CH4 - Spectroscopic Methods Flashcards
(17 cards)
To be IR active?
The oscillating electric field of the incident IR must be able to interact directly with a changing molecular dipole moment. No change in dipole moment (e.g. centre of symmetry), IR inactive.
To be Ramen active?
Incident light should induce an instantaneous dipole moment. If displacement corresponds to a vibrational mode, Ramen active. Magnitude depends on polarizability (how easily it can be deformed).
When are IR and Ramen useful?
When a mineral changes structure due to temp/pressure, vibrational spectrum changes and can be analysed using these techniques. This can monitor change in a more sensitive way than diffraction
Minerals respond to thermal excitation by changing vibrational states. These changes can be monitored
What is Ramen spectroscopy?
Non-destructive process based upon the interaction of monochromatic light with the chemical bonds in a material
What information does Ramen provide?
- Chemical structure and identity
- Phase and polymorphism
- Intrinsic stress/strain
- Contamination and impurity
Why is Ramen qualitative?
The spectrum profile provides a unique chemical fingerprint which can be used to identify the material
Why is Ramen quantitative?
The intensity of a spectrum is directly proportional to concentration. With mixtures, relative peak intensities provide information about relative concentration of components. Absolute peak intensities used for absolute concentration information
What materials can be Ramen spectra be obtained from?
Nearly all samples with true molecular bonding
- Solids
- Powders
- Slurries
- Liquids
- Gels
Gases - specialised equipment with higher powered lasers
Why is the energy loss (stokes) process more probable?
As at normal temperatures the initial population of n=1 excited state is low
What is IR spectroscopy?
Measures intensity of absorption and active vibrational modes in a sample
- IR radiation is passed through a sample (powder/crystal)
- Intensity of transmitted beam is function of frequency
- Coupling often occurs
Absorbance = transition from ground to higher energy state
What information is provided by IR?
Identify specific molecular groups (e.g. hydroxyl, water molecules)
Orientation of the crystal
Why is IR quantitative?
Main theory of IR quantification is Beer-Lambert law. From this we can figure out the relationship between absorbance and concentration, since there is a particular molar absorptivity at a particular wavelength
Factors influencing IR?
Isotope effects - reduced mass may effect rotational and vibrational behaviour
Solvent effects - polarity influences compound
Advantages of IR?
- High scan speed
- High resolution
- High sensitivity
- Wide range of application
- Non-destructive
Disadvantages of IR?
- Sample constraint - cannot contain water as this absorbs IR light
- Spectrum complication
- Quantification
Coupling often occurs so peaks cannot be related to an isolated molecular group
What happens in an IR spectra if the OH is strongly bonded?
Weakens the O-H bond, thus the stretch vibration occurs at a lower energy
How can the orientation of an O-H bond in a crystal be determined?
Using polarized incident radiation and observing the change in the absorption for different orientations. O-H very strongly absorbs.
