Electronic, Vibrational and Rotational Spectroscopy

Question 1

What 3 ways can EM radiation interact with matter?

Answer 1

Emission, Absorption and Scattering

Question 2

What is a photon?

Answer 2

A specific amount (quantum) of energy

Question 3

Low energy waves have a short wavelength. True or false?

Answer 3

False

Question 4

Which region of the EM spectrum gives rise to a change in spin?

Answer 4

Radio Waves

Question 5

Which region of the EM spectrum gives rise to a change in shape?

Answer 5

Infrared

Question 6

Which region of the EM spectrum gives rise to a change in electronic distribution?

Answer 6

UV-vis and X-rays

Question 7

Which region of the EM spectrum gives rise to a change in orientation?

Answer 7

Microwaves

Question 8

What is meant by an amount of energy being Quantized?

Answer 8

It can only exist in discrete quantities

Question 9

How is an absoption or emission line identified in a spectra?

Answer 9

The photon emitted or absorbed has a frequency which has an energy which corresponds to the energy gap between two quantized energy levels

Question 10

What would a monochromator be used for?

Answer 10

To select a specific wavelength

Question 11

What is detected in an absorption spectrometer?

Answer 11

Intensity of the photons passing through the sample

Question 12

How do you calculate Transmittance (T)?

Answer 12

Intensity / Starting Intensity

Question 13

How do you calculate Absorbance (A)?

Answer 13

log(1/T)

Question 14

In an emission spectrometer, what is the purpose of the excitation source?

Answer 14

To electronically excite the sample to allow it to emit a photon when the sample returns to it’s relaxed state

Question 15

If energy levels are close together, how will transitions be distributed?

Answer 15

In a Boltzmann Distribution

Question 16

If energy levels are widely spread, how might this affect the spectrum produced?

Answer 16

The majority of the sample will be in the same energy state, so the spectrum may only show one band

Question 17

How could you increase the number of molecules encountered by the photons in a spectrometer? Give 2 ways.

Answer 17

Increase the pathlength of the sample and increase the concentration of the sample

Question 18

In the context of Rotational Spectroscopy, how is quantization applied?

Answer 18

Each molecule can rotate with a discrete amount of rotational energy

Question 19

There are small differences in energy between the different rotational energy levels. True or False

Answer 19

True. Microwaves cause these transitions, so must be low energy.

Question 20

E = BJ(J+1) is the equation which defines permitted rotational energy levels. What do B and J denote?

Answer 20

B: Rotational Constant
J: Rotational Quantum Number

Question 21

E = BJ(J+1) is the equation which defines permitted rotational energy levels. What would the energy change be for levels 3=>4

Answer 21

E(3)= 3B(3+1) = 12B
E(4)= 4B(4+1) = 20B
E(3=>4)= 8B

Question 22

How does the Moment of Inertia (I) relate to the Rotational Constant (B)?

Answer 22

B is proportional to 1/I

Question 23

If a molecule is heavy and has long bonds, will it’s rotational energy gaps be large or small?

Answer 23

Small. The Rotational Constant will be lowered by the increased moment of inertia

Question 24

What is the Gross Selection Rule for rotation?

Answer 24

There must be a change in dipole on rotation for an emission or absorption to occur

Question 25

Can carbon monoxide display a rotational spectrum?

Answer 25

Yes. It has a permanent dipole, so it complies with the gross selection rule

Question 26

What is the Specific Selection rule for Rotational Spectroscopy?

Answer 26

Changes in rotational energy levels can only increase or decrease by 1

Question 27

Can a molecule start with a rotational energy level greater than 0?

Answer 27

Yes. Any molecule can start with a higher energy level. This gives rise to specific energy patterns in a spectrum