Consider the following statements as they apply to infrared spectroscopy. Decide if each is true or false. True False The quantized nature of molecular vibrations are generally understood in terms of a rigid rotor model. True False Absorption bands in infrared spectroscopy are due to vibrational and rotational-vibrational transitions in molecules. True False For a given pair of atoms, the vibrational frequency expected if they are joined by a single bond should be higher than if they are joined by a double bond. True False Because molecules in condensed phases (solids and liquid) are not free to rotate, rotational-vibrational transitions are not observed in their infrared spectra. For this reason, even at high resolution, a condensed phase spectrum is characterized by broad bands. True False Heteronuclear diatomic molecules such as HCI do not absorb infrared radiation.
The Correct Answer and Explanation is :
Correct Answers:
- False – The quantized nature of molecular vibrations is not understood in terms of the rigid rotor model but rather the harmonic oscillator model.
- True – Absorption bands in infrared spectroscopy are due to vibrational and rotational-vibrational transitions in molecules.
- False – A single bond will have a lower vibrational frequency compared to a double bond due to differences in bond strength.
- True – Molecules in condensed phases experience restricted rotations, leading to broad infrared absorption bands in the spectrum.
- False – Heteronuclear diatomic molecules, such as HCl, do absorb infrared radiation because they possess a permanent dipole moment.
Explanation:
Infrared spectroscopy studies how molecular vibrations absorb infrared light. These vibrations are quantized, meaning only specific energy levels are allowed. However, they are modeled using a harmonic oscillator model, not the rigid rotor model (which applies to rotational transitions).
Absorption bands arise from transitions between vibrational states, often coupled with rotational transitions (rotational-vibrational transitions). These transitions depend on the molecule’s dipole moment changing during vibration, making such transitions detectable.
The vibrational frequency of a bond correlates with the bond strength and the masses of the atoms. Double bonds are stronger than single bonds and thus vibrate at higher frequencies. This contradicts the statement that single bonds have higher vibrational frequencies.
In condensed phases like liquids and solids, molecular rotations are constrained due to intermolecular interactions. This restriction eliminates distinct rotational-vibrational transitions, and as a result, spectra display broader bands compared to gases, where rotational features are more resolved.
Finally, heteronuclear diatomic molecules (e.g., HCl) absorb IR radiation due to their changing dipole moment during vibration. Homonuclear diatomic molecules, like O₂ or N₂, do not absorb IR because their vibrations do not produce a dipole change.