Electronic Spectra Flashcards
What is the wavelength range for ultraviolet radiation?
10 to 400 nm
Define the term ‘HOMO’
HOMO stands for the Highest Occupied Molecular Orbital, which is the highest energy orbital containing electrons underground state conditions
Define the term LUMO
LUMO stands for Lowest Unoccupied Molecular Orbital, which is the lowest energy orbital that electrons can be excited to during electronic transitions.
What is the principle behind the selection rules in electronic spectroscopy?
The selection rules govern which transitions are allowed based on the symmetry properties of the molecular orbitals involved, ensuring conservation of angular momentum and parity in the molecule.
What is meant by ‘crystal field splitting’ in transition metal complexes?
Crystal field splitting refers to the energy difference created when degenerate orbitals in a free metal ion are split into different energy levels upon coordination with ligands.
Why do conjugated compounds tend to be colorful?
Conjugated compounds have delocalized π electrons which allow for lower energy transitions between molecular orbitals, resulting in absorption in the visible spectrum and thus colorful appearance.
How does the electronic structure of a molecule affect its absorption properties in UV-Vis spectroscopy?
The electronic structure, particularly the energy gap between HOMO and LUMO, determines the wavelength of light that a molecule can absorb. Smaller gaps typically result in absorption of longer wavelengths (visible light).
Why are transition metal complexes often highly colored?
Transition metal complexes are colored due to the d-d transitions within the metal and charge transfer transitions between the metal and its ligands, which occur in the visible range.
Discuss how molecular symmetry affects electronic transitions.
Molecular symmetry determines the selection rules for electronic transitions. For example, in centrosymmetric molecules, transitions between orbitals of the same symmetry (g to g or u to u) are forbidden, influencing the observed spectra.
What factors could influence the electronic spectra of a compound other than its molecular orbitals?
Factors include the molecular environment (solvent effects), temperature, pressure, and presence of other interacting molecules, which can all alter the energy levels and transition probabilities.
What is the range of visible radiation in nanometers?
400 to 700 nm.
Which electron transitions are typically involved in electronic spectroscopy?
Transitions from filled to empty (virtual) molecular orbitals.
What are the selection rules for allowed electronic transitions in molecules with an inversion center?
Transitions must be from a symmetric (g) to an asymmetric (u) orbital or vice versa.
Describe the difference in energy between HOMO and LUMO in terms of electronic transitions in ethylene (C2H4).
The difference is approximately 0.1279 Hartrees, corresponding to an energy of about 343 kJ/mol.
What is the significance of the HOMO-LUMO gap in determining the color of a compound?
The gap determines the wavelength of light absorbed, which in turn influences the color of the compound as perceived when the light is visible.
