Week 9
MOLECULAR SPECTROSCOPY
Part 2
by Tan Ching Mun (16002662)
Ultraviolet and Visible (UV/Vis) Absorption Spectroscopy
Organic UV/Vis Absorption
The first video below explains the type of bonding and transitions of electrons.
Next, the second video talks about the technique used to analyze molecules with electrons in pi orbitals and nonbonding(n) orbitals.
- widely used for quantitative analysis
- region of spectrum
- UV: 185-400nm
- visible: 400-700nm - generally results from excitation of electrons
- valuable in identifying functional groups in a molecule or compound containing absorbing group
Organic UV/Vis Absorption
The first video below explains the type of bonding and transitions of electrons.
Next, the second video talks about the technique used to analyze molecules with electrons in pi orbitals and nonbonding(n) orbitals.
Type of bonding and transitions of electrons
Technique used to analyze molecules
Below shows some common organic absorption spectra
Absorption characteristic of some common chromophores
Effect of Solvent
- Each solvent has its own characteristic polarity
- Will affect the position and intensity of absorption bands
Inorganic UV/Vis Absorption
- A number of inorganic anions exhibit UV absorption bands.
- E.g.: nitrate, nitrite, and chromate.
Qualitative Analysis
UV/Vis absorption spectroscopy is the most useful tool available for quantitative analysis.
Important characteristic of spectrophotometric method:
- wide application to both organic & inorganic systems
- typical detection limits to 10^-4 to 10^-5 M.
- good accuracy (1-3%)
- ease and convenient
Molecular Fluorescence Spectroscopy
Measured by exciting the sample at the absorption wavelength (excitation wavelength), and measuring the emission at a longer wavelength -- fluorescence wavelength.
Less widely applicable
Relaxation Processes
Two most important processes:
Two most important processes:
- Nonradiative Relaxation
-vibration
-internal conversion - Fluorescence Emission
Molecular fluorescence bands: longer in wavelength, lower in frequency & lower in energy than absorbed radiation (excitation) wavelength.
Quantum yield (0~1):
- no. of molecules that fluoresce / total no. of excited molecules
- photons emitted/ photons absorbed
Highly fluorescent molecules (Quantum yield approach 1)
Nonfluorescent species (Quantum yield approach 0)
Factors affecting Quantum Yield:
Nonfluorescent species (Quantum yield approach 0)
Factors affecting Quantum Yield:
- aromatic functional groups ( intense & useful)
- The rigidity of structure (the more rigid, the better)
- Temperature ( high temperature, increase collisional relaxation, quantum efficiency decrease)
- Solvent viscosity ( the lower the viscosity, the lower the quantum efficiency)
Reflection
Learning UV/Vis Absorption made me recall knowledge that I learned during foundation Chemistry II and Organic Chemistry. After the lecture, I have a clearer understanding of the transitions of electrons and techniques to analyze molecules. Not only that, I learned the effect of solvent, qualitative and quantitative analysis, and molecular fluorescence.
We do not have a tutorial session for this week.
