Infrared and Raman Spectroscopy. Flashcards
What does Infrared and Raman Spectroscopy measure?
Vibrational energy
How does a molecule absorb vibrational energy?
A molecule can absorb vibrational energy if it is vibrated in such a way that the charge distribution (Electron dipole moment) changes during the vibration.
What are the two main modes of vibration in molecules?
Stretching and Bending
What two factors lead to are higher vibrational energy within molecules?
If the molecule has strong/short bonds in it’s system.
If the molecule mass is lighter then there is an increase vibrational energy aswell.
Would the bond between carbon and hydrogen have a high or low vibrational energy?
Higher, there is a short bond between these two atoms, creating a vibrational energy of about 3000 reciprocal centimetres.
Would a single bond or double bond have higher vibrational energy?
A double bond would have a higher energy as the bond is stronger.
What type of instrumentation is used in Infared spectroscopy?
FTIR - Fourier Transform IR
How does FTIR work?
By having a broadband of IR frequencies pass through the analyte in which the resultant absorbance is measured. The Instrument changes the frequency of the broadband rapidly and allows for a quick measurement of the IR spectra.
How does a Michelsons Interferometer work?
This instrument uses a stationary and moving mirror as a means to enhance and nullify wavelengths that are being passed through the sample. The moving mirror will change the distribution of the wavelength composition causing the a spectrum of IR to be measured.
What common compounds affect the accuracy of FTIR?
Water and carbon dioxide both absorb vibrational energy quite well and so the presence of these molecules will affect the absorption measurement.
How can we account for atmospheric or chemical contamination of samples in FTIR?
We can run a FTIR with juts the atmosphere, also we can multiple scans to allow us to subtract contamination
Is FTIR qualitative or quantitative?
FTIR can provide both concentration and the fingerprinting of a molecule and therefor is both qualitative and quantitative.
Why is Raman Spec complementary to IR Spec?
Because Raman can observe symmetrical vibrations not observed in IR.
Some vibrational modes are only visible in Raman, some only in IR and some in both.
What are the advantages of raman spec?
1) Requires little sample prep
2) Water and alcohols have weak raman absorption and therefor samples can be prepped into aqueous solutions.
3) You can measure in glass and plastic containers. (measure your products that have been packaged)
How do we prepare a IR spec sample?
We cannot use glass, quartz or plastic containers to hold our sample as they absorb IR energy. Therefor we have to use halide salts cells in order to prep the sample. H2O is also a excellent IR absorber so we cannot use solutions.
