Stretching Vibrations

Definition

Stretching vibrations refer to the oscillatory motion of atoms or molecules along the axis of a chemical bond, resulting in changes in the bond length. These vibrations are a fundamental aspect of infrared spectroscopy, as they contribute to the absorption and emission of infrared radiation by molecules.

5 Must Know Facts For Your Next Test

1. Stretching vibrations can be either symmetric, where the bond lengths change in phase, or asymmetric, where the bond lengths change out of phase.
2. The frequency of a stretching vibration is inversely proportional to the square root of the reduced mass of the atoms involved and the force constant of the bond.
3. Stretching vibrations typically occur in the mid-infrared region of the electromagnetic spectrum, between 4000 and 400 cm^-1.
4. The intensity of a stretching vibration band in an infrared spectrum is related to the change in the dipole moment of the molecule during the vibration.
5. Stretching vibrations can be used to identify functional groups and provide information about the structure and bonding in organic molecules.

Review Questions

• Explain the relationship between stretching vibrations and the absorption of infrared radiation by molecules.
  • Stretching vibrations in molecules occur when the atoms oscillate along the axis of a chemical bond, causing changes in the bond length. These changes in bond length lead to a corresponding change in the dipole moment of the molecule, which can result in the absorption of infrared radiation at specific frequencies. The absorption of infrared radiation by stretching vibrations is a fundamental aspect of infrared spectroscopy, as it allows for the identification of functional groups and the determination of molecular structure.
• Describe the factors that influence the frequency of a stretching vibration.
  • The frequency of a stretching vibration is determined by the strength of the chemical bond and the masses of the atoms involved. Specifically, the frequency is inversely proportional to the square root of the reduced mass of the atoms and the force constant of the bond. Stronger bonds and lighter atoms will result in higher frequency stretching vibrations, while weaker bonds and heavier atoms will lead to lower frequency vibrations. Understanding these factors is crucial for interpreting infrared spectra and assigning observed absorption bands to specific functional groups or structural features.
• Analyze how the symmetry of a stretching vibration can affect its appearance in an infrared spectrum.
  • Stretching vibrations can be either symmetric or asymmetric in nature. Symmetric stretching vibrations, where the bond lengths change in phase, typically result in stronger absorption bands in the infrared spectrum compared to asymmetric stretching vibrations, where the bond lengths change out of phase. This is because symmetric stretching vibrations often involve a larger change in the dipole moment of the molecule, which is directly related to the intensity of the absorption band. Recognizing the symmetry of a stretching vibration and its impact on the infrared spectrum is essential for interpreting and analyzing infrared data to gain insights into molecular structure and bonding.

Related terms

Bending Vibrations:

Bending vibrations involve the change in bond angles between atoms, causing the molecule to bend or distort.

Rocking Vibrations: Rocking vibrations occur when a group of atoms in a molecule moves back and forth, causing the molecule to rock or sway.

Torsional Vibrations: Torsional vibrations involve the twisting or rotation of a molecule around a single bond, leading to changes in the dihedral angle.