Electron-donor-acceptor-complexes

Background and Rational

Electronegativity is a chemical property that describes the ability of an atom (or, a functional group) to attract electrons (or electron density) towards itself in a covalent bond. In the development of valence bond theory, it has been shown to correlate with a number of other chemical properties.1-6 The most commonly used unit for electronegativity is a dimensionless quantity, commonly referred to as the Pauling scale, on a relative scale running from 0.7 to 4.0 (hydrogen = 2.2). In organic chemistry, electronegativity is also associated more with different functional groups than with individual atoms. The terms group electronegativity and substituent electronegativity are used synonymously. In this experiment the electronegativity of several atoms and functional groups are compared in the context of electron-donor-acceptor-complexes.

Noncovalent interactions are interactions between molecules that stabilize the association of two or more molecules, but are not new ionic or covalent bonds between the molecules. For example, hydrogen bonding is a type of noncovalent interactions that associate molecules by electrostatic forces. Noncovalent interactions involving aromatic rings play a major role in determining the stability of biological systems. In addition to their prominent role in DNA and RNA structures, aromatic stacking interactions have been observed in the complexes of medicinal drugs and the targeted enzymes. Several fundamental forces including: (1) electrostatic, (2) charge-transfer, (3) dispersion (or van der Waals), (4) ion-mediated, and (5) hydrophobic interactions contribute to noncovalent molecular interactions.

This experiment deals with the charge-transfer forces. An electron-donor-acceptor- complex (i.e., charge-transfer complex) is an association of two molecules, in which the attraction between the molecules is created by an electronic transition into an excited electronic state, such that a fraction of electronic charge is transferred between the molecules. The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor, and the receiving molecule is called the electron acceptor, hence the name, electron-donor-acceptor-complex. The nature of the attraction in an electron-donor-acceptor-complex is not a stable chemical bond and is much weaker than covalent forces, rather it is better characterized as a X X O O X X O O X X Y X X Y Electron donor Electron acceptor Complex (Y = donor group) (X = electron withdrawing group)

Figure 1. An illustration of a donor-acceptor complex. Benzoquinones with electron- withdrawing groups are common electron acceptors and benzene rings with electron-donating groups are usually electron donors.

weak electron resonance. As a result, the excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum. This produces the usually intense colors characteristic for these complexes. These optical absorption bands are often referred to as charge-transfer bands, or CT bands. Optical spectroscopy is a powerful technique to characterize charge-transfer bands. In this experiment, we will examine the electron-donor-acceptor-complex by visual inspection and by computational chemistry.

Based on frontier molecular orbital (FMO) theory, the interactions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are most important. A CT band is usually observed when the energy gap between the HOMO of the donor and the LUMO of the acceptor is relatively small. Electron-donating groups (EDG) raise the energy level of the HOMO and Electron-withdrawing groups (EWG) lower the energy level of the LUMO. Therefore, EDGs contribute to make a better electron-donor and EWGs contribute to provide a better electron-acceptor.

ct

LUMO

hυ ct HOMO

Figure 2. Molecular orbital diagram for a simple electron-donor-acceptor complex. LUMO is the unoccupied molecular orbital of the acceptor molecule and HOMO is the highest occupied molecular orbital of the donor molecule. In this experiment, the preparation of electron-donor-acceptor complexes uses a combinatorial approach. The structure variation is shown for each donor and acceptor molecules in Figure 3. Each aromatic ring can be diversified by substitution patterns. Each group is assigned a unique combination of the donor/acceptor pair on the basis of his/her position in the lab (see Figure 3). The entire class will perform the same experiment with a pair of students preparing two versions of a complex. The assay and the identification of individual complexes rely upon visual inspection and comparison of the results from other students following the mixing experiment. The color of the complex serves as the final assay for this experiment.

A1 B1 C1 D1

OMe O Cl CN 1 A Cl CN OMe O O B2 C2 D2 A2 Cl Cl 2 B OMe Cl Cl O OMe MeO O stock B3 C3 Cl Cl D3 -room 3 C OMe O OMe O MeO OMe H H D4 A4 B4 C4 4 H H D OMe O

Figure 3. Your hood position in the lab determines the donor and acceptor molecules to use for making the complex solution. Each student should compare the structure of the donor and acceptor with those used by other students in the lab. At the end of this experiment, the colors of the complexes from the entire class should be compared. Any conclusions from the correlation of the donor and acceptor structures to the colored complexes should be discussed in the lab report. In the laboratory, the positions of the lab hoods are labeled with a combination of letters (A-D) and numbers (1-4, see Figure 3). Different lab hood positions will be using different combinations of aromatic donors and benzoquinones. You need to first find out your hood position in the lab according to Figure 3. Based on your hood position, you will be able to decide which two reactants to use for this lab. Your combination of starting materials should be unique and should produce a unique complex (A1-D4, see Figure 3). A sample collection of the solutions from a previous version of this experiment is shown in Figure 4. Depending on the time of exposure to the air, some solutions may appear to have slightly different colors. Check with your lab instructor before you start the experimental procedures.

Figure 4. A display of the parallel mixing of donor and acceptor solutions. Recall that the donor acceptor complex is relatively weakly associated, compared to ionic or covalent bonds, so the donor acceptor complex formation might be represented by the following equilibrium equation:

O X X X X O O + X X X X Y O Y

Complex Acceptor Donor Increasing the concentration of the Electron Donor may form more of the donor acceptor complex as the equilibrium is shifted toward the product. So following your first investigation, you will repeat your same experiment with a ten times more concentrated solution of your assigned electron donor.

There are two parts to this experiment, the experimental part and the computational part. First you will perform the experimental part by following the two procedures below. After you have recorded the experimental results, you will be performing computations to calculate the lowest unoccupied molecular orbitals (LUMO) of the acceptor molecules.

Part 1. Experimental procedure:

Locate the unique combination of your bench position (i.e., A1, B2, etc.). Use a pipette to take approximately one ml of each of the stock dichloromethane solutions of the donor (conc. 10 mM) and the acceptor (conc. 10 mM) molecules according to your bench position and combine 7 the two solutions in a small Erlenmeyer flask. Complex formation should appear immediately upon mixing and may result in a solution color change or precipitation. Record your observations. It is possible that there will be no change for your specific combination. After you have completed the mixing of the acceptor and the donor solutions, proceed to record your experimental evidence of the donor acceptor complex formation. Report changes in your report (light yellow solution to dark purple solution, etc.) both in words and photographs.

Probe your charge-transfer complex formation further by mixing one ml of a 10 times more concentrated donor solution (conc 100 mM) with one ml of the acceptor (same 10 mM conc) in another small Erlenmeyer flask. Again, report changes in the color of the solution or precipitation. Compare these results to your previous results, and report the changes in both words and photographs.

A picture should be taken of the entire lab’s collection of solutions arranged in order of hood positions, see your laboratory instructor for coordination.

Warning • anisole and methoxybenzenes are irritants • benzoquinones are irritant. Avoid skin contact with any chemicals used in the lab.

Part 2. Computations using Gaussian programs via WebMO The WebMO computational software facilitates computational chemistry via the WWW. WebMO permits the access of Gaussian software packages, which are installed on the Miami High Performance Computers (HPC), over the WWW using a web browser. Computational chemistry jobs can be created, queued, run, viewed, downloaded, and deleted all within the context of the WebMO interface. The following instructions allow you to perform calculations.

1. Use the URL link

http://miamioh.edu/webmo

to navigate to the Web MO login page;

2. The first time you login, after entering your unique ID and password, you will be prompted to select a group. They should select chm255spring13 and enter the password chm255s13 when prompted;

3. From the Job Manager screen, using the pull-down menu

New Job --Create a New Job

to start a new window for building your molecule (Java must be enabled on your laptop)

4. In the Build Molecule screen, draw or use a template to place benzoquinone on the screen to be computed; Use the pull-down menu Clean-up --Add hydrogen

to correct carbon atoms' valences.

Use the pull-down menu

Clean-up--Mechanics--Optimize

to optimize your molecule before submission for Gaussian calculations.

5. Ignore the pop up warning about symmetry by click on OK;

6. Now your molecule is almost ready for calculations by the Gaussian program. Click the Job options button on the left panel;

Enter the following choices:

Job name: benzoquinone (or any other acceptors)

Calculation: Geometry Optimization

Theory: Hartree-Fock

Basis Set: Routine: 6-31G(d)

Charge: 0

Multiplicity: Singlet

7. On the left panel, click Submit Job.

8. You will be directed back to the Job Manager screen, where you should be able to see the status of your Gaussian calculation. It takes from several seconds to a few hours to optimize the geometry of the molecule at the HF/6-31G(d) level of theory depending on the size of the molecule.

9. Once the status indicates complete, click on the right hand side button Actions--View to display the results of the calculations.

10. The displayed molecule is now geometry optimized. You can use this geometry to calculate a number of different properties of the molecule. What we need for the donor- acceptor experiment are the molecular orbital energies. At the bottom center of the screen, click the button New Job using this geometry.

11. On the next screen, click Job options. Change the Geometry optimization to Molecular orbitals. Then click Submit job. This should only take less than a minute. 12. Once the status turns to complete, click on the right hand side button of Actions--View to display the results of the calculations. Scroll down pass the graphical display, pass the other results including partial charges section. You should see a section labeled Molecular Orbitals. Find the HOMO and LUMO and copy the energies to your notebook. The occupied orbitals have occupancy of 2 and the unoccupied orbitals have occupancy of 0. The listed energies are in the unit of Hartrees. Convert them to kcal using the conversion factor 1 Hartree = 627.51 kcal. Negative values indicate lower energies.

13. Repeat steps 1-12 for all four acceptor molecules.

Help is available Miami Redhawk cluster computer help: anyone who has questions can contact [email protected]. E-mail to this address will go to people involved in administrating the cluster computer.

Hand In Description of your experiment and the observation of your complex should be written carefully. A picture of the entire class’ solutions should be attached.

The calculated LUMO energies for acceptors should be compiled and included in your report.

Answer the following questions:

(1) Assuming one more different benzoquinone and one more donor molecule was used, how many different combinations of complexes could be obtained?

(2) Compare the LUMO energies for all acceptors. Discuss in your report any trend between substituents and LUMO energies.

(3) Assuming the HOMO energies of the donors are identical at -180 kcal/mol, compare the HOMO-LUMO energy gaps among donor-acceptor pairs. Discuss in your report any trend between the HOMO-LUMO energy gaps and the observed color. Discuss the possible reasons behind the differences in the color intensity by different benzoquinone molecules.

(4) Based on the experimental colors of the class results, suggest an order of HOMO energies for the donor molecules from lowest (most negative) to highest (least negative). (5) From the class results and the HOMO-LUMO energy gaps, suggest another organic compound that would be expected to form a colored charge transfer compound with any one of the acceptor molecules in this experiment. Give reasons for your choice of compound.

References

1. Jensen, W. B. Journal of Chemical Education 2003, 80, 279-287, "Electronegativity from Avogadro to Pauling: II. Late nineteenth- and early twentieth-century developments". 2. Drago, R. S.; Wong, N. M. Journal of Chemical Education 1996, 73, 123-129, "The role of electron-density transfer and electronegativity in understanding chemical reactivity and bonding". 3. Jensen, W. B. Journal of Chemical Education 1996, 73, 11-20, "Electronegativity from avogadro to Pauling .1. Origins of the electronegativity concept". 4. Cao, C. Z. Journal of Chemical Education 1993, 70, 25-28, "The Relation Between The Ionization-Potential And The Molecular Electronegativity Of Organic Homologs". 5. Liu, H.; Wang, Q. W.; Liu, L. X. Journal of Chemical Education 1992, 69, 783-784, "An Improvement On The Formula For Group Electronegativity". 6. Reed, J. L. Journal of Chemical Education 1992, 69, 785-790, "Electronegativity And Atomic Charge". 7. Hanko, R.; Hammond, M. D.; Fruchtmann, R.; Pfitzner, J.; Place, G. A. Journal of Medicinal Chemistry 1990, 33, 1163-70, "Design, synthesis, and 5-lipoxygenase- inhibiting properties of 1-thio-substituted butadienes".