Rotational & Vibrational Spectra Flashcards
What are the typical energy ranges (in kJ/mol) for rotational, vibrational, and electronic transitions?
Rotational transitions are around 0.1 kJ/mol, vibrational transitions about 10 kJ/mol, and electronic transitions approximately 1000 kJ/mol.
Define the term ‘moment of inertia’ in the context of rotational spectroscopy.
The moment of inertia is a measure of an object’s resistance to changes in its rotation and is dependent on the mass distribution relative to the axis of rotation.
What is meant by a molecule being a “polar molecule” in spectroscopy?
A polar molecule has a permanent electric dipole moment due to the uneven distribution of electrons between bonded atoms, which allows it to undergo rotational transitions.
Explain the significance of the rotational constant (B’).
The rotational constant is a measure related to the moment of inertia and the bond lengths in molecules; it appears in the energy expression for rotational levels.
What is the Morse potential in vibrational spectroscopy?
The Morse potential is a mathematical model used to describe the potential energy of a bond as a function of displacement, simulating bond stretching and compressing.
Why might a molecule show a high intensity in its vibrational spectrum for certain modes but not for others?
Intensity variations in vibrational spectra can be due to differences in how much a particular vibrational mode changes the dipole moment or the polarizability of the molecule.
How do molecular symmetry and dipole moments affect the spectroscopic behavior of a molecule?
Molecular symmetry influences selection rules and thus whether certain transitions are allowed. Dipole moments affect whether molecules can be studied by certain types of spectroscopy, like rotational spectroscopy.
Discuss the impact of molecular complexity on the vibrational spectra of larger organic molecules.
As molecules become larger and more complex, their vibrational spectra become more crowded and complex due to the increased number of vibrational modes and possible interactions between them.
What role does the quantum mechanical model of a molecule play in predicting its spectroscopic properties?
Quantum mechanical models help predict energy levels, transition probabilities, and interaction with electromagnetic radiation, directly influencing spectroscopic properties and interpretations.
Explain why some molecular vibrations are more sensitive to anharmonic effects than others.
Anharmonic effects become significant at higher energies or larger atomic displacements, especially in bonds that are less stiff, leading to deviations from the ideal harmonic oscillator model.
What are the selection rules for rotational transitions?
The molecule must have a permanent electric dipole moment, and the quantum numbers must satisfy ΔJ = ±1 and ΔM = 0 or ±1.
Describe how vibrational spectroscopy utilizes the harmonic oscillator model.
In vibrational spectroscopy, the harmonic oscillator model approximates the vibrational motion of molecules, especially at low energies, where the energy levels are quantized as E = (ν + ½)ħω.
How does infrared spectroscopy differ from Raman spectroscopy in terms of selection rules?
Infrared spectroscopy requires a change in the dipole moment of the molecule (IR active), whereas Raman spectroscopy involves changes in polarizability and can observe vibrations that are IR inactive.
How are bond lengths determined using rotational spectra?
Bond lengths are calculated from rotational spectra by using the rotational constant and the moment of inertia, which depend on the mass and the square of the bond length.
What information can be obtained from the vibrational force constant?
The vibrational force constant, which reflects the stiffness of a chemical bond, can provide insights into bond strength and energy
