Which of the following statements accurately describes the characteristic molecular vibrations of IR spectroscopy? A.)The stretch frequencies are lower than the frequencies of corresponding bending. B.) The stretch frequencies appear mainly in the fingerprint region. C.)Triple bonds have lower stretch frequencies than corresponding double bonds, which in turn have lower frequencies than single bonds. D.) Hydrogen bonds have higher stretch frequencies than larger heavy atoms.
Which of the following statements accurately describes the characteristic molecular vibrations of IR spectroscopy?
A.)The stretch frequencies are lower than the frequencies of corresponding bending.
B.) The stretch frequencies appear mainly in the fingerprint region.
C.)Triple bonds have lower stretch frequencies than corresponding double bonds, which in turn have lower frequencies than single bonds.
D.) Hydrogen bonds have higher stretch frequencies than larger heavy atoms.
The Correct Answer and Explanation is :
The correct answer is D.) Hydrogen bonds have higher stretch frequencies than larger heavy atoms.
Explanation:
Infrared (IR) spectroscopy relies on the interaction of infrared radiation with molecular vibrations, which include stretching and bending motions of chemical bonds. Different bonds and groups exhibit characteristic frequencies, allowing IR spectroscopy to identify functional groups in a molecule. Let’s analyze the options:
A.) The stretch frequencies are lower than the frequencies of corresponding bending.
- Incorrect. Stretching vibrations generally occur at higher frequencies compared to bending vibrations. This is because stretching involves the movement of atoms along the bond axis, which requires more energy than bending (a deformation motion).
B.) The stretch frequencies appear mainly in the fingerprint region.
- Incorrect. The fingerprint region of the IR spectrum (600–1500 cm⁻¹) is primarily associated with complex bending and combination vibrations, not stretching. Stretch frequencies of key bonds like O-H, N-H, C-H, and triple bonds are observed in the functional group region (1500–4000 cm⁻¹).
C.) Triple bonds have lower stretch frequencies than corresponding double bonds, which in turn have lower frequencies than single bonds.
- Incorrect. Triple bonds have higher stretch frequencies than double bonds, which are higher than single bonds. This is because the bond strength increases from single to triple bonds, requiring more energy to stretch.
D.) Hydrogen bonds have higher stretch frequencies than larger heavy atoms.
- Correct. Bonds involving hydrogen (e.g., O-H, N-H, and C-H) have higher stretch frequencies due to hydrogen’s low mass. According to Hooke’s Law ((v \propto \sqrt{\frac{k}{\mu}})), the vibrational frequency is inversely proportional to the square root of the reduced mass ((\mu)). Bonds with hydrogen atoms (small (\mu)) vibrate at higher frequencies compared to bonds with heavier atoms.
In conclusion, IR spectroscopy is a powerful tool for identifying molecular vibrations. The key takeaway is that lighter atoms, such as hydrogen, exhibit higher vibrational frequencies than heavier atoms, supporting Option D as the correct answer.