atoms (protons) of ethyl ethanoate occupy three different chemical environments so that the H-1 proton low resolution NMR spectra should show 3 peaks (diagram above for ethyl ethanoate).

CH3COOCH2CH3

Note the ratio 3:2:3 of the three colours of the protons in the three chemically different environments.

Although there are 8 hydrogen atoms in the molecule, there only 3 possible chemical environment for the hydrogen atoms in ethyl ethanoate molecule.

The proton ratio of 3:2:3 observed, corresponds with the structural formula of ethyl ethanoate.

 

4.

Example 4. Only considering a low resolution 1H NMR spectrum of 1-chlorobutane

You can illustrate this with a coloured structural formula of 1-chlorobutane.

CH3CH2CH2CH2Cl

(note the 4 colours indicating the 4 different chemical environment of the hydrogen atoms).

The integrated proton signal ratio is 3 : 2 : 2 : 2 for the four different proton environments giving four principal and different principal chemical shift peaks - ignoring the splitting for the moment.

The hydrogen atoms (protons) of 2-chlorobutane occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for 2-chlorobutane - ignoring the line splitting for the moment).

Note that 1-chlorobutane (3:2:2:2) can be distinguished from the 2-chlorobutane (3:1:2:3) isomer by the different proton ratios.

 

5.

Example 5. Only considering a low resolution 1H NMR spectrum of 2-chlorobutane

The hydrogen atoms (protons) of 2-chlorobutane occupy 4 different chemical environments so that the low resolution NMR spectra should show 4 principal peaks of different H-1 NMR chemical shifts (diagram above for 2-chlorobutane).

CH3CHClCH2CH3

Note the proton ratio 3:1:2:3 of the 4 colours of the protons in the 4 chemically different environments

Chemical shifts (a) to (d) on the H-1 NMR spectrum diagram for 2-chlorobutane - ignoring the line splitting for the moment.

Although there are 9 hydrogen atoms in the molecule, there are only 4 possible different chemical environments for the hydrogen atoms in 2-chlorobutane molecule.

The integrated signal proton ratio 3:1:2:3 observed in the high resolution H-1 NMR spectrum, corresponds with the structural formula of 2-chlorobutane.

Note that 1-chlorobutane (3:2:2:2) can be distinguished from the 2-chlorobutane (3:1:2:3) isomer by the different proton ratios.


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(e) Interpreting 1H (proton) NMR spectra and field splitting effects (high resolution spectra)

Origin of the n + 1 rule

You will notice in the spectra analysed at low resolution in (d) that there are rather more 'lines' that you might expect!

Let us assume the external fixed magnetic field is orientated -N p S- with 'proton magnet' between these poles.

We can now consider:

(i) the various possible variations in orientation/alignments of the proton and then ...

(ii) their effect on neighbouring equivalent protons e.g. an adjacent carbon atom with equivalent protons -CHx where x = 1, 2 or 3 and this is known as coupling.

Protons attached to the same carbon are usually equivalent to each other.

n = number of protons causing the coupling effect.

1 proton e.g. -CH- group (n = 1)

(i) There are two possible orientations for one proton:

Aligned with the external field S-N and against the external field N-S.

(ii) This results in splitting the 1NMR resonance signal of neighbouring equivalent protons in two - they 'sense' two slightly different magnetic fields.

This results in a doublet line of equal height intensity 1:1 (n + 1 = 2).

Reminder: The interactions between protons on neighbouring carbon atoms is called coupling.

2 protons e.g. -CH2- group (n = 2)

(i) There are three possible alignments for the two protons are

S-N  S-N both aligned with the external magnetic field

S-N  N-S one aligned with, and one against, the external magnetic field

N-S  N-S both aligned against the external magnetic field

(ii) The coupling results in splitting the 1NMR resonance signal of neighbouring equivalent protons in three - they 'sense' three slightly different magnetic fields.

This results in a triplet of lines of intensities in the ratio 1:2:1 (n + 1 = 2).

3 protons e.g. -CH3 group (n = 3)

(i) There are four possible alignments for the three protons are

S-N  S-N  S-N all three aligned with the external magnetic field

S-N  S-N  N-S two aligned with, and one against, the external magnetic field

S-N  N-S  N-S one aligned with, and two against, the external magnetic field

N-S  N-S  N-S all three aligned against the external magnetic field

(ii) The coupling results in splitting the 1H NMR resonance signal of neighbouring equivalent protons in four - they 'sense' four slightly different magnetic fields.

This results in a quartet of lines of intensities in the ratio 1:3:3:1 (n + 1 = 3).

EXTRA NOTES:

You not have to work out the theoretical intensity ratios quoted above and in the table below.

These are the result of all the probabilities that are possible from the couplings of the magnetic moments between sets of neighbouring protons.

You can see that the number of resonance lines produced for a particular hydrogen nucleus (1H, proton) is n + 1 where n = the number of adjacent protons doing the splitting

This is the theoretical origin of the n+1 rule - which is applied in in this section below,

The splitting pattern of resonance lines becomes more complex for a given -CHx, if two or three neighbouring carbon atoms are bonded to protons e.g -CH2-CH2-CH3 where n = 5.

The n+1 rule still applies, but n can be anything from 2 to 9 and the signal intensity ratios also get more complicated - but don't worry about it.

The intensity of the integrated NMR resonance signals can be expressed as the simplest whole number ratio of the protons in their different chemical environments - this may or may not be the actual relative numbers of protons in the molecule.

The possible field splitting patterns from the n+1 rule

(and extending the argument from above, but NOT working out all the possible N-S alignments)

Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1

How to apply the n + 1 rule (using the same five spectra analysed at low resolution in section (d)

5 examples interpreted with explanation in terms of a high resolution 1H NMR spectra, applying the n+1 rule

1.

Example 1. The high resolution 1H NMR spectrum of ethanal

CH3CHO 

All low and high resolution spectra of ethanal show 2 groups of protons and in the ratio of 3 : 1 expected from the formula of ethanal.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of ethanal - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin filed splitting effects.

So, using the chemical shifts and applying the n+1 rule to ethanal

Chemical shift 2.21 ppm for the CH3 protons

The methyl proton resonance is split into a 1 : 1 doublet by the single proton of the aldehyde CHO group (1 proton, n+1 = 2 = doublet).

Evidence for the presence of a CH group in the molecule of ethanal

Chemical shift 9.79 ppm for the CH proton

The aldehyde group proton resonance is split into a 1 : 3 : 3 : 1 quartet by the methyl group protons (3 protons, n+1 = 4 = quartet).

Evidence for the presence of a CH3 group in the molecule of ethanal

 

2.

Example 2. The high resolution 1H NMR spectrum of ethanol

CH3CH2OH

Note the ratio of the 3 colours for the 3 proton chemical environments in ethanol.

In terms of the H-1 chemical shifts for ethanol (a) to (c) and applying the n+1 rule:

(a) Centred at 1.22 ppm, the CH3 protons are split by the CH2 protons into a 1 : 2 : 1 triplet (n+1 = 3).

(b) Centred at 3.69 ppm, the CH2 protons are split by the CH3 protons into a 1 : 3 : 3 : 1 quartet (n+3 = 4).

(c) The hydroxy proton O-H gives a chemical shift of 2.61 ppm and shows no significant splitting.

Normally the O-H proton resonance is not split by adjacent protons and neither does it, in turn, split the resonance of the same adjacent carbon atom protons.

The integrated NMR proton ratio observed of 3 : 2 : 1 corresponds with the structural formula of ethanol.

 

3.

Example 3. The high resolution 1H NMR spectrum of ethyl ethanoate

The high resolution spectrum of ethyl ethanoate: proton ratio 3:2:3

CH3COOCH2CH3

So, using the chemical shifts and applying the n+1 rule to ethyl ethanoate

At 0.92 ppm the CH2 protons split the 'right-hand' CH3 shift into 1:2:1 triplet.

Evidence of the CH2 group in the molecule of ethyl ethanoate

At 2.04 ppm the 'left-hand' CH3 protons show no evident splitting shift by adjacent protons

Evidence that one of the CH3 groups is not joined to another carbon atom with protons - as is the case with the ethyl ethanoate molecule.

The proton ratio in the spectrum allows the deduction of two methyl groups in the molecule.

At 4.10 ppm the CH2 proton line is split into a 1:3:3:1 quartet by the adjacent methyl group.

Evidence that there is indeed another CH3 group joined to another carbon atom with protons - as is the case with ethyl ethanoate molecule.

 

4.

Example 4. The high resolution 1H NMR spectrum of 1-chlorobutane

CH3CH2CH2CH2Cl

There are four different chemical environment of the 9 protons in 1-chlorobutane and all resonances will be split by coupling with neighbouring protons

As you can see, the high resolution spectrum of 1-chlorobutane is complex when applying the n+1 rule

The left-hand end CH3 is split by the adjacent CH2 into a 1 : 2 : 1 triplet at 0.92 ppm (n+2 = 3).

The right-hand end CH2 is split into a 1 : 2 : 1 triplet by the adjacent CH2 at 3.42 (n+2 = 3).

However, the two 'inner' sets of CH2 protons are split on both sides by adjacent non-equivalent protons into multiple resonance lines.

The 1.41 ppm chemical shift:

From the n+1 rule, the 'left-hand' CH2 protons (H2) are split by CH3 protons (H3) and by the middle CH2 protons (H2), (5 protons in total), into a 1:5:10:10:5:1 sextet of resonance lines (n+5 = 6).

This is pattern of resonances is a good indication of a propyl group (CH3CH2CH2).

The 1.68 ppm chemical shift:

The middle CH2 protons (H2) are split on both sides by CH2 protons (H2 and H2), (4 protons in total), into a 1:4:6:4:1 quintet of resonance lines (n+4 = 5).

 

5.

Example 5. The high resolution 1H NMR spectrum of 2-chlorobutane

CH3CHClCH2CH3

All low and high resolution spectra of 2-chlorobutane show 4 groups of proton resonances and in the ratio expected from the formula of 2-chlorobutane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 2-chlorobutane - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on 2-chlorobutane below.

So, using the chemical shifts and applying the n+1 rule to 2-chlorobutane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 1.50 ppm, CH3 protons: CH3CHClCH2CH3

This 1H resonance is split by the adjacent CH proton into a 1:1 doublet (n+1 = 2).

Evidence for the presence of a CH3 group in the molecule of 2-chlorobutane

(b) 1H Chemical shift 3.97 ppm, CH proton :  CH3CHClCH2CH3

This 1H resonance is split by the adjacent CH3 and CH2 protons into a 1:5:10:10:5:1 sextet (n+1 = 6).

Evidence for the presence of a CH3-C-CH2 grouping in the molecule of 2-chlorobutane

(c) 1H Chemical shift 1.71 ppm, CH2 protons : CH3CHClCH2CH3

This 1H resonance is split by the adjacent CH3 and CH protons into a 1:4:6:4:1 quintet (n+1 = 5).

Evidence for the presence of a CH3-C-CH group in the molecule of 2-chlorobutane

(d) 1H Chemical shift 1.02 ppm, CH3 protons : CH3CHClCH2CH3

This 1H resonance is split by the adjacent CH2 protons into a 1:2:1 triplet (n+1 = 3).

Evidence for the presence of a CH2 group in the molecule of 2-chlorobutane


Key words and phrases: what is the theoretical basis of 1H NMR? How do you interpret a 1H NMR spectrum What is the n+1 rule? How do you apply the n + 1 rule? What is the difference between low resolution and high resolution 1H proton spectra?


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