Spectroscopy 2 Flashcards
Raman Spectroscopy (1)
When radiation passes through a transparent medium, most of it is elastically scattered (Rayleigh scattering).
This is where light is scattered at the same frequency to the incident radiation.
Raman Spectroscopy (2)
Around one in a million photons are inelastically scattered (Raman scattering). This is where light is scattered at a different frequency to the incident radiation.
This often uses high-powered lasers, which ensure that more photons are scattered inelastically. Lasers are a monochromatic source so the wavelength can be tuned to avoid fluorescence.
Raman Spectroscopy (3)
A decrease in frequency is known as Stokes emission. An increase in frequency is known as anti-Stokes emission.
Typically we see Stokes emission, given that anti-Stokes emission requires the molecule to be in an excited state initially.
Raman Spectroscopy (4)
Bond vibration leads to a change in frequency.
The difference between the incident and scattered ration will fall in the IR region and so provides information on molecular structure.
RS Selection Rules
Raman Spectroscopy is a complementary technique, with different selection rules to IR, showing different vibrations.
Gross selection rule:
There must be a change in molecular polarizability on vibration.
Specific selection rule:
The change in vibrational energy level must be Δν = ±1.
Motion of Molecules on RS
There are different ways in which molecules may move:
Symmetric stretch
Asymmetric stretch
Wagging
Scissoring
Raman Spectroscopy - Benefits
Generally little sample prep is required.
Can handle any state of sample.
Preferable to FTIR for inorganic analysis.
Raman Spectroscopy - Limitations
Often more expensive than FTIR.
Less detailed information due to the low numbers of photons that scatter inelastically.
UV-Vis Spectroscopy (1)
This type of spectroscopy shows electronic excitation of a molecule.
It can be very useful for determining concentration of coloured solutions containing organic compounds with a high level of pi-conjugation.
Relies on the Beer-Lambert Law:
A = εcl
UV-Vis Spectroscopy (2)
Limited to detecting molecules with strong UV-Vis absorbance.
If a sample is too concentrated, there will be a flattening of the peak, and accurate calculations are impossible.
Matrix mathematics is required for mixtures, which is doable but complicated.
UV-Vis Spectroscopy (3)
Can be used as a presumptive test, and is useful to determine instrument settings for confirmatory testing.
Fluorescence Spectroscopy
This is complementary to UV-Vis spectroscopy.
Some molecules absorb in the UV region and then emit light at a longer wavelength, often in the visible region.
It is very useful for organic molecules.
Nuclear Magnetic Resonance Spectroscopy (NMRS) (1)
Nuclei have an intrinsic property called “spin” - when placed in a magnetic field, the nuclei either align with or against the field.
Applying a radio frequency to the sample will cause the spins to be excited to the higher spin state.
Nuclear Magnetic Resonance Spectroscopy (NMRS) (2)
Not all nuclei are NMR active, they must have an overall spin that is non-integer.
The most common forms of NMR use spin -1/2 nuclei.
Nuclear Magnetic Resonance Spectroscopy (NMRS) (3)
Depending on which atoms they are bonded to, nuclei respond differently and have a “chemical shift”, meaning we can identify functional groups much like in IR and Raman spectroscopy.
