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?

  • AOut-of-plane twisting
  • BOut-of-plane wagging
  • CIn-plane scissoring
  • DAsymmetric stretching
  • EIn-plane rocking


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

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


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

  • A 3 ๐‘› โˆ’ 5
  • B 3 ๐‘› โˆ’ 6
  • C 2 ๐‘› โˆ’ 6
  • D 2 ๐‘› โˆ’ 5


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


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

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


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


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

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