State the definition of spectroscopy.
Absorption, emission or scattering of electromagnetic radiation by atoms or molecules.
State the 5 types of spectroscopy and what they refer to.
- NMR - refers to a quantum change of spin.
- Infrared - refers to a quantum change of configuration, it excites vibrational levels.
- UV/vis - refers to a quantum change of electron distribution.
- Vibrational.
- Rotational.
- -> both vibrational and rotational are usually much smaller than electronic separations, transitions between occur at different wavelengths or frequencies.
State the Beer-Lambert Law and annotate.
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
State the equation for transmittance + absorbance.
T = it/io A = -log(T) --> T = 10^-A
How does absorption spectroscopy work? (3).
- Radiation is passed through a sample in a container of known length.
- A certain amount of radiation is absorbed by the sample depending on its concentration.
- We record the reduced amount of radiation transmitted through.
State the different energy types in order from the highest to lowest.
Electronic > Vibrational > Rotational > Translational
What does a bigger energy correspond to? (3).
- Higher frequency
- Increasing wavenumber
- Decreasing wavelength
What is the “allowed vs. forbidden” selection rule?
Only transitions between certain energy levels can take place, ones with very low probability are referred to as forbidden transitions.
What is the “gross selection rule”?
Establishes which properties of the molecule are needed in order to absorb radiation.
What is the “specific selection rule”?
Determines between which levels transitions are allowed.
State the 3 factors that determine the amount of light absorbed by a sample.
- Number of molecules in the sample that absorb light (c)
- The transition probability
- The length of the sample container
State the equation that converts wavelength to frequency (v).
v = c/wavelength
Where c is the speed of light in m s-1.
State the equation that converts frequency (v) into wavenumber (v~ in cm-1).
v~ = v/c
Where c is the speed of light in cm s-1.
State the equation that converts wavenumber (v~ in cm-1) into energy per photon (E).
E = hcv~
Where c is the speed of light in cm s-1.
State the equation that converts energy per photon into energy per mole (E).
E x Na
Where Na is Avogadro’s number.
