1. Spectroscopy
Chem honors

2. Spectroscopy
Is the interaction between matter and electromagnetic radiation (light)
Spectroscopy is commonly used in chemistry to detect, identify, and quantify information about different atoms and molecules
We’ve already seen a couple experiments that dealt with spectroscopic studies
Photoelectron effect – Albert Einstein
Explanation of the atomic structure – Niels Bohr

3. Analysis
Quantitative Analysis – determination of the absolute or relative abundance of one, several, or all particular substance(s) present in a sample
Qualitative Analysis – seeks to find the elemental composition of compounds

4. Spectroscopy
Infrared (IR) Spectroscopy – molecules absorb infrared radiation at specific frequencies that are characteristic of their structure [qualitative]
Ultraviolet-Visible (UV-vis) Spectroscopy – molecules/atoms/ions absorb ultraviolet-visible radiation at specific frequencies and undergo electronic transitions (transition from the ground state to an excited state) [qualitative / quantitative]

5. Infrared Spectroscopy
Infrared Spectroscopy allow chemists the ability to determine the identity of a given molecule
Infrared radiation is absorbed by a given molecule’s stretching and bending frequencies in most covalent bonds
The energy absorbed serves to increase the amplitude of the vibrational motions of the bonds in the molecules (We perceive this vibration as heat)
Only bonds that have a dipole moment (difference in electronegativity) that changes as a function of time are capable of absorbing infrared radiation

6. Molecular Vibrations

7. IR Spectrum

8. Absorption Frequency of Major Functional Groups

9. UV-vis Spectroscopy
Absorption Spectroscopy in the UV-visible region (200nm – 800nm)
Molecules will undergo electric transitions
Measures transition between the ground state to the excited state
Can be used to collect qualitative and quantitative data

10. UV-vis Spectroscopy

11. Electronic States Important Jumps: From π – π* From non-bonding to π*

12. Determination of Analytes
Transition Metal – can be colored because d- electrons within the metal can be excited from one electronic state to another. The color of the metal is strongly affected by the type of ligand bound to the central atom.
Organic Molecules – especially those with a high amount of conjugation.
Methyl Orange
Beta-Carotene

13. Transition Metal Salt Solutions

14. Complementary Colors
Colors opposite of one another are considered complimentary:
Red – Cyan
Yellow – Blue
Green – Magenta
When a specific wavelength of color is absorbed by a molecule from white light, what is seen is the complementary color
The wavelength absorbed is known as your λmax

15. UV-vis Spectrum of beta-carotene
Beta-Carotene has a λmax between nm
Absorbs at about 470nm = Around blue and cyan
Complementary color is between yellow and red
Therefore the color you see is orange!!!!

16. Absorbance vs. Concentration
Absorbance is directly proportional to concentration
The more concentrated a substances is, the more molecules are present to absorb light in the UV-Visible region
Due to this relationship, we can quantitatively determine the concentration of solution based off of its absorbance values.

17. Beer’s Law
Beer’s Law describes the relationship between concentration and absorbance:
A = εbc
A = absorbance (value collected from the spectrometer)
c = concentration of solution
ε = molar absorptivity (probability of the electronic transition)
b = path length (the distance light travels through the cuvette – commonly cm)

18. Beer’s Law Experiment
Prepare a set of solutions with a known concentration
Determine the λmax of the solution
Place each solution into the spectrophotometer
Collect data (notice the linear relationship between data points)
Graph the data and apply a best fit line
The slope of the line is equal to the molar absorptivity
After collecting the molar absorptivity for your sample set, you can determine the concentration of an unknown solution by collecting its absorbance values from the spectrophotometer