CH04 - IR, NIR and Raman Spectroscopy Flashcards
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Range of wavelenths for source in Raman spectroscopy?
NIR (800-2500nm or 4000-10000cm-1) and Vis (400-800nm)
Range of wavelengths in Raman spectrum?
Same as in Mid-IR (400-4000 cm-1) since scattered radiation results from molecule relaxing to ground vibrational state.
3 ways in which Raman spectroscopy complements mid-IR spectroscopy?
- Weak absorption lines in mid-IR may appear as strong lines in Raman spectroscopy (e.g Cl-Cl are IR incative, but raman active).
- Spectrae for a given molecule does not depend on the sample’s physical form (e.g powder or pill).
3.Raman can differentiate between substances with little chemical difference like isomers.
How is polarizability related to the selection rule for raman activity?
To be raman active vibration must be accompanied by a change in polarizability
Range of Wavenumbers for mid-IR?
400-4000cm-1
Range of Wavenumbers for near IR (NIR)?
4000-100000 cm-1
NIR Absorption lines result from ________ or ____ bands of fundamental vibrational transitions in mid-IR.
overtones, combination
What is NIR spectroscopy?
A spectroscopic method which relies on absorption of radiation in the NIR range (4000-10000 cm^(-1))
What is a fundamental vibrational transition?
The term that describes the Δn=±1 change between vibrational energy states E = 0 to E = 1.
What are overtones?
Transitions that appear with decreasing intensity at fundamental vibrational frequencies Δn=±2,±3… etc
Fundamental transition bands appear as ____ bands in IR spectrum.
Intense
Overtone transition bands appear as ____ bands in IR spectrum.
Weak
The fundamental absorption bands in mid IR are used for _______analysis, whereas absorption bands in NIR are used for _______ analysis.
qualitative, quantiative
Condition for NIR activity?
(dμ_e)/dx≠0 (same as in mid IR).
Disadvantages of NIR compared to mid IR?
- Low signal intensity
- Limited structural information (The bands that occur in this region are generally due to OH, NH, and CH bonds).
Advantages of NIR compared to mid IR (list 6)?
- Tungsten halogen light source provides intense and stable lighting over the entire NIR range.
- Fused silica and quartz can be used for lenses and sample holders (those materials absorb mid IR so cannot be used there - mid IR uses water-soluble salts instead).
- NIR spectroscopy uses PbS detectors, which do not require cooling by liquid nitrogen.
- No need\few need for sample preparation (solvents or reagents not required). Enables faster analysis with less interference and nondestructive process monitoring.
- High dynamic concentration range.
- Shorter wavelength enables deep sample penetration.
Possible effect of isotopic abundance on IR spectrum?
Elements with common isotopes can form doublet absorption bands with intensity ratio corresponding to relative abundance.
E.g. HCl vibration band may appear as a doublet with intensity ratio 1:3 (due to Cl)
Primary alchohols create C-C→O bands in _______ cm-1
1048 cm-1
S=C=S gives a strong absorption line in _____ cm-1.
1500cm-1
Quantitative analysis in IR range is performed using _______ law.
Beer’s law (A=abc, where a and b are constants, c is determined).
Sample holders and optics in mid IR analysis are made from____ or ____ which are _____ soluble.
KBr, NaCl (since quartz and fused silica absorb mid IR). These salts are water soluble.
Even at electronic ground state, a molecule still has nonzero ______ energy.
Vibrational (because E_vib=h_bar*ν(n+1/2))
In Raman and Rayleigh scattering, electrons are exited to ______ states.
Virtual (thus it is a “Scattering” and not an “Absorption”)
“An elastic scattering process that occurs when photons of monochromatic radiation
with energy strikes a molecule, and the scattered radiation contains photons of the same energy as that of the incident radiation”
rayleigh scattering
