Instrumental Lecture 13 pt 2

Factors Affecting Number of Bands

• Symmetry can impact IR activity; symmetric stretches generally do not change dipole moment, making them IR inactive but Raman active.
• Degenerate vibrations (same energy) can overlap and cause fewer observed bands.
• Low absorption intensity may render bands undetectable.
• Vibrational energy at wavelengths beyond instrument capability may not be observed.

General Rule of Mutual Exclusion

• IR bands typically do not appear in Raman spectra and vice versa.

Solvent Considerations

• Water is suitable for Raman but not for IR due to the OH stretch interfering in the IR spectrum.
• IR typically uses salt (KBr) for solids, while Raman can use glass or quartz.

Instrument Differences

• Raman instruments utilize laser sources for quick and versatile sampling, able to analyze various states (solids, liquids, gases).
• IR uses thermal detectors which are less sensitive and slower compared to Raman detectors.

Spectra Comparison

• Raman spectra tend to be simpler than IR spectra, facilitating easier interpretation.
• Totally symmetric vibrations are observed in Raman but not often in IR.

Laser Sources for Raman

• Key lasers include Argon (488, 514.5nm), Krypton (530.9, 647.1nm), Helium-Neon (632.8nm), and Neodymium YAG (1064nm), with 785nm and 1064nm being most practical to minimize sample fluorescence interference.