Using Computational Chemistry to Explore Concepts in General Chemistry

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Using Computational Chemistry to Explore Concepts
in General Chemistry Mark Wirtz, Edward Ehrat,
David L. Cedeno Department of Chemistry,
Illinois State University, Box 4160, Normal, IL
61790-4160
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• Introduction
• Students find that some topics and concepts in
  General Chemistry are difficult and challenging
  because they require some extent of
  abstractionism.
• These concepts may be rationalized better if
  students have the opportunity to explore and
  play with them through a visual methodology.
• Computational chemistry modeling software
  provides appropriate ways of visualizing
  concepts. We are utilizing readily available and
  accessible software to create educational
  modules.
• Modules development integrate the instructor and
  volunteer students who have been (or are)
  enrolled in the class. Modules may be implemented
  as a laboratory exercise or made available as a
  tutorial aid.
• Three modules have been developed so far
  Polarity of Bonds and Molecules Resonance
  (Delocalized Bonding) Molecular Orbital Theory

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• Methodology
• We have developed modules based upon students own
  experiences.
• Students decide on which methods are worth and
  brainstorm ideas with each other and the
  instructor on how to implement them.
• Each module is been tested by the developers,
  volunteer students and other instructors.
• After development each module will be tested in a
  small scale (25 students) before implementing it
  throughout the entire class (up to 350 students)
• Computational chemistry software utilized is
  ArgusLab 3.1 by Planaria-Software
  (www.planaria-sotware.com) inexpensive, easily
  accessible to student, user friendly and has a
  nice graphical user interface with surface
  display capabilities.

Illinois State University has licensed 18 copies
of ArgusLab, which are installed in a new
laboratory classroom.
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Description of a typical Module Modules are being
developed using an approach that involves three
steps Warm-up Introduces the student to a
calculation to familiarize him/her with job
commands, visualization tools and gathering
results. The modules rely on visualization of
chemical properties that are related to physical
properties such as electrostatic potentials,
molecular orbitals, etc. Playroom Students are
assigned to calculate properties in a series of
molecules. The are inquired about what they see
and how that relates to a given property
(resonance, molecular polarity, etc.). They are
also asked to tabulate data (dipole moments, bond
lengths, molecular energy, etc.) and correlate
these to the properties mapped on the
screen. Challenge Specific questions are posed
to rationalize other systems. Students are
encouraged to explore these using the software.
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• Polarity of Bonds and Molecules Module (A
  two-part module)
• Part 1-Polarity of Bonds To learn how the
  difference of electronegativity between two atoms
  influence the polarity of a chemical bond.
• Warm-up Student is guided by building,
  minimizing and visualizing H2. The following
  properties are calculated (PM3 level) Dipole
  moment, bond length, Mulliken atomic charges,
  electrostatic potential mapped on the electron
  density.
• This is what the student sees
• Warm-up questions
• Record the dipole moment. What does the dipole
  moment measures?
• Why is it zero for the H2 molecule?
• Record the atomic charges. Should they be equal?
  Why?
• Record the H-H Bond length

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• Playroom Students are asked to explore HF, HCl,
  HBr, and HI.
• This is what students would see
• Playroom Questions
• How is the electron density distributed in these
  different molecules?
• Based on your comparison of the electron density
  distributions, which molecule should have the
  most polar bond, and which one the least polar?
  Arrange the molecules in increasing order of
  polarity.

HF
HCl
HBr
HI
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• Students are asked to fill in the following
  table
• What trends do you see in the properties
  tabulated? Provide reasonable explanations for
  these trends
• What kind of correlations can be made from the
  properties tabulated and the electron density
  surfaces?
• Challenge Students are challenged to extend
  their acquired knowledge to other systems or to
  explain other results
• There seems to be a discrepancy between the
  experimental and calculated dipole moments.
  Provide a feasible explanation
• What do you expect for the following ions HO-,
  HN2-, HS-?

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• Part 2-Polarity of Molecules To understand that
  the polarity of a molecule is defined by the
  polarity of the bonds and the molecular geometry
• Warm-up Students are guided in building,
  optimizing and visualizing CH4. Same properties
  from bond polarity module are calculated (PM3).
  Students will see
• Warm-up questions
• Record the Dipole moment. What does the dipole
  moment measures for this molecule?
• Why is it zero for the CH4 molecule?
• Record the atomic charges.

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• Playroom Students are asked to explore CH3Cl,
  CH2Cl2, CHCl3, CCl4 (all tetrahedral).
• This is what students see
• Playroom Questions
• Describe how is the electron density distributed
  in these different molecules? Based on your
  comparison of the electron density distributions,
  which molecule(s) should be the most polar, and
  which one(s) the least polar?
• Arrange the molecules in increasing order of
  polarity.

CH3Cl
CHCl3
CCl4
CH2Cl2
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• Students are asked to fill in the following table
• What trends do you see in the properties
  tabulated? Provide reasonable explanations for
  these trends
• What kind of correlations can be made from the
  properties tabulated and the electron density
  surfaces?
• Challenge
• What would the polarity of a square planar
  CH2Cl2? Note that there could be two possible
  isomers. Would you be able to differentiate them
  based on polarity?

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• Resonance (Delocalized Bonding)
• To understand the concept of resonance based on a
  comparison of non-resonant and resonant Lewis
  Structures.
• Warm-up Student is guided by building,
  minimizing and visualizing O3 (resonant and
  non-resonant). The following properties are
  calculated (PM3 level) Molecular energy, bond
  lengths, Mulliken atomic charges, electrostatic
  potential mapped on the electron density.
• This is what the students see
• Warm-up questions
• What do you see? In which oxygen is the electron
  density higher, in which is lower? Does the
  surface correlate with the Mulliken charges?
• Why is the resonant structure different from the
  non-resonant structure?
• Which of the structures is the most stable
  (compare heats of formation)?
• What happened to the bond lengths and charges?

Non-resonant
Resonant
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Playroom Students are asked to explore benzene,
NO3-, nitrobenzene This is what students see
NO3-
Benzene
Nitrobenzene
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• Playroom Questions
• Students are asked to fill in the following table
• What can you conclude about resonance and the
  stability of molecules?
• Challenge
• Compare Naphtalene (C10H8) and 9,10-dihydronaphtal
  ene (C10H10). Is resonance expected in these
  compounds, why?

C10H8
C10H10
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• Molecular Orbital Theory
• To understand the concept of molecular orbital
  theory.
• Warm-up Student is guided on creating and
  visualizing the valence atomic orbitals of carbon
  and oxygen (EHT). They are also asked to get the
  energy of each orbital
• This is what the student sees
• Warm-up questions
• Arrange the atomic orbitals in order of
  increasing energy.
• What shapes do the orbitals have?
• Are there any orbitals with the same energy?

2s
2p (x, y, z)
carbon
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• Playroom Students are asked to build the
  Molecular orbitals of C2 and O2.
• This is what students see
• Playroom Questions
• Arrange the atomic and molecular orbitals in
  order of increasing energy.
• How many orbitals are per molecule?
• Can you distinguish the bonding from the
  antibonding MOs?
• Challenge
• Build MO energy diagrams for C2 and O2 using the
  atomic and molecular orbitals you generated.
  Determine their magnetic properties, spin
  multiplicities and bond orders.

s2s
p2px
p2py
s2p
s2s
p2px
p2py
s2p
C2
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• Conclusions (so far) and Future Work
• Students involved in the development of the
  modules gained a better understanding of the
  concepts they dealt with. They also became aware
  of other applications as they went into advanced
  courses.
• Some volunteer general chemistry students (not
  involved in development) had very positive
  experiences with the module they tried. They were
  also useful in suggesting some changes related to
  instructions.
• Other general chemistry instructors are
  evaluating one of the modules.
• Next stage is to test the polarity module in a
  small scale 15-25 students.
• Large scale testing (gt 100 students) is expected
  in the future.
• Modules will be available for implementation at
  other educational levels secondary, non-science
  majors.
• Development of modules to be used for organic
  chemistry, inorganic chemistry, physical
  chemistry, etc.

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• Acknowledgments
• Funding
• Illinois State University
• Faculty Technology Support Services (software)
• College of Arts and Science (hardware)
• Department of Chemistry (hardware)
• People
• Past General Chemistry Students Teresa
  Arrospide, Lynda Bugos, Kathy Elger
• Dr. Mark Thompson at Planaria-Software
• Dr. Pilar Mejia at Illinois State University
• The Honors program at Illinois State University