Infrared spectroscopy Knowledge-Based Download PDF

Questions

1. 1.

   In infrared (IR) spectroscopy, what is the primary spectral effect observed when a hydrogen atom in a carbon–hydrogen (C–H) bond is replaced by a deuterium atom (C–D)?

   • A) The absorption band shifts to a higher wavenumber because the heavier isotope strengthens the chemical bond.
   • B) The absorption band completely disappears because isotopic substitution eliminates the molecular dipole moment.
   • C) The absorption band shifts to a lower wavenumber because the reduced mass of the vibrating atom pair increases.
   • D) The absorption band remains at the identical wavenumber because isotopic substitution does not alter atomic charge.
2. 2.

   In infrared (IR) spectroscopy, which factor primarily explains why a carbon-carbon triple bond absorbs radiation at a higher wavenumber than a carbon-carbon double bond?

   • A) A greater bond force constant resulting from the higher bond order
   • B) A decrease in the reduced mass of the vibrating carbon atoms
   • C) An increase in the permanent dipole moment of the alkyne bond
   • D) A transition from a fundamental stretching vibration to an overtone
3. 3.

   In infrared (IR) spectroscopy, which spectral region, typically spanning wavenumbers below $1500\text{ cm}^{-1}$, exhibits a complex pattern of absorptions characteristic of the molecule as a whole and is primarily used for definitive compound identification against a reference library?

   • A) The functional group region
   • B) The overtone region
   • C) The fingerprint region
   • D) The diagnostic region
4. 4.

   Which condition must be satisfied for a fundamental molecular vibration to be infrared (IR) active and produce an absorption band in an infrared spectrum?

   • A) The vibration must result in a net change in the molecular dipole moment.
   • B) The vibration must produce a net change in the molecular polarizability.
   • C) The molecule must possess an odd number of total valence electrons.
   • D) The vibration must involve an electronic transition between different orbital energy levels.
5. 5.

   In infrared (IR) spectroscopy, which functional group is characteristically identified by an intense, sharp stretching absorption band in the region of $1680\text{--}1750\text{ cm}^{-1}$?

   • A) Hydroxyl group ($-\text{OH}$)
   • B) Carbonyl group ($\text{C}=\text{O}$)
   • C) Alkyne group ($\text{C}\equiv\text{C}$)
   • D) Amine group ($-\text{NH}_2$)