14 Questions
What is the wavelength range of infrared radiation?
0.8 - 1000 µm
What unit is commonly used to measure infrared radiation?
Wavenumbers (cm-1)
What is the relationship between wavenumber and wavelength?
Wavenumber is inversely proportional to wavelength
What is the order of energy levels from highest to lowest?
Electronic > Vibrational > Rotational
What type of bond is usually IR-active?
Polar bond
What happens when molecules are exposed to IR light of the same frequency as the frequency of its origin vibrations?
The molecules absorb energy
What is the X-axis of an IR spectrum typically plotted against?
Wave number
What is the purpose of a Monochromator in an IR spectrometer?
To break down the radiation of light to different wavenumber regions
What is the temperature of the Tungsten halogen lamp used as an IR radiation source?
2000oC
What is the purpose of using a small amount of solid sample in a Potassium Bromide disk?
To reduce the amount of sample required for analysis
What is the type of radiation emitted by Laser diodes?
Broad spectral light distribution from visible to infrared region
What is the purpose of using a drop of the sample between two potassium bromide or sodium chloride circular plates for liquid sample preparation?
To produce a thin capillary film
What is the principle of operation of a Thermocouple detector in IR spectrometer?
Temperature difference between two junctions results in a potential difference
What is the purpose of the Globar in an IR spectrometer?
To provide a continuous thermal radiation source
Study Notes
Infrared Radiation
- Infrared radiation has a wavelength region of 0.8 - 1000 µm.
- Wavenumbers (cm-1) are commonly used units, which are proportional to energy (E) and frequency (ν), and reciprocal to wavelength.
Regions of Infrared Radiation
- Near Infrared
- Middle Infrared
- Far Infrared
Processes of Absorption
- Absorption of radiation leads to higher rotational energy levels.
- Absorption of energy leads to increased vibrational energy levels (vibrational transition).
- Electronic transition: electrons of molecules are raised to a higher electron energy level.
- Energy levels in order: electronic > vibrational > rotational.
Bond Vibrations
- Covalent bonds vibrate at only certain allowable frequencies.
- Types of vibrations:
- Stretching: change bond length (requires high energy).
- Bending: change bond angle (requires low energy).
Principle of IR Absorption
- Polar bonds are usually IR-active (e.g., C-O).
- Nonpolar bonds in symmetrical molecules absorb weakly or not at all (e.g., C-C).
- Molecules absorb IR light of the same frequency as their origin vibrations.
IR Spectrum
- Plot of wavenumber (X-axis) against percentage of light transmitted (Y-axis).
Components of IR Spectrometer
- IR Radiation Source:
- Tungsten halogen lamp (2000°C).
- Globar (1100°C).
- Laser diodes (broad spectral light distribution).
- Sample:
- Solids: Potassium Bromide disk.
- Liquids: thin capillary film between potassium bromide or sodium chloride plates.
- Monochromator:
- Breaks down radiation into different wavenumber regions.
- Detector:
- Thermocouples.
- Bolometers.
- Pyroelectric Detectors.
- Photoconductive detector.
- Display.
Explore the properties of infrared radiation, including its wavelength region, units, and absorption processes in molecules. Learn about the three main regions of infrared radiation and how it relates to molecular energy levels.
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