General Spectroscopy

Question 1

State the definition of spectroscopy.

Answer 1

Absorption, emission or scattering of electromagnetic radiation by atoms or molecules.

Question 2

State the 5 types of spectroscopy and what they refer to.

Answer 2

1. NMR - refers to a quantum change of spin.
2. Infrared - refers to a quantum change of configuration, it excites vibrational levels.
3. UV/vis - refers to a quantum change of electron distribution.
4. Vibrational.
5. Rotational.
   - -> both vibrational and rotational are usually much smaller than electronic separations, transitions between occur at different wavelengths or frequencies.

Question 3

State the Beer-Lambert Law and annotate.

Answer 3

A = -log(it/io) = ecl

e = molar absorption coefficient, mol-1 dm3 cm-1
c = concentration, moldm-3
l = path length of light through sample, cm
i = length of the sample container

Question 4

State the equation for transmittance + absorbance.

Answer 4

T = it/io
A = -log(T) --> T = 10^-A

Question 5

How does absorption spectroscopy work? (3).

Answer 5

1. Radiation is passed through a sample in a container of known length.
2. A certain amount of radiation is absorbed by the sample depending on its concentration.
3. We record the reduced amount of radiation transmitted through.

Question 6

State the different energy types in order from the highest to lowest.

Answer 6

Electronic > Vibrational > Rotational > Translational

Question 7

What does a bigger energy correspond to? (3).

Answer 7

1. Higher frequency
2. Increasing wavenumber
3. Decreasing wavelength

Question 8

What is the “allowed vs. forbidden” selection rule?

Answer 8

Only transitions between certain energy levels can take place, ones with very low probability are referred to as forbidden transitions.

Question 9

What is the “gross selection rule”?

Answer 9

Establishes which properties of the molecule are needed in order to absorb radiation.

Question 10

What is the “specific selection rule”?

Answer 10

Determines between which levels transitions are allowed.

Question 11

State the 3 factors that determine the amount of light absorbed by a sample.

Answer 11

1. Number of molecules in the sample that absorb light (c)
2. The transition probability
3. The length of the sample container

Question 12

State the equation that converts wavelength to frequency (v).

Answer 12

v = c/wavelength

Where c is the speed of light in m s-1.

Question 13

State the equation that converts frequency (v) into wavenumber (v~ in cm-1).

Answer 13

v~ = v/c

Where c is the speed of light in cm s-1.

Question 14

State the equation that converts wavenumber (v~ in cm-1) into energy per photon (E).

Answer 14

E = hcv~

Where c is the speed of light in cm s-1.

Question 15

State the equation that converts energy per photon into energy per mole (E).

Answer 15

E x Na

Where Na is Avogadro’s number.