Chemistry 2000 Slide Set 3: LCAO-MO theory for heteronuclear - - PowerPoint PPT Presentation

Chemistry 2000 Slide Set 3: LCAO-MO theory for heteronuclear diatomic molecules

Marc R. Roussel January 2, 2020

Marc R. Roussel Heteronuclear diatomics January 2, 2020 1 / 7

Heteronuclear diatomics

Key ideas used in constructing LCAO-MOs of heteronuclear diatomics:

1 Only valence orbitals of similar energy (±1 Ry) mix to form an MO. 2 We can separate AOs into two sets:

σ-type AOs: s and pz π-type AOs: px and py Only σ-type AOs are involved in σ bonding. Only π-type AOs are involved in π bonding.

3 Number of MOs constructed = number of AOs mixed Marc R. Roussel Heteronuclear diatomics January 2, 2020 2 / 7

LCAO-MO treatment of HF

Valence AO energies:

• 4
• 3.5
• 3
• 2.5
• 2
• 1.5
• 1
• 0.5

F H εi /RH 1s 2s 2p

Develop MO diagram, compare to Lewis diagram.

Marc R. Roussel Heteronuclear diatomics January 2, 2020 3 / 7

LCAO-MO treatment of HF (continued)

New elements in the MO diagram: We get nonbonding (nb) orbitals. We generally drop the antibonding labels because this becomes harder to decide from the MO diagram alone.

In a simple case like this (σ orbitals arise from mixing one AO on each atom), it is generally the case that the lower combination is bonding and the upper is anti-bonding. Otherwise, we need to look at pictures from an MO program like Gaussian, although even that isn’t always very clear. Better: look at dependence of orbital energy on distance between nuclei.

If the orbital energy increases as the nuclei move apart, the orbital is bonding; decreases, antibonding.

Soon: experiments to the rescue!

Marc R. Roussel Heteronuclear diatomics January 2, 2020 4 / 7

LCAO-MO treatment of CO

Valence AO energies:

• 4
• 3.5
• 3
• 2.5
• 2
• 1.5
• 1
• 0.5

O C εi /RH 2s 2p 2s 2p

(Develop MO diagram on board)

Marc R. Roussel Heteronuclear diatomics January 2, 2020 5 / 7

CO molecular orbitals

3σ ×2 1π 2σ 1σ

Marc R. Roussel Heteronuclear diatomics January 2, 2020 6 / 7

LCAO-MO treatment of CO

The orbitals are polarized. As a rule, there is a larger contribution to the MO from the nearest AO in energy. Use this to discuss the Lewis acidity of CO. The ordering of the σ and π orbitals is not immediately obvious. To determine this, we can use a quantum chemistry program like Gaussian. We can however take a reasonable guess based on the fact that CO is isoelectronic with N2: In N2, the 1π orbitals are below the 3σ. This turns out to be the case for CO as well.

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