Electronic Spectra

Question 1

What is the wavelength range for ultraviolet radiation?

Answer 1

10 to 400 nm

Question 2

Define the term ‘HOMO’

Answer 2

HOMO stands for the Highest Occupied Molecular Orbital, which is the highest energy orbital containing electrons underground state conditions

Question 3

Define the term LUMO

Answer 3

LUMO stands for Lowest Unoccupied Molecular Orbital, which is the lowest energy orbital that electrons can be excited to during electronic transitions.

Question 4

What is the principle behind the selection rules in electronic spectroscopy?

Answer 4

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.

Question 5

What is meant by ‘crystal field splitting’ in transition metal complexes?

Answer 5

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.

Question 6

Why do conjugated compounds tend to be colorful?

Answer 6

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.

Question 7

How does the electronic structure of a molecule affect its absorption properties in UV-Vis spectroscopy?

Answer 7

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).

Question 8

Why are transition metal complexes often highly colored?

Answer 8

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.

Question 9

Discuss how molecular symmetry affects electronic transitions.

Answer 9

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.

Question 10

What factors could influence the electronic spectra of a compound other than its molecular orbitals?

Answer 10

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.

Question 11

What is the range of visible radiation in nanometers?

Answer 11

400 to 700 nm.

Question 12

Which electron transitions are typically involved in electronic spectroscopy?

Answer 12

Transitions from filled to empty (virtual) molecular orbitals.

Question 13

What are the selection rules for allowed electronic transitions in molecules with an inversion center?

Answer 13

Transitions must be from a symmetric (g) to an asymmetric (u) orbital or vice versa.

Question 14

Describe the difference in energy between HOMO and LUMO in terms of electronic transitions in ethylene (C2H4).

Answer 14

The difference is approximately 0.1279 Hartrees, corresponding to an energy of about 343 kJ/mol.

Question 15

What is the significance of the HOMO-LUMO gap in determining the color of a compound?

Answer 15

The gap determines the wavelength of light absorbed, which in turn influences the color of the compound as perceived when the light is visible.

Question 16

What is the role of conjugation in the electronic properties of molecules like beta-carotene?

Answer 16

Conjugation allows for lower energy electronic transitions, typically π → π* transitions, influencing the visible color and absorption properties of the molecule.

Question 17

Define ‘chromophores’ in the context of electronic spectroscopy.

Answer 17

Chromophores are components of molecules that absorb light within the visible spectrum, leading to coloration.

Question 18

Explain the concept of ‘ligand field theory’ and its application to transition metal complexes.

Answer 18

Ligand field theory explains the splitting of degenerate d orbitals in a transition metal complex due to interactions with surrounding ligands, influencing the electronic transitions and colors of the complexes.

Question 19

What are ‘charge transfer’ transitions in the context of transition metal complexes?

Answer 19

Charge transfer transitions are electronic transitions where electrons are transferred between the metal and its ligands, either from metal to ligand or ligand to metal, often resulting in intense coloration.

Question 20

How does the electronic transition in a Fe(NH3)4(Cl)2 complex occur?

Answer 20

The transition is a metal to ligand charge transfer, where electrons move from the HOMO, which is concentrated on Fe, to the LUMO, which is spread across the ligands.