Infrared spectroscopy
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What region of the electromagnetic spectrum is used in infrared spectroscopy?
Infrared spectroscopy uses the infrared (IR) region of the electromagnetic spectrum. This region lies between the microwave and visible regions.
What property of a molecule causes it to absorb IR radiation?
Molecules absorb IR radiation when the frequency of the radiation matches the vibrational frequency of a bond within the molecule. This causes a change in the amplitude of the vibration.
What is wavenumber, and what are its units?
Wavenumber is the reciprocal of wavelength (1/λ) and is proportional to the frequency of vibration. It is commonly expressed in units of cm⁻¹.
List the characteristic absorption range (wavenumber) for the O-H stretch in alcohols.
The O-H stretch in alcohols typically absorbs in the range of 3200-3600 cm⁻¹. This band is usually broad due to hydrogen bonding.
List the characteristic absorption range (wavenumber) for the C=O stretch in carbonyl compounds.
The C=O stretch in carbonyl compounds (aldehydes, ketones, carboxylic acids, esters) typically absorbs in the range of 1680-1750 cm⁻¹.
List the characteristic absorption range (wavenumber) for the C-H stretch in alkanes.
The C-H stretch in alkanes typically absorbs in the range of 2850-3000 cm⁻¹.
What information can be obtained from the fingerprint region of an IR spectrum?
The fingerprint region (below 1500 cm⁻¹) contains complex vibrational modes that are unique to each molecule. It can be used to identify a specific compound by comparing its IR spectrum to a reference spectrum.
How can IR spectroscopy distinguish between a carboxylic acid and an alcohol?
Both show a broad O-H stretch. However, carboxylic acids exhibit a very broad O-H stretch (2500-3300 cm⁻¹) due to strong hydrogen bonding and also a C=O stretch (1680-1750 cm⁻¹).
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