10 Questions
What is the main difference between Raman and IR spectroscopy in terms of sample preparation?
Raman does not require fancy sample preparation, while IR does.
What is the advantage of Raman spectroscopy over IR spectroscopy in biological samples?
Raman spectroscopy can use water as a solvent, which is suitable for biological samples in their native state.
What is the requirement for a molecule to be active in Raman spectroscopy?
A change in the polarizability of the molecule during vibration.
What is the percentage of Raman scattering compared to the incident light source?
0.001% of the incident light source.
Why is a laser light source used in Raman spectroscopy?
To provide the required intensity of light for reasonable Raman scattering.
What is the main difference between Raman and IR spectroscopy in terms of the frequency of the light source?
Raman spectroscopy can use a visible light source, while IR spectroscopy uses an IR light source.
What is the advantage of Raman spectroscopy over IR spectroscopy in terms of solvent interference?
Raman spectroscopy is less sensitive to solvent interference.
What is the type of Raman scattering that is typically used in Raman spectroscopy?
Anti-Stokes lines.
What is the main application of Raman spectroscopy?
Fingerprinting and probing molecular symmetry.
Why is Raman spectroscopy suitable for biological samples?
Because it can use water as a solvent, which is suitable for biological samples in their native state.
Study Notes
Raman Spectroscopy
- Based on inelastic scattering of a monochromatic excitation source, with a routine energy range of 200-4000 cm⁻¹.
- Complementary to IR spectroscopy, with selection rules dictating which molecular vibrations are examined.
Principles
- Radiation at a certain frequency is scattered by the molecule with shifts in the wavelength of the incident beam.
- Observed frequency shifts are related to vibrational changes in the molecule.
- Raman scattering spectrum resembles IR absorbance spectrum, but with different mechanisms.
Comparison with IR Spectroscopy
- IR: change in dipole moment, asymmetric vibrations (active), and vibration of electrons leads to stretching/bending.
- Raman: change in polarizability, symmetric vibrations (active), and vibration of electrons leads to momentary distortion of electrons distributed around the bond.
- Some vibrational modes are both IR and Raman active.
Spectra
- CO²: Raman: 1335 cm⁻¹, IR: 2349 cm⁻¹ and 667 cm⁻¹.
- H²O: Raman + IR: 3657 cm⁻¹, 3756 cm⁻¹, and 1594 cm⁻¹.
Types of Raman Scattering
- Rayleigh scattering: elastic, no change in energy.
- Raman scattering: inelastic, with a net change in energy.
- Stokes: energy is transferred from photon to molecule (E = hn - DE).
- Anti-Stokes: energy is transferred from molecule to photon (E = hn + DE).
Raman Spectrum
- Probability of emission is proportional to observed intensity.
- Raleigh scattering >> Stokes >> anti-Stokes.
Advantages and Applications
- A vibrational spectroscopy technique for assessing molecular motion and fingerprinting species.
- IR tends to emphasize polar functional groups, while Raman emphasizes aromatic and carbon backbone.
- Raman does not "see" many common polar solvents, making it suitable for use with aqueous samples.
- Advantages over IR: can use water as a solvent, suitable for biological samples in native state.
Learn about the principles of Raman spectroscopy, including its energy range, selection rules, and how it relates to IR spectroscopy. Understand how Raman scattering works and how it provides information about molecular vibrations.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
