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.

Q1:

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

Q2:

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

Q3:

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

Q4:

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

Q5:

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.

Q6:

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

Q7:

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

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