Principles of Organic Chemistry lecture 6, page 1

• A bonding Model of CH4 (continued from lecture 5)

If there are four equivalent orbitals in methane does this mean that the orbital diagram is the following?

o

∆Ε

o Felix and others call the above view of methane and its orbitals ‘localised’.

• Four equivalent bonding orbitals and four equivalent antibonding orbitals separated by some energy.

• As Felix points out (p. 34), this is not what is observed by photo electron spectroscopy (PES).

• The orbital diagram above is more sensible because there are three peaks in the PE spectrum.

hν = T + EB

Electromagnetic radiation (hν) impinges on the sample. A photon is ejected and its kinetic energy is measured (T). The difference between the energy of the radiation and the kinetic energy is the energy of the electron in the molecule.

• You should think of an orbital as a place that is low-energy for an electron, a hole or a sink.

• With this view point PES and its interpretation makes a lot of sense.

∆Ε

o Felix and others call this view of methane and its orbitals ‘delocalised’.

The term is understandable when we have a quick look at what the orbitals in this view of methane look like.

Principles of Organic Chemistry lecture 6, page 2

o Let’s use the arguments in J. Chem. Ed. 1977, 54, 590-595. To construct a delocalized

molecular orbital picture of methane.

see above.

In (a) the ‘generator’ 2S orbital of the carbon atom and the ‘generator’ 1S orbitals of the hydrogen atoms (atomic) orbitals combine to make molecular orbitals.

• The bonding (BMO-σ) and antibonding (ABMO-σ∗) results of this orbital mixing is shown in (e) and (f).

• Below (and in lecture) I give the molecular orbital results of generator combination (atomic orbital) (b) in the figure.

• I am going to do it in my own handwriting so you feel more comfortable with playing around with this kind of media.

•

• BMOπs in your text are simply linear combinations of the BMOπ above and others like them.

o I did this on the board and made the analogy to the LCAO approach with the allyl system.

Instructor shows the output of computation in isovalue projections of methane molecular orbitals generated by the program Gaussian03.

o With such a picture of the methane molecule, how can all the bonds be equal . . . as they must for a tetrahedral geometry?

Principles of Organic Chemistry lecture 6, page 3

The electrons are not localized in any one orbital.

• The 2S orbital of the carbon atom contributes equally to the bonding of all four C-H bonds!

• Likewise the BMOπ orbitals contribute to the bonding of all four C-H bonds!

To appreciate the structure and the spectra of methane one needs to consider symmetry-corrected delocalized orbitals.

• Molecular Orbital Calculations

o Can answer a few questions

What are the main features in the IR, NMR, Mass Spectrum, or UV spectrum?

• In my research I have been recently playing with the calculation of circular dichroism spectra.

What is the stability of the molecule?

• How happy are the electrons in the molecule?

o How deep of a well did the kittens fall into?

• We wrestled with this issue when we were using Benson group values to calculate ∆H.

o When we were making these calculations we were trying to express how stable the electrons were around the nuclei from a bond and ‘group’ perspective.

How does the energy change when two molecules interact?

o We don’t have a chance of answering this question with Benson group values. and a pi molecular orbital.