Worksheet: Vibrational Modes in IR Spectra

In this worksheet, we will practice linking infrared signals to particular vibrations of functional groups, including stretching and bending modes.


Which of the following is not used to describe the bending modes in infrared spectroscopy?

  • AIn-plane scissoring
  • BOut-of-plane twisting
  • CAsymmetric stretching
  • DOut-of-plane wagging
  • EIn-plane rocking


What does Hookeโ€™s law dictate that IR stretching frequencies are dependent on?

  • AThe number of lone pairs and dipole moment of the bond
  • BBond strength and molar masses of the atoms
  • CThe effective nuclear charge and polarizability of the bond
  • DThe magnetic spin and hybridization of the atoms


In nonlinear molecules, how many fundamental vibrations may exist? let ๐‘› = the number of atoms.

  • A2๐‘›โˆ’6
  • B2๐‘›โˆ’5
  • C3๐‘›โˆ’6
  • D3๐‘›โˆ’5


How many fundamental vibrations would exist for ethyne (HCCH)?


Why are vibrational lifetimes of molecular functional groups difficult to study directly?

  • AThe vibrations occur over picosecond timescales.
  • BThe electron-donating abilities of functional groups interfere with detection.
  • CThe vibrational lifetimes depend on the rotational quantum number, which is highly variable.
  • DThe functional groups undergo rapid isomerizations.
  • EThe intense colors of the molecules prevent transmission of the source radiation.


How many normal modes of vibration occur in the nonlinear molecule CHCl242?


How many normal modes of vibration occur in the benzene molecule (CH66) ?


Which of the following vibrational modes of the carbon dioxide molecule CO2 will be IR active?

  • AV2, V3
  • BV1, V2
  • CV1, V2, V3
  • DAll vibrational modes are IR inactive


An absorption spectrum for the diatomic molecule hydrogen bromide HBr appeared at the IR region as shown below. Which of the following is the most likely origin for the observed spectrum?

  • ARotational-vibrational transition
  • BPure rotational transition
  • CElectronic transition
  • DPure vibrational transition

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