Nuclear Magnetic Resonance Spectroscopy Revision Notes for VCE SSCE Chemistry

Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most powerful techniques for determining the structure of complex molecules. When combined with infrared (IR) spectroscopy and other analytical methods, NMR can help you work out the exact structure of an unknown organic compound.

What is NMR spectroscopy?

NMR spectroscopy uses electromagnetic radiation in the radio frequency range to gather information about molecular structure. Unlike other forms of spectroscopy you've studied, the energy of radio waves is too low to cause electronic, vibrational, or rotational transitions in molecules. Instead, NMR detects changes in nuclear spin states.

Medical application: MRI

infoNote

A form of NMR technology called Magnetic Resonance Imaging (MRI) is used in medicine to create highly detailed pictures of internal body structures. MRI can detect diseased tissue without using harmful radiation like X-rays.

Principles of nuclear magnetic resonance spectroscopy

Nuclear spin

For atoms to be detected by NMR spectroscopy, their nuclei must have a property called nuclear spin. Only nuclei with an odd number of protons and/or neutrons possess nuclear spin. The most important examples for organic chemistry are:

Hydrogen-1 ( $^1$ H) - one proton, no neutrons
Carbon-13 ( $^{13}$ C) - six protons, seven neutrons
Phosphorus-31 ( $^{31}$ P) - fifteen protons, sixteen neutrons

The odd number of nuclear particles causes these nuclei to behave like tiny bar magnets.

Energy transitions in a magnetic field

When placed in an external magnetic field, nuclei with spin can adopt one of two orientations:

Lower energy state: aligned with the magnetic field
Higher energy state: aligned against the magnetic field

Inside an NMR spectrometer, nuclei normally exist in the lower energy state. A radio frequency transmitter provides energy to 'flip' nuclei into the higher energy state. Over time, the nuclei naturally return to the lower energy state, releasing a pulse of energy as they do so. This released energy is detected and presented as an NMR spectrum.

chatImportant

The energy difference between the two spin states depends on:

The type of nucleus being examined
The chemical environment surrounding the nucleus

This dependence on chemical environment is what makes NMR such a powerful tool for structure determination!

Types of NMR spectroscopy

Proton ( $^1$ H) NMR spectroscopy is the most common form. It examines hydrogen-1 nuclei and provides information about any molecule containing hydrogen atoms. Note that chemists often use the terms 'hydrogen' and 'proton' interchangeably when discussing NMR.

Carbon-13 ( $^{13}$ C) NMR spectroscopy examines carbon-13 nuclei and is valuable for investigating the carbon backbone of organic molecules.

Chemical environments

What is a chemical environment?

A chemical environment is made up of the atoms and electrons that surround a specific atom in a molecule. Atoms are in the same chemical environment if they are:

Attached in the same way
Bonded to the same types of atoms
Positioned symmetrically within the molecule

Atoms in the same chemical environment are called equivalent atoms. All equivalent atoms produce a single signal in an NMR spectrum.

chatImportant

Key Principle: The number of signals in an NMR spectrum equals the number of different chemical environments.

Examples of chemical environments

lightbulbExample

Worked Example: Identifying Chemical Environments

Ethane ( $\text{CH}_3\text{CH}_3$ ):

All six hydrogen atoms are equivalent (each is part of a $-\text{CH}_3$ group)
The two carbon atoms are equivalent
Result: ONE hydrogen signal and ONE carbon signal

Propane ( $\text{CH}_3\text{CH}_2\text{CH}_3$ ):

Six hydrogen atoms in the $-\text{CH}_3$ groups are equivalent
Two hydrogen atoms in the central $-\text{CH}_2-$ group form a different environment
Result: TWO hydrogen signals and TWO carbon signals

2,2-dichlorobutane:

Three different hydrogen environments
Four different carbon environments
Result: THREE hydrogen signals and FOUR carbon signals

infoNote

General rule: Molecules with more symmetry have fewer different chemical environments. The more planes of symmetry a molecule has, the fewer unique chemical environments it contains.

How NMR spectrometers work

An NMR spectrometer contains:

A powerful magnet that creates a strong external magnetic field
A spinning sample tube (ensures uniform magnetic field exposure)
A radio frequency transmitter (provides energy to flip nuclear spins)
A detector (measures energy released as nuclei return to lower energy states)
A computer (analyses signals and produces the NMR spectrum)

The sample is dissolved in a special solvent that doesn't produce NMR signals. Common solvents include $\text{CDCl}_3$ , $\text{D}_2\text{O}$ , and $\text{CD}_3\text{Cl}$ . These contain deuterium ( $^2$ H), oxygen-16, or carbon-12, which don't show up in proton or carbon-13 NMR spectroscopy.

Interpreting proton NMR spectra

A proton NMR spectrum provides four key pieces of information:

Number of signals - shows how many different hydrogen environments exist
Relative peak area - indicates the number of hydrogen atoms in each environment
Signal splitting pattern - reveals information about neighbouring hydrogen atoms
Chemical shift - helps identify the type of chemical environment

Peak area and integration

The area under each signal (called the peak area) is proportional to the number of hydrogen atoms in that environment. The ratio of peak areas tells you the ratio of hydrogen atoms in each environment.

chatImportant

You may need to multiply the ratio by a factor to account for all hydrogen atoms in the molecule.

lightbulbExample

Worked Example: 2,2,4,4-tetrachloropentane

This molecule shows two signals with relative peak areas of 1:3.

The 1:3 ratio must be multiplied by 2 to account for all 8 hydrogen atoms
Therefore: 2 hydrogen atoms produce one signal, 6 hydrogen atoms produce the other signal

Chemical shifts and the TMS reference

Different NMR spectrometers operate at different magnetic field strengths, which would make comparing results difficult. To solve this problem, chemists use a reference compound called tetramethylsilane (TMS).

TMS has several useful properties:

Chemically inert (doesn't react with samples)
Contains only one chemical environment for both hydrogen and carbon
Produces a single, sharp peak
Its signal appears away from most organic compound signals

The chemical shift ( $\delta$ ) measures how far a signal is from the TMS reference peak. Chemical shift is measured in parts per million (ppm). By definition, TMS has a chemical shift of zero ( $\delta = 0$ ppm).

Why do chemical shifts vary?

The electrons surrounding each nucleus have their own magnetic properties. These electrons shield the nucleus from the external magnetic field. The amount of shielding depends on the nearby atoms in the molecule.

Different functional groups create different amounts of electron shielding, which affects the energy needed for nuclear spin changes. This is why hydrogen atoms in different chemical environments have different chemical shifts.

infoNote

For example:

Hydrogen in a $-\text{CH}_3$ group has a different chemical shift than hydrogen in a $-\text{CH}_2-$ group
Hydrogen in an $-\text{OH}$ group has a different chemical shift again

Typical proton NMR chemical shifts

Type of hydrogen	Chemical shift (ppm)
$\text{R}-\text{CH}_3$	0.9-1.0
$\text{R}-\text{CH}_2-\text{R}$	1.3-1.4
$\text{RCH}=\text{CH}-\text{CH}_3$ (allylic)	1.6-1.9
$\text{R}_3\text{CH}$	1.5
$\text{CH}_3-\text{C}(=\text{O})-\text{OR}$ or $\text{CH}_3-\text{C}(=\text{O})-\text{NHR}$	2.0
$\text{R}-\text{CH}_2-\text{C}(=\text{O})-$	2.1-2.7
$\text{R}-\text{CH}_2-\text{X}$ (X = F, Cl, Br, I)	3.0-4.5
$\text{R}-\text{CH}_2-\text{OH}$ or $\text{R}_2-\text{CH}-\text{OH}$	3.3-4.5
$\text{R}-\text{C}(=\text{O})-\text{OCH}_3$	3.7-4.8
$\text{R}-\text{O}-\text{H}$	1-6 (varies widely)
$\text{R}-\text{NH}_2$	1-5
$\text{RHC}=\text{CHR}$ (alkene)	4.5-7.0
Amide N-H	5.5-8.5
Aldehyde H	9.4-10.0
Carboxylic acid O-H	9.0-13.0

infoNote

Study tip: Methyl groups ( $-\text{CH}_3$ ) typically have the lowest chemical shift values in a spectrum.

Signal splitting and the $n+1$ rule

In high-resolution NMR spectra, signals often split into multiple peaks. This splitting occurs because of neighbouring hydrogen atoms. A neighbouring environment is one that is up to three bonds away from the hydrogen atoms producing the signal.

chatImportant

The $n+1$ rule: The number of peaks in a signal equals $n + 1$ , where $n$ is the number of equivalent hydrogen atoms in neighbouring environments (but not equivalent to the hydrogen atoms producing the signal).

Common splitting patterns:

Singlet (1 peak): no neighbouring hydrogen atoms
Doublet (2 peaks): one neighbouring hydrogen atom ( $n = 1$ , so $1 + 1 = 2$ )
Triplet (3 peaks): two neighbouring hydrogen atoms ( $n = 2$ , so $2 + 1 = 3$ )
Quartet (4 peaks): three neighbouring hydrogen atoms ( $n = 3$ , so $3 + 1 = 4$ )

lightbulbExample

Worked Example: Chloroethane

In chloroethane:

The blue $-\text{CH}_3$ $- CH_{3}$ group has 2 neighbouring red hydrogen atoms
- Splitting: $2 + 1 = 3$ (triplet)
The red $-\text{CH}_2-$ $- CH_{2} -$ group has 3 neighbouring blue hydrogen atoms
- Splitting: $3 + 1 = 4$ (quartet)

The peak area ratio is 3:2, matching the number of hydrogen atoms in each environment.

lightbulbExample

Worked Example: Methyl propanoate

Red $-\text{CH}_3$ attached to oxygen: more than 3 bonds from other hydrogens → singlet
Blue $-\text{CH}_2-$ : has 3 neighbouring hydrogens in the yellow $-\text{CH}_3$ → quartet
Yellow $-\text{CH}_3$ : has 2 neighbouring hydrogens in the blue $-\text{CH}_2-$ → triplet

Special cases in signal splitting

infoNote

Equivalent neighbouring atoms don't cause splitting: If neighbouring hydrogen atoms are in the same chemical environment as the hydrogen atoms producing the signal, they won't cause splitting.

Hydroxyl groups: The hydrogen atom in alcohol $-\text{OH}$ groups:

Usually doesn't split neighbouring hydrogen signals
Usually appears as a singlet itself
Doesn't count as a neighbour when applying the $n+1$ rule

Multiplets: Sometimes you may see more complex splitting patterns called multiplets. These occur when hydrogen atoms have non-equivalent neighbouring groups, leading to overlapping splitting patterns.

Systematic interpretation of proton NMR spectra

Follow these steps to identify an unknown compound from its proton NMR spectrum:

Step 1: Count the number of signals to determine how many different hydrogen environments exist.

Step 2: Examine the relative peak areas to find the ratio of hydrogen atoms in each environment. Remember to scale this ratio to match the total number of hydrogens in the molecular formula.

Step 3: Analyse the splitting pattern of each signal using the $n+1$ rule to identify neighbouring hydrogen atoms.

Step 4: Use the chemical shift values and Table 14.3.1 to identify the types of functional groups present.

Step 5: Combine all the information to propose a molecular structure. Check that your structure:

Has the correct molecular formula
Produces the observed number of signals
Shows the correct peak area ratios
Displays the expected splitting patterns
Has chemical shifts matching the observed values

lightbulbExample

Worked Example: Identifying Dichloroethane Isomers

Problem: A compound has the molecular formula $\text{C}_2\text{H}_4\text{Cl}_2$ . Its proton NMR spectrum shows:

A doublet at 2.1 ppm (relative area = 3)
A quartet at 5.9 ppm (relative area = 1)

Solution:

Step 1 - Possible structures: Two isomers are possible:

1,2-dichloroethane: $\text{ClCH}_2\text{CH}_2\text{Cl}$
1,1-dichloroethane: $\text{CH}_3\text{CHCl}_2$

Step 2 - Analyse the spectrum:

Chemical shift (ppm)	Peak splitting	Relative peak area
2.1	Doublet	3
5.9	Quartet	1

Step 3 - Number of environments: Two signals = two different hydrogen environments

Step 4 - Number of hydrogens:

Peak area ratio 1:3 directly gives the number of hydrogens (total = 4)
One environment has 1 hydrogen, the other has 3 hydrogens
This indicates a $-\text{CH}$ group and a $-\text{CH}_3$ group

Step 5 - Interpret splitting patterns:

Doublet (2.1 ppm): $n + 1 = 2$ , so $n = 1$ neighbour → this is the $-\text{CH}_3$ group
Quartet (5.9 ppm): $n + 1 = 4$ , so $n = 3$ neighbours → this is the $-\text{CH}$ group

Step 6 - Conclusion: The molecule must be 1,1-dichloroethane ( $\text{CH}_3\text{CHCl}_2$ ) because:

It has a $-\text{CH}_3$ group adjacent to a $-\text{CH}-$ group
The splitting patterns match: the methyl group splits the $-\text{CH}-$ signal into a quartet, and the $-\text{CH}-$ splits the methyl signal into a doublet
1,2-dichloroethane would show only one signal (all hydrogens equivalent)

Carbon-13 NMR spectroscopy

Principles of carbon-13 NMR

Carbon-13 ( $^{13}$ C) is a naturally occurring isotope that makes up only 1.1% of all carbon atoms. Unlike carbon-12 (the most abundant isotope), carbon-13 has nuclear spin and can be detected by NMR spectroscopy.

Carbon-13 NMR identifies different carbon atom environments within a molecule. Like proton NMR, it uses TMS as a reference standard (defined as $\delta = 0$ ppm).

Key differences from proton NMR

chatImportant

1. Chemical shift range: Carbon-13 chemical shifts range from 0 to about 220 ppm, much wider than the proton NMR range (0-13 ppm).

2. No splitting patterns: Because carbon-13 is only 1.1% abundant, it's extremely unlikely that two carbon-13 atoms will be next to each other in a molecule. Therefore, carbon-13 NMR spectra don't show splitting by other carbon atoms. All signals appear as single peaks.

3. Peak areas: The peak areas in carbon-13 NMR are NOT directly proportional to the number of carbon atoms in each environment. You cannot use integration to count carbons.

Typical carbon-13 NMR chemical shifts

Type of carbon	Chemical shift (ppm)
$\text{R}-\text{CH}_3$	8-25
$\text{R}-\text{CH}_2-\text{R}$	20-45
$\text{R}_3\text{CH}$	40-60
$\text{R}_4\text{C}$	36-45
$\text{R}-\text{CH}_2-\text{X}$ (X = F, Cl, Br, I)	15-80
$\text{R}_3\text{C}-\text{NH}_2$ or $\text{R}_3\text{C}-\text{NR}_2$	35-70
$\text{R}-\text{CH}_2-\text{OH}$	50-90
$\text{RC}\equiv\text{CR}$ (alkyne)	75-95
$\text{R}_2\text{C}=\text{CR}_2$ (alkene)	110-150
$\text{RCOOH}$ (carboxylic acid)	160-185
Ester carbon	165-175
Ketone/aldehyde carbon	190-220

infoNote

Study tip: Carbonyl carbons ( $\text{C}=\text{O}$ ) have the highest chemical shifts, appearing at 160-220 ppm.

Comparing proton and carbon-13 NMR

For the same molecule (such as chloroethane shown above):

Both types of NMR show the same number of signals (equal to the number of different environments)
Proton NMR shows splitting patterns; carbon-13 NMR doesn't
Proton NMR peak areas are proportional to hydrogen count; carbon-13 peak areas are not reliable for counting carbons
Chemical shift ranges are different (0-13 ppm for $^1$ H, 0-220 ppm for $^{13}$ C)

Remember!

bookmarkSummary

Key Points to Remember:

NMR spectroscopy uses radio frequency radiation to detect nuclear spin changes in molecules, providing detailed structural information.
The number of signals in an NMR spectrum equals the number of different chemical environments. Equivalent atoms (those in the same chemical environment) produce one signal.
In proton NMR, four key features help identify structures: (1) number of signals shows hydrogen environments, (2) peak area ratio shows relative number of hydrogens, (3) chemical shift indicates functional groups, and (4) splitting pattern reveals neighbouring hydrogens.
The $n+1$ rule states that a signal splits into $n+1$ peaks, where $n$ is the number of equivalent neighbouring hydrogen atoms within three bonds. Common patterns: singlet (no neighbours), doublet (1 neighbour), triplet (2 neighbours), quartet (3 neighbours).
Carbon-13 NMR shows the number of different carbon environments but differs from proton NMR in three important ways: wider chemical shift range (0-220 ppm), no splitting patterns, and peak areas not proportional to carbon count.

Nuclear Magnetic Resonance Spectroscopy (VCE SSCE Chemistry): Revision Notes