KBr is inactive in IR because it is transparent to infrared light and does not absorb in the IR region, making it an ideal matrix for preparing samples for IR spectroscopy.
Explanation:
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Transparency to Infrared Light: KBr, or potassium bromide, is a salt that is transparent to infrared radiation. This means that it does not absorb the wavelengths of light that are used in infrared spectroscopy, which typically range from about 2.5 to 25 micrometers (corresponding to wavenumbers from 4000 to 400 cm⁻¹). This transparency is crucial because it allows the infrared light to pass through the sample without interference from the KBr itself.
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Use as a Matrix for Sample Preparation: In IR spectroscopy, samples are often prepared in a matrix of KBr to facilitate the analysis of solids. The KBr pellet method involves mixing a small amount of the sample (usually about 1% by weight) with KBr powder, which is then compressed under high pressure to form a transparent pellet. This pellet is then placed in the spectrometer for analysis. The KBr serves as a carrier for the sample and provides a uniform, transparent medium through which the infrared light can pass.
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Absence of IR-Active Vibrations: The chemical bonds in KBr do not undergo vibrational modes that correspond to the wavelengths used in IR spectroscopy. In molecules, IR-active vibrations occur when the change in dipole moment during the vibration is non-zero, which leads to absorption of IR light. Since KBr is a symmetric ionic compound, its vibrational modes do not result in a change in dipole moment and therefore do not absorb IR radiation.
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Practical Considerations: The use of KBr in IR spectroscopy is also practical due to its availability and ease of use. However, it is important to note that KBr is hygroscopic, meaning it absorbs moisture from the air. This can affect the quality of the IR spectrum if not properly handled, as the absorbed water can introduce additional peaks in the spectrum. Therefore, it is recommended to handle KBr in a controlled environment, such as a glovebox, to prevent moisture absorption.
In summary, KBr is inactive in IR because it is transparent to the wavelengths used in IR spectroscopy and does not absorb in this region. This property makes it an excellent choice for preparing samples for IR analysis, as it allows the spectroscopic analysis of the sample without interference from the matrix itself.
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