1. Spectroscopy
a laboratory method of analyzing matter using electromagnetic radiation

2. The electromagnetic spectrum

3. Example of spectroscopy
Radiation
Scale of 
Absorption involves:
Example of spectroscopy
Gamma rays pm
Nuclear reactions
x-rays
0.1 nm
Transitions of inner atomic electrons
Photoelectron spectroscopy (PES)
UV/vis nm
Transitions of outer atomic electrons
UV-Vis spectroscopy, Atomic Emission Spectroscopy,
Colorimetry IR mm
Molecular vibrations
IR, FTIR, Raman
microwave
Molecular rotations
Rotational spectroscopy
Radar, radio waves
cm  >>m
Oscillation of mobile or free electrons
NMR

4. Some spectroscopic methods
Used to elucidate structures of crystals and organic compounds
NMR
Nuclear Magnetic Resonance
Often uses carbon-13 (organic chemistry)
Like an “MRI of the molecule” IR
Infra-red spectroscopy
a type of absorption spectroscopy

5. Types of motion (vibrations) caused by IR radiation
Stretching
Symmetrical and assymetrical
Scissoring
Rocking
Wagging
Twisting

6. Different “pieces” of molecules absorb different IR wavelengths

7. The presence of IR absorption of specific wavelengths indicate that molecular “piece” is present in the molecule

8. Microwaves cause molecular rotations

9. Spectroscopy
a laboratory method of analyzing matter using electromagnetic radiation

10. Photoelectron Spectroscopy
(PES)

11. Photoelectron Spectroscopy
PES apparatus:
iramis.cea.fr

12. Photoelectron Spectroscopy
How it works:
Sample is exposed to EM radiation
Electrons jump out of sample and go through analyzer

13. Image source: Inna M Vishik

14. Kinetic Energy Analyzer
X-ray or UV
Source
Kinetic Energy Analyzer
6.26
0.52
Binding Energy (MJ/mol) 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

15. Energy to remove an electron
PES Data for Neon
Each peak represents the electrons in a single sublevel in the atom
The bigger the peak , the more electrons
Electrons generally farther from the nucleus
Energy to remove an electron
“binding energy”
often measured in eV (electron volts)
1 eV = × 10-19 J
Electrons
generally closer to the nucleus

16. Hydrogen vs. Helium
The helium peak is farther to the right (higher energy) thus more energy is needed to remove
the 1s electrons in helium. They must be held more tightly because there is a higher effective
nuclear charge. (Helium has 2 protons pulling on 1s but hydrogen only has 1)
The helium peak is twice as tall because there are twice as many electrons in the 1s sublevel

17. Oxygen (1s22s22p4) 4 electrons in 2p 2 electrons in 2s

18. Scandium (1s22s22p63s23p64s23d1)
*Notice that it takes more energy to remove an electron from 3d than from 4s.
This is because as electrons are added to 3d they shield 4s thus it’s easier (takes less energy) to remove 4s electrons compared to 3d electrons.

19. Transition metals…
Remember: when transition metals become cations - it’s the s electrons that are lost first!
Fe [Ar] 4s23d6
Fe2+ [Ar] 3d6
Fe3+ [Ar] 3d5
The most weakly held electrons are the first to be lost
The “order of filling” is not necessarily the same as the “order of removal”!!

20. Ex1: Identify the element whose PES data is shown
Sodium
Why is one peak much
Larger than the others?
This peak represents
6 electrons in the 2p
sublevel
The other peaks
represent only 1 or 2
electrons
In which sublevel are the electrons
Represented by peak A? 3s A

21. Ex1: Identify the element whose PES data is shown
Molybdenum
Why is one peak much
Larger than the others?
In which sublevel are the electrons
Represented by peak A?
Represented by peak B? A B

22. Example 2:
oxygen
nitrogen
#e-
#e-
increasing energy →
increasing energy →
The PES data above shows only the peak for the 1s electrons. Why is the peak for Nitrogen farther to the left?
It takes less energy to remove a 1s electron from nitrogen because it has a lower effective nuclear charge (fewer protons) than oxygen

23. Ex3: Sketch the expected PES spectrum for Aluminum

24. Colorimetry UV/Vis Spectroscopy

25. Colorimetry and uv/vis spectroscopy - used to find the concentration of colored solutions
Many compounds absorb ultraviolet (UV) or visible (Vis.) light.
When dissolved in water, the absorption of some of the frequencies (colors) of light causes them to transmit others, and the solutions are thus colored.
For example aqueous Cu2+ solutions often appear blue, because the Cu2+ ion absorbs visible light in the 600 – 650nm range.

27. Co(II) ion
Co2+ solutions often appear red to our eyes, because the Co2+ ion absorbs visible light in the 500 – 510nm range (blue-green to green)
The “perceived” color is then red.

28. The diagram below shows a beam of monochromatic radiation of radiant power P0, directed at a sample solution.
Absorption takes place and the beam of radiation leaving the sample has radiant power P.

29. The amount of radiation absorbed may be measured in a number of ways: Transmittance, T = P / P0 % Transmittance, %T = 100 T Absorbance A = log10 P0 / P A = log10 1 / T A = log / %T A = 2 - log10 %T

30. The last equation, A = 2 - log10 %T , is worth remembering because it allows you to easily calculate absorbance from percentage transmittance data.
The relationship between absorbance and transmittance is illustrated in the following diagram:

31. Co(II) ion
The relationship between absorbance and transmittance

32. Choose the wavelength with maximum absorbance when analyzing a solution

33. Beer’s Law
Now let us look at Beer’s law - the equation representing the law is straightforward:
A=abc
Where A is absorbance (no units, since A = log10 P0 / P )
a is the molar absorbtivity with units of M-1 cm-1
b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained. We will express this measurement in centimeters.
c is the concentration of the compound in solution, expressed in M

35. The reason why we prefer to express the law with this equation is because absorbance is directly proportional to the other parameters, as long as the law is obeyed. (We are not going to deal with deviations from the law.)

36. Beer’s Law – typical scenario
Measure the absorbance of a several samples of a solution of known concentrations
Plot A vs concentation (usually molarity)
This is called a “calibration curve”
Should be linear

37. 3. Measure the absorbance of a solution of the same chemical, but with unknown concentration 4. Using the calibration curve, determine the molarity of the unknown solution

38. Spectrometry
a laboratory method of analyzing matter using electromagnetic radiation.

39. Mass Spectrometry
Determines the relative abundance of the different isotopes of an element
Used to determine the average atomic mass of an element

40. a Mass Spectrograph for Ne

41. Portions adapted from
AP 2003 FRQ #5
Chemistry, Chang, 10th edition
APSI 2013 OU presentation; J. Beninga
Wikipedia: IR spectroscopy gifs