Handout

Testing Lewis Formulas and VSEPR Models with Quantum Theory
Copyright  2014 John W. Keller
Experiment Sections
I
Draw Lewis formulas for three ions or molecules in one group. Predict each
molecular geometry using VSEPR theory.
II
Use quantum theory (WebMO) to test your geometry predictions.
III
Build plastic models of one or more molecules or ions in Part I.
IV
Use WebMO to investigate a Lewis formula containing resonance forms.
V
Display a sigma (σ) molecular orbital and a pi (π) molecular orbital.
VI
Advanced options. Analyze bond angles and dipole moment in a molecule from
Part I; or use the Internet to find information on complex molecular shapes.
I. Lewis formulas and VSEPR predictions
Draw the Lewis formula for the molecule or ion in the shaded box in your assigned group, plus
TWO OTHERS in that group. Use 3 worksheets, one for each molecule or ion. First draw the
Lewis formula, and then predict the molecular geometry using VSEPR theory. In Part II test
your predictions using quantum theory calculations. You may wish to test each prediction by
going immediately to Part II, then returning to draw the next Lewis formula on a fresh
worksheet.
Group
“br” = branched; “nonbr” = not branched; atoms in a row.
(Where it may be ambiguous, the underlined and BOLD atom is central.)
1
BFCl+ (nonbr)
COCl2 (br)
SeO32- (br)
SCl2F2 (br)
2
ClFO3 (br)
FCCF (nonbr)
H3S+ (br)
SCN- (nonbr)
3
ClClO (nonbr)
CS(NH2)2 (br)
NCO- (nonbr)
SSO22- (br)
4
ClF4- (br)
CNH(NH2)2 (br)
FNNF (nonbr)
SClFS (br)
5
FCO(OH) (br)
NCS- (nonbr)
SiH2F2 (br)
ONOO- (nonbr)
6
CNO- (nonbr)
CO(OH)2 (br)
SiClF3 (br)
SSSS2- (nonbr)
1
II. Test molecular geometry using WebMO and PM3
1) Click the “24 molecules and ions” link at this experiment’s website (obtain the URL or
hyperlink from your instructor). Follow the link of an assigned molecule to a webpage
showing two or three 3D structures. Keep that page open for the next step.
2) Use WebMO and the PM3 quantum method to calculate the heat of formation of all
geometries shown (some have 2, others 3). Record each heat of formation on the
worksheet for that molecule or ion.
3) Now: Is your VSEPR predicted geometry the same as the geometry with the lowest
energy? Hopefully, yes. Your back-of-the-envelop sketch has been verified by quantum
theory!
4) If not, check your original Lewis formula and VSEPR prediction. Perhaps revise the Lewis
formula, or re-calculate “electron pair geometry” and “molecular geometry” so that
they are consistent with the structure having the lowest energy.
(No need to erase your original formula: there is no penalty for getting an incorrect
geometry. In fact, comparing correct and incorrect formulas is a great way to learn
about—and remember—the VSEPR theory.)
III. Build plastic models
Choose at least one of the (correct) geometries and build a plastic model. See the handout
HGSandScholARmodels.pdf on the ex periment website. For resonance stabilized ions or
molecules, only one resonance form can be built at a time.
IV. Resonance
Background. It seems that fairly often, especially when molecules contain double or triple
bonds and nearby lone pairs, the bonds and lone pairs can be arranged in different ways to
satisfy the Octet Rule. If these 2 or 3 formulas are reasonable, then the actual bonding is
considered to be a hybrid of all these formulas. This is an admittedly confusing situation, but it
arises because some molecules contain a bond that stretches across 3, 4 or more atoms. A
multi-atom bond like this cannot be represented by the customary line between 2 atoms, so
drawing 2 or more resonance forms is the
best way to represent these bonds with
Lewis formulas. In fact, the same atoms
where the variable bonds are placed are
the same ones that contain the multi-atom
bond. A multi-atom bond is a pi-type bond
(π-molecular orbital) which can be thought
of as the summation of several parallel p-orbitals.
2
Group
Resonance-stabilized
molecule (or ion)
1
COCl2
2
SCN-
3
NCO-
4
CNH(NH2)2
5
ONOO-
6
CO(OH)2
1. Anyway, if you have not done so already, draw as many resonance forms as possible (that
obey the Octet Rule) for the ion or molecule in your group as shown above. Be sure to include
in your formulas the correct formal charge for any atom with a non-zero formal charge. See
section 9.7 in Chang for more help with formal charges.
***Your TA should check your resonance formulas first before continuing to WebMO.****
2. Do these formulas actually represent physically different ions or molecules? We can provide
a definitive answer using quantum theory. First, use WebMO and PM3 to optimize the
geometry of one resonance form.
3. Next, optimize the geometry of the other resonance form(s). Compare the energies, bond
lengths, and charge distributions of the 2 (or 3) jobs. Are they the same, or different? If they
are indeed resonance forms, their energies and other parameters should be identical.
(If the energy of the second resonance form WebMO job is different, it’s possible that a
setting on the input page of WebMO was incorrect, such as the charge or multiplicity. Or, in
the case of ONOO-, it has two possible shapes: curved or zig-zag. Either one is OK, but make
sure both calculations use the same shape.)
3
V. Sigma (σ) and Pi (π) Molecular Orbitals
In WebMO again, open one of your resonance jobs, click “New Job Using This Geometry”
at the bottom of the window. Continue. Continue. Now under Calculation, choose
Molecular Orbitals. Continue. When completed, see the results by clicking the job name,
and go to the table near the bottom of page. The bonding molecular orbitals (MOs) are
the ones with “population = 2 electrons”. Draw, or print out, one sigma and one pi bond
for your molecule.
A pi (π) bond is two-sided, with electron density zones appearing above and below the
plane of the bonded atoms. Pi bonds are formed by overlap of atomic p-orbitals. MO 6 in
the video example is a π-bond. A sigma (σ) bond lies in the plane of the bonded atoms.
Sigma bonds are formed by overlap of s- or p-orbitals.
In some molecules or ions in this experiment, the type of MO is already labeled “σ” or “π”
in column 2 of the results page. View an MO by clicking the magnifying glass icon on the
right side of the table. The style of the MO display is controlled by the Preferences menu.
If the MOs are not labeled “σ” or “π”, just choose one that is several steps in energy lower
than the highest occupied MO. If that is not to your liking, display another one by clicking
a different MO number in the left column.
This button opens the
Preferences box.
Click a
number
here to
display
another
MO.
MO 6 of thioformateion is a π-bond
extending over 3 atoms.
The volume of the orbital, or equal
probability surface, is controlled by the iso
value: 0.03 gives a reasonable surface.
The surface style or opacity is controlled by the
Opacity slider. There 4 choices (l-to-r): none, mesh
(shown here), translucent, and opaque.
4
Review Topics
Review the topics below in your textbook. You may also wish to bring the text to lab, or open
these sections online. However, it is vital that you have some skill in drawing Lewis formulas
and calculating formal charge before coming to lab.
o
Topic
Heat of Formation ∆fH
Drawing Lewis formulas and formal charge
Valence Shell Electron Pair Repulsion (VSEPR) theory
The Schrödinger Equation and quantum theory of atoms
Resonance
Molecular orbitals
5
Sections in Chang
6.6
9.6, 9.7
10.1
7.5
9.8
10.6, 10.7, 10.8