Chemical shifts

Question 1

What is the reason NMR is so useful?

Answer 1

Different atoms in a molecule have different resonance frequencies

Question 2

Why do different atoms in a molecule have different resonance frequencies?

Answer 2

Electrons in molecules interact with an applied magnetic field to produce smaller induced local magnetic fields

Question 3

What causes electrons to move in a magnetic field?

Answer 3

they are charged so they experience a force which causes them to move

Question 4

What does a local diamagnetic current do?

Answer 4

Opposes the applied field and has the effect of reducing the effective magnetic field at any nearby nucleus

Question 5

When there is a local diamagnetic current, what happens to the resonant frequency? What is this called?

Answer 5

The resonant frequency of the nucleus will therefore change slightly depending on the electronic environment
Chemical shift

Question 6

What does the chemical shift tell you about?

Answer 6

The environment of the atom in the molecule

Question 7

Why is the chemical shift different for different atoms?

Answer 7

They exist in slightly different electronic environments within a molecule

Question 8

What does chemical shift depend on?

Answer 8

The elements present and the type of bonding

Question 9

How does the magnetic field at nucleus compare to the applied field?

Answer 9

It is generally less than the applied field

Question 10

What are differences in resonance frequencies like?

Answer 10

They are very small

Question 11

What equation is used to calculate the chemical shift?

Answer 11

δ = v-v(reference) / v(measuring frequency)

Question 12

What are the features of a molecule with chemical shift lower than the reference?

Answer 12

Shielded

Low frequency

Question 13

What are the features of a molecule with chemical shift higher than the reference?

Answer 13

Deshielded

High frequency

Question 14

What is the name of the reference compound used for NMR?

Answer 14

TMS

Tetramethylsilane

Question 15

What is the chemical shift of the reference compound?

Answer 15

0

Question 16

Why is TMS used as a reference compound?

Answer 16

All of the protons and carbons are more shielded than other compounds
Gives one sharp peak at 0 because all hydrogens are in the same environment

Question 17

How does an NMR spectrum excite different protons at the same time?

Answer 17

A short powerful burst of energy is applied which excites all magnetic nuclei of the same species being observed in one go

Question 18

Generally, what pulse of energy is given?

Answer 18

90 MHz

Question 19

What happens to the nuclei after the pulse?

Answer 19

They relax, losing energy and returning to their original spin state

Question 20

What happens when the nuclei relaxes?

Answer 20

It emits electromagnetic radiation

Question 21

Why are there many different frequencies of electromagnetic radiation released?

Answer 21

There are many different nuclei relaxing

Question 22

What is the emission from relaxing nuclei called?

Answer 22

free-induction decay (FID)

Question 23

What does it mean if there is one absorption in proton NMR?

Answer 23

All protons are chemically equivalent

Question 24

What does it mean if there are 2 absorptions in proton NMR?

Answer 24

There are two different sets of chemically equivalent protons

Question 25

What does it mean if there are 3 absorptions in proton NMR?

Answer 25

Three different sets of chemically equivalent protons

Question 26

What do different types of protons have?

Answer 26

Different chemical shifts

Question 27

What does each type of proton have?

Answer 27

A characteristic value of chemical shift

A limited range of δ values over which it gives a resonance

Question 28

What can the δ value be used for?

Answer 28

As a clue as to which type of proton has given rise to the signal

Question 29

How does electronegativity affect the chemical shift?

Answer 29

δ increases as the electronegativity increases

Question 30

Why does δ increase with electronegativity increasing?

Answer 30

Electronegative atoms withdraw electron density
Reduces valence electron density around protons that are attached to the same carbon
This reduces shielding by the electrons of the applied magnetic field and the proton is said to be deshielded

Question 31

Within what range of ppm are protons attached to an sp3 proton found?

Answer 31

0-2ppm

Question 32

What effect does an sp2 C-H bond have on the ppm?

Answer 32

Higher than the sp3

Question 33

Why is sp2 different to sp3?

Answer 33

In sp2, the carbon atom has more s character which makes it more electronegative than sp3 carbon
So there is less shielding

Question 34

Where do sp hydrogens appear on ppm?

Answer 34

2-3 ppm

Question 35

Where do carboxylic acid protons resonant?

Answer 35

10.5 - 12ppm

Question 36

What effect do acidic protons have? (H NMR)

Answer 36

Resonance and the electronegativity effect of oxygen withdraw electrons from the acidic protons

Question 37

What effect does hydrogen bonding have? (H NMR)

Answer 37

Protons which can exhibit H-bonding resonate at variable positions
The more hydrogen bonding, the more deshielded a proton is

Question 38

What are magnetic anisotropy effects?

Answer 38

There can be both shielding and deshielding effects on atoms from electron motion in other parts of the molecule

Question 39

How are magnetic anisotropy effects caused?

Answer 39

By anisotropic electron circulation

Question 40

What does moving electrons generate?

Answer 40

a magnetic field

Question 41

What does anisotropic field mean?

Answer 41

A non-uniform field

Question 42

What happens to pi electrons in an aromatic ring when they are placed in a magnetic field?

Answer 42

They are induced to circulate around ring and this creates a ring current