Topic 9 - Spectroscopy

Question 1

What are the energy graduations across the Electromagnetic spectrum from gamma to radio spectra?

Answer 1

Gamma, x-ray, ultraviolet, visible, infrared, microwaves, radio waves.

Question 2

What causes the nucleus of an atom to have a magnetic moment?

Answer 2

Atomic nuclei both spin on an axis and are positively charged creating a magnetic moment (causing it to behave to some extent like a bar magnet)

Question 3

What is the spin quantum number?

Answer 3

The nuclear spin and therefore the magnetic moment can be quantized - this value is the spin quantum number.

Question 4

What is the spin quantum number of nuclei containing an odd number of protons or neutrons or both?

Answer 4

1/2

Question 5

What two very common nuclei have a spin quantum number of 0 (due to having zero spin and therefore zero magnetic moment)?

Answer 5

¹²C and ¹⁶O because they have even numbers of protons and neutrons.

Question 6

Describe the orientation of the nuclei before being subjected to a magnetic field.

Answer 6

The nuclei are orientated randomly.

Question 7

Describe the orientation of the nuclei in the presence of a magnetic field.

Answer 7

The orient themselves to either align with the magnetic field or oppose it. (Slightly nuclei more align than oppose).

Question 8

What is the spin state of a nucleus that orients itself to oppose the magnetic field?

Answer 8

• 1/2

Question 9

What is the spin state of a nucleus that orients itself to align with the magnetic field?

Answer 9

1/2

Question 10

Which of the two orientations of the nuclei in NMR is higher energy?

Answer 10

The one which opposes the magnetic field.

Question 11

Which of the two orientations of the nuclei in NMR is lower energy?

Answer 11

The one which aligns with the magnetic field.

Question 12

What effect does increasing either the strength of the magnetic field is or the frequency of the radio waves (one must remain constant) have on the nuclei undergoing NMR spectroscopy?

Answer 12

The nuclei which are aligned with the magnetic field change their orientation to the higher energy state (so that they now oppose the magnetic field).

Question 13

What is nuclear magnetic resonance and how is it detected?

Answer 13

This is a measure of the energy required in the form of electromagnetic radiation (radio waves) to cause the nuclei to “flip”.

When the electromagnetic radiation is removed, the energized “beta” (-1/2) nuclei that flipped to opposed the spin lose their excess energy and once again return to the lower “alpha” (1/2) spin state. This causes fluctuations in the magnetic field which can be picked up and measured.

Question 14

What does the nuclear magnetic resonance for an atomic nucleus depend on?

Answer 14

The identity (type and structural position) of the nucleus and the strength of the electromagnetic field.

Question 15

How are variations in the strength of the magnetic field in different NMR machines accounted for.

Answer 15

By using a “standard” chemical as a baseline. Tetramethylsilane is used (in both ¹³C and ¹H NMR) because all the hydrogen (or C) atoms are in the same environment; I.e bonded in the same way. And hence gives a single line on the spectrum. The magnetic field in increased until the exact condition of resonance is obtained and that value is used as a basis to compare the resonances of other compounds with.

Question 16

What is the chemical shift?

Answer 16

This is the difference between the field strength needed to bring the protons in a substance into resonance and that needed for TMS.

Question 17

What is the chemical shift measured in?

Answer 17

Parts per million.

Question 18

What range of values in parts per million do most organic molecules fall into?

Answer 18

0-10

Question 19

What is NMR used for?

Answer 19

It is widely used to identify substances and only a few milligrams of substance are needed.

Question 20

Give a medical use of NMR.

Answer 20

An MRI scan uses the same principle to give a scan of body tissues as protons (hydrogen nuclei) in lipids, carbohydrates, proteins and water all give different signals as they are in different environments (bonded differently).

Question 21

Describe the d orbitals of a transition metal which is not involved in chemical bonding and what name is given to this?

Answer 21

They are all at the same energy level and are said to be degenerate; showing a five fold degeneracy.

Question 22

Describe the d orbitals of a transition metal as part of a complex ion.

Answer 22

The d orbitals differ slightly in energy because of the repulsion between the electrons in the ligands and the d orbitals of the metal ion.

Question 23

What is a ligand?

Answer 23

An ion of functional group that binds to a central metal atom to form a coordination complex.

Question 24

Into what two sets are non degenerate d orbitals put into?

Answer 24

A lower lying(energy) threefold degenerate set and a higher lying(energy) twofold degenerate set.

Question 25

What happens to electrons in the lower lying(energy) degenerate set when they absorb energy (usually from the visible region of the EM spectrum)?

Answer 25

They move from the lower energy degenerate set to the higher energy degenerate set.

Question 26

What happens to the energy that is not absorbed?

Answer 26

It is reflected. Wavelengths in the visible region that are reflected can be picked up by our eyes as colour.

Question 27

How do ions that contain transition metals that have either full (e.g Cu⁺ and Zn²⁺) or empty (e.g Se³⁺) d orbitals appear and why?

Answer 27

The ion will appear colourless as no transitions between the higher and lower degenerate sets are possible.

Question 28

What is a chromophore?

Answer 28

These are groups of atoms that absorb in the visible and ultraviolet regions of the spectrum.

Question 29

What is the orange colour of carrots caused by?

Answer 29

It is caused by the absorption of blue light by β carotene through its system of conjugated alternating single and double bonds. The presence of high concentrations of electrons causes colours to be absorbed

Question 30

What are azo dyes?

Answer 30

Azo dyes contain the chromophore -N=N- which is derived from azo benzene. These are orange or red in colour due to absorption in the blue region of the visible spectrum. The presence of high concentrations of electrons causes colours to be absorbed.