1

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

2

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

3

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.

4

Description of a typical ModuleModules 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.

5

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

6

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

7

•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-?

Property H-H H-F H-Cl H-Br H-I Calculated dipole (D) 0.00 1.41 1.67 1.27 0.97

Experimental Dipole (D) 0.00 1.82 1.08 0.82 0.44 Calculated Bond Length (Å) 0.699 0.94 1.26 1.47 1.68

Experimental Bond Length (Å) 0.74 0.92 1.27 1.41 1.61 Atomic Charge H 0.000 0.18 0.24 0.14 0.11

Atomic Charge halogen ---- -0.18 -0.24 -0.14 -0.11

Electronegativity difference 0.00 1.9 0.9 0.7 0.4

8

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.

9

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 CH2Cl2CHCl3 CCl4

10

•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?

Property CH4 CH3Cl CH2Cl2 CHCl3 CCl4 Calculated dipole (D) 0.00 1.70 1.68 1.26 0.00

Atomic Charge C 0.00 -0.21 -0.10 -0.03 0.03 Atomic Charge H 0.000 0.11 0.13 0.16 --- Atomic Charge Cl ---- -0.13 -0.08 -0.04 -0.01

11

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

12

Playroom: Students are asked to explore benzene, NO3-, nitrobenzene

This is what students see:

Benzene NO3-

Nitrobenzene

13

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-dihydronaphtalene (C10H10). Is

resonance expected in these compounds, why?

C10H8 C10H10

Structure Energy (kcal/mol)

Mulliken Charges in C

Mulliken Charges in N

Mulliken Charges in O

O3 – resonant 57.7 ---- ---- -0.26; 0.52; -0.26

O3 – non res. 47.8 ---- ---- -0.30; 0.48; -0.18

C6H6 – res. 53.6 -0.19 (all) ---- ----

C6H6 – non res.

33.6 -0.19 (all) ---- ----

(NO3)- -- res -74.6 ---- 1.30 -0.77

(NO3)- -- non

res -54.0 ---- 1.22 -0.69; -0.76; -0.76

C6H5NO2 – res.

56.0 -0.09; -0.37; -0.09; -0.22; -0.12; -0.22

1.08 -0.54

C6H5NO2 – non res.

45.8 -0.05; -0.40; -0.13; -0.23; -0.13; -0.20

1.17 -0.51; -0.61

14

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 orbitalThis 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

15

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.

2s

2s

2px

2px

2py

2py

2p

2p

C2

16

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 (> 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.

17

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