The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. a. Assuming this electronic transition corresponds to the HOMO–LUMO transition, what is the HOMO in ethylene?

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Identify the type of molecular orbitals involved in ethylene. Ethylene (C2H4) has a double bond between the two carbon atoms, which includes a sigma (σ) bond and a pi (π) bond.

Understand the terms HOMO and LUMO. HOMO stands for Highest Occupied Molecular Orbital, and LUMO stands for Lowest Unoccupied Molecular Orbital.

Recognize the bonding and antibonding orbitals in ethylene. The π bond formed by the sideways overlap of p orbitals results in a π bonding orbital and a π* antibonding orbital.

Determine the HOMO in ethylene. Since the π bonding orbital is the highest occupied molecular orbital in the pi bonding system of ethylene, it is the HOMO.

Relate the electronic transition to the absorption of a photon. When a photon is absorbed, an electron in the HOMO (π bonding orbital) can be excited to the LUMO (π* antibonding orbital).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Orbitals

Molecular orbitals (MOs) are formed by the combination of atomic orbitals when atoms bond together. In ethylene, the sideways overlap of p orbitals creates two types of MOs: bonding and antibonding. The bonding molecular orbital is lower in energy and can accommodate electrons, while the antibonding orbital is higher in energy and destabilizes the molecule when occupied.

HOMO and LUMO

The Highest Occupied Molecular Orbital (HOMO) is the molecular orbital that contains the highest energy electrons in a molecule, while the Lowest Unoccupied Molecular Orbital (LUMO) is the lowest energy orbital that is unoccupied. In the context of electronic transitions, when a photon is absorbed, an electron can be promoted from the HOMO to the LUMO, facilitating various chemical reactions and properties.

Electronic Transitions

Electronic transitions refer to the movement of electrons between different energy levels or orbitals within a molecule. In ethylene, the absorption of a photon can promote an electron from the bonding HOMO to the higher-energy LUMO. This transition is crucial for understanding the molecule's reactivity and its interaction with light, which is fundamental in spectroscopy and photochemistry.

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Azo dyes are organic dyes that are used for many applications, such as the coloring of fabrics. Many azo dyes are derivatives of the organic substance azobenzene, C12H10N2. A closely related substance is hydrazobenzene, C12H12N2. The Lewis structures of these two substances are

(Recall the shorthand notation used for benzene.) (b) How many unhybridized atomic orbitals are there on the N and the C atoms in each of the substances? How many unhybridized atomic orbitals are there on the N and the C atoms in hydrazobenzene?

Textbook Question

(Recall the shorthand notation used for benzene.) (c) Predict the N¬N¬C angles in each of the substances.

Textbook Question

Carbon monoxide, CO, is isoelectronic to N2. (d) Would you expect the p2p MOs of CO to have equal atomic orbital contributions from the C and O atoms? If not, which atom would have the greater contribution?

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Textbook Question

The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. b. Assuming this electronic transition corresponds to the HOMO–LUMO transition, what is the LUMO in ethylene?

Textbook Question

The energy-level diagram in Figure 9.40 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the to the molecular orbital. c. Is the bond in ethylene stronger or weaker in the excited state than in the ground state? Why?