Principles of Organic Chemistry
lecture 8, page 1
ORGANIC STRUCTURE
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A bonding Model of CH4 (continued from lecture 6)
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If there are four equivalent orbitals in methane does this mean that
the orbital diagram is the following?
∆Ε
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o Felix and others call the above view of methane and its orbitals ‘localised’.
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Four equivalent bonding orbitals and four equivalent
antibonding orbitals separated by some energy.
o The lowest lying orbital is the carbon 1s core.
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As Felix points out (p. 34), this is not what is observed by
photo electron spectroscopy (PES).
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The orbital diagram above is more sensible because there are
three peaks in the PE spectrum.
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hν = T + EB
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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.
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You should think of an orbital as a place that is
low-energy for an electron, a hole or a sink.
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With this view point photoelectron spectroscopy PES and its
interpretation makes a lot of sense.
Principles of Organic Chemistry
lecture 8, page 2
Energy
o Felix and others call this view of methane and its orbitals ‘delocalised’.
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There is no need to hybridize orbitals in the delocalized model.
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Pure atomic orbitals at carbon and grouped atomic orbitals
of hydrogen generate the minimalist ‘back of the envelope’
molecular orbitals of methane.
The term delocalized is understandable when we have a quick look
at what the orbitals in this model of methane look like.
o Let’s use the arguments in J. Chem. Ed. 1977, 54, 590-595. To construct a
delocalized molecular orbital picture of methane.
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see above.
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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.
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The bonding (BMO-σ) and antibonding (ABMO-σ∗) results
of this orbital mixing is shown in (e) and (f).
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Below I give the molecular orbital results of generator
combination (atomic orbital) (b) in the figure.
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I am going to do it in my own handwriting so you feel more
comfortable with playing around with this kind of media.
Principles of Organic Chemistry
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lecture 8, page 3
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.
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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?
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The electrons are not localized in any one orbital.
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The 2S orbital of the carbon atom contributes equally to the
bonding of all four C-H bonds!
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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 these symmetry-corrected delocalized orbitals.
Molecular Orbital Calculations
o Should be able to answer all chemistry questions!
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What are the main features in the IR, NMR, Mass Spectrum, or UV
spectrum?
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What is the stability of the molecule?
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How happy are the electrons in the molecule?
o How deep of a well did the kittens fall into?
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We wrestled with this issue when we were using Benson
Principles of Organic Chemistry
lecture 8, page 4
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.
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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.