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
  • BIn-plane rocking
  • COut-of-plane twisting
  • DAssymetric stretching
  • EOut-of-plane wagging


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

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


In non-linear molecules, how many fundamental vibrations may exist (let ๐‘› = the number of atoms)?

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


How many fundamental vibrations would exist for ethyne ( H C C H )?


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

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


How many normal modes of vibration occur in a non-linear molecule C H C l 2 4 2 ?


How many normal modes of vibration occur in the benzene molecule ( C H 6 6 ) ?

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