Bond Stretching Vibrations
Definition and meaning of Bond Stretching Vibrations in chemistry.
Bond stretching vibrations are periodic oscillations in the length of a chemical bond, in which the two bonded atoms move closer together and farther apart along the bond axis.
In more detail
To a good approximation, a bond behaves as a simple harmonic oscillator obeying Hooke's law, so its stretching frequency depends on the bond's force constant (a measure of bond stiffness) and the reduced mass of the two atoms: stiffer bonds and lighter atoms vibrate faster. Vibrational energy is quantized, and transitions between these levels are probed by infrared (IR) spectroscopy, since IR photons carry roughly the right energy to excite them. A stretch only absorbs IR light if it changes the molecule's dipole moment, so some symmetric stretches are IR-inactive.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing model | Simple harmonic oscillator (Hooke's law) |
| Frequency formula | ṽ = (1/2πc)√(k/μ) |
| Typical IR range | 4000–400 cm⁻¹ (mid-infrared) |
The C=O stretch of a ketone gives a strong, sharp IR absorption near 1715 cm⁻¹, a signature chemists use to confirm the presence of a carbonyl group in an unknown sample.
Frequently asked questions
Why do stronger bonds and lighter atoms vibrate at higher frequency?
Stretching frequency is proportional to the square root of k/μ, where k is the bond force constant and μ is the reduced mass of the two atoms; a stiffer bond (higher k) or lighter atoms (lower μ) raise the frequency.
Does every bond stretch absorb infrared light?
No. Only vibrations that produce a net change in the molecule's dipole moment are IR-active; for example, the symmetric C≡C stretch in a symmetrical alkyne causes no dipole change and is IR-inactive.