Lecture 4 - Infrared spec (vibrational) Flashcards
Is infrared absorption or emission?
Absorption
-A molecule absorbs a discrete amount of energy
-E = hv, E = hv1-hv0
-Selection rule (+/-) 1
How in an IR spectrum done?
- Plot of IR transmission % vs against cm-1
- Over range 400-4000cm-1
-Usually 4000-2000 and 2000-400.
-100% is no absorption, 0% is total absorption.
How do the number of bands work?
3N-6 for non-linear and 3N-5 for linear, which gives number of vibrational peaks.
However not all are shown in a spectrum. Multiple vibrations can occur at same wavenumber.
What kind of vibration in the strongest and explain why.
- Asymmetric vibrations are the strongest as they give the largest change in dipole moment.
-Asym for CO2 around 2300
-Sym for linear molecules (ex CO2) symmetric stretch isn’t IR active whereas non-linear does (ex H2O)
How does IR work for larger molecules?
- Multiple vibrations can occur at the same wavenumber (like C-H’s) and give group frequencies.
- Some can occur at frequencies too low that they aren’t on the IR.
-Not all are IR active, so there might not be peaks the same as 3N-6/5
-Example all C-H stretches will be together as 1 peak.
How do overtones work?
-Usually goes from v0->v1
-Overtones break the rule so are very weak peaks, example v0->v2 (which occurs at wl/2 nm), v0->v3 (wl/3 nm).
-The peaks are very small in height, they become smaller as the overtones become higher.
- The difference between the peaks isn’t exactly half due to morse curve but small difference.
What is a hot bands?
-When particle is in excited state at the start (not in GS)
-Not likely at room temp, usually happens if instrumentation is heated.
What does Hooke’s law predict about position of peaks?
k in the equation is the force constant between a and b.
u is the reduced mass.
- Higher k means a higher frequency (so light atoms have strong bonds so higher frequency)
- Heavier atoms give a lower frequency.
What are the IR selection rules?
- Deltav = (+/-) 1but overtones can happen
- Involves the absorption of radiation normally as the molecule moves from one vibrational state to another.
- MUST be a change in dipole during vibration for absorption to occur - Asym the strongest.
How does IR Instrumentation work?
- The light source is a nerst filament or globar
- A monochromator separates the wavelengths using a prism or a diffraction grating.
-Can also have interferometric
How does the sampling work?
- Glass, quartz and plastic cant be used, all opaque to due radiation
-No water can be present - due to H bonding causes a massive problem and overrides organic molecules
-Water gives strong stretching and bending peaks.
-NaCl plates used (liwuid applied neat, solid dispersed and ground in nijol) - The solid is usually grinded with KBr and made into a disc
-Solution cells are made of NaCl.
How does ATR-IR work? (Attenuated Total Reflection in Fourier Transform Infrared)
-Has a crystal (usually germanium, zinc selenide, Si or diamond - the better the crystal the less it scratches but more expensive)
- Uses total internal reflection resulting in an evanescent wave
- Infrared beam can interact with the sample once or multiple times (depending on the set-up) before leaving the cell
- Light penetrates from 0.5 to 2 microns into the sample
- Pour a liquid sample onto the surface or press a solid sample onto the surface
What are the disadvantages of using IR?
- Can be time consuming
- No water (sample handling)
- Aqueous samples not done easily
- Sample destroyed or hard to recover
What can IR be used for?
- Identification of functional group and structure elucidation
- Identification of substances
- Reaction/process monitoring
- Detection of impurities
- Quantitative analysis
- Biomedical analysis
What are the different peaks in the spectrum?
- 4000-2500 is single bonds
-2500-2000 triple bonds - 2000-1500 double bonds
- <1500 fingerprints region
Position given by Hooke’s law!
