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?

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

Q2:

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

Q3:

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

Q4:

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

Q5:

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.

Q6:

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

Q7:

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

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