The solvent normally used in IR spectroscopy for preparing solid samples is dichloromethane (CH2Cl2). This solvent is chosen for its ability to dissolve a wide range of organic compounds, making it suitable for preparing concentrated solutions of the sample.
Explanation:
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Solubility: Dichloromethane is a versatile solvent that can dissolve many organic compounds, which is crucial for preparing a concentrated solution of the sample. This solubility ensures that the sample can be effectively analyzed in the IR spectroscopy setup.
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Absorption Bands: It is important to note that all solvents, including dichloromethane, have their own characteristic absorption bands in the IR spectrum. However, dichloromethane is often preferred because its absorption bands do not typically interfere with the important bands of the sample. This is particularly important when obtaining a spectrum of the solvent as a baseline to automatically subtract it from the sample spectrum, ensuring that the resulting spectrum is clear and interpretable.
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Avoidance of Water: The reference mentions that solvents containing water should be avoided as they can dissolve KBr plates or cause them to fog up, and the broad water band might mask important bands of the compound. Dichloromethane is anhydrous, making it a suitable choice for IR spectroscopy where water interference is a concern.
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Practicality: The use of dichloromethane is also practical in a laboratory setting. It is readily available, and its handling is well-understood by chemists. Additionally, the method of preparing the sample by either placing a small amount directly on the plates and adding one drop of solvent or dissolving it in a small test tube first and transferring the solution with a pipet onto the IR plates is straightforward and commonly used.
In summary, dichloromethane is the solvent normally used in IR spectroscopy for solid samples due to its solubility properties, minimal interference with the sample's IR spectrum, and practical considerations in the laboratory.
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