Determine whether each of the following statements is true or false. If the stat
ID: 1065940 • Letter: D
Question
Determine whether each of the following statements is true or false. If the statement is false, modify and rewrite it so that it is a true statement.
a. When a molecule has two, degenerate, “infrared active”, vibrational modes, the two vibrational modes will show absorptions at different frequencies in the infrared spectrum.
b. For a given substance, strong intermolecular forces between molecules of the substance can cause peak broadening of some of the absorptions in the infrared spectrum of the substance.
Explanation / Answer
a. When a molecule has two, degenerate, “infrared active”, vibrational modes, the two vibrational modes will show absorptions at different frequencies in the infrared spectrum.
Answer: False
Reason: Vibrations with same frequency are said to be doubly degenerate (two, degenerate), they are identical in frequency but in perpendicular directions and these multiple vibrations only result in one infrared absorption band being seen in the spectrum.
rewritten: When a molecule has two, degenerate, “infrared active”, vibrational modes, the two vibrational modes will show absorptions at one frequency in the infrared spectrum.
b. For a given substance, strong intermolecular forces between molecules of the substance can cause peak broadening of some of the absorptions in the infrared spectrum of the substance.
answer: True
Reason: Let us consider methanol, ethanol, water - all will show a broad peak around ~ 3200-3600 cm-1 in the infrared spectrum for O-H stretching frequency. The broadening is due to the presence of intermolecular force hydrogen bonding. Hydrogen bonding lowers the wavelength of absorption maximum than free O-H stretching frequency.
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