Charge Distributions

Abstract:

        The unsymmetrical distribution of charge within a molecule gives rise to the experimental property called dipole moment (m).  Polar molecules have permanent dipole moments, and nonpolar molecules in the presence of external fields can acquire induced dipole moments.  The nonlinear optical properties of certain conjugated organic molecules with electron donating and accepting groups have exhibited very interesting chemical behavior due to the redistribution of charge upon excitation.  The effect of donor and acceptor strength on the electronic spectra and related properties for several substituted benzene compounds was investigated in this exercise.  Changes in charge distribution by variations in position using the same substituents was also explored.  The molecular properties of interest are the energy (wavelength), intensity of the transition (oscillator strength), and the change in dipole moment upon excitation.
 

Introduction:

        The purpose of this lab was to calculate the dipole moments of different molecules using PM3, CNDO, and AM1 semi-empirical methods as well as the vector addition method.  A log file was used to determine the wavelength, oscillator strength, and State Dipole moments for several molecules with electron donating and accepting groups.  Molecular mechanics (MM+) and semi-empirical methods AM1 and ZINDO/S were used to generate the log file.
 

Experimental Method:

Experiment 37
        Each molecule (HCl, chlorobenzene, acetonitrile, and dimethyl ether) was initially built and optimized with molecular mechanics (MM+).  Next, the dipole (m) was calculated for each molecule by using PM3, CNDO, and AM1 semi-empirical methods.  The root mean square (RMS) deviation was also computed for each of these semi-empirical methods.  The second and third set of molecules (o-dichlorobenzene, m-dichlorobenzene, chloromethane, and dichloromethane), (dimethyl ether, diethyl ether, and tetrahydrofuran) were built and geometry optimized using MM+ and the PM3 semi-empirical method.  The angle and dipole moment were measured for each molecule; the PM3 calculated dipole moment for each molecule was compared to the dipole moment calculated using the vector addition method.

Experiment 40
        The following molecules (aniline, p-nitroaniline, m-nitroaniline, p-nitromethoxy benzene, m-nitromethoxy benzene, m-nitrophenyl acetylene, and p-nitrophenyl acetylene) were built and geometry optimized using MM+ and AM1 methods.  Then, a log file was started; a semi-empirical ZINDO/S single point calculation was run on each molecule.  The ZINDO/S single point calculation included the Configurations interaction and the singly excited method (the orbital criterion was set to 3 for both the HOMO and LUMO).  After the calculation on each molecule was completed, the log file was opened and the wavelength, oscillator strength, and Sate Dipole moments were extracted for the most intense interactions.  From the tabulated data, the charge-transfer band and associated states were determined for each molecule.
 

Results and Discussion:

Experiment 37

A table summarizing this experiment's results is listed at the end.

        The PM3 semi-empirical method tended to give the lowest dipole moment, followed by the AM1 and the CNDO methods.  The AM1 semi-empirical method had the largest method root mean square deviation followed by the CNDO and PM3 methods.

o-Dichlorobenzene
 

m-Dichlorobenzene
 

Chloromethane
 

Dichloromethane
 
 

Summary

        According to a set of quantum mechanics calculations, chloromethane had the highest dipole moment of l.37339 debye.  Dichloromethane came in second at 1.36339 debye, o-dichlorobenzene had the third lowest at 1.35081 debye, and m-dichlorobenzene came in last at 0.87987 debye.

Dimethyl Ether
 

Diethyl Ether
 

Tetrahydrofuran

Summary

        The previous three compounds were expected to have approximately the same dipole moment due to two polar C-O bonds.  According to a semi-empirical PM3 calculation, all three molecules have different sized dipole moments.  Dimethyl ether has a dipole of 1.2549 debye, and diethyl ether has a dipole of 1.1498 debye.  Finally, tetrahydrofuran has a dipole moment of 1.6551 debye.

Experiment 40

A table summarizing this experiment's results is listed at the end.
 
 

        Aniline has a fairly small Dm for both transitions, but p-nitroaniline seems to have a very lage Dm in the 26 ® 27  transition.  The slightly different m-nitroalinine molecule also has a big Dm at the 26 ® 27 transition. Consider p-nitromethoxy benzene.  It also contains a large Dm at the 29 ® 30 transition.  A similar isomer, m-nitromethoxy benzene, has a large Dm transition state (28 ® 30).  Now, let's look at Dm regarding m-nitrophenyl acetylene; it has a large Dm at the 26 ® 28 transition.  Finally, p-nitrophenyl acetylene has a large Dm at the 27 ® 28 state of transition.

    Marder, S. R.; Beratan, N. N.; Cheng, L. -T. Science. 1991, 252, 103.

    McClellan, A. L. Tables of Experimental Dipole Movements. Freeman: San Fransisco, 1963.

    Shoemaker, D. P.; Garland, C. W. Experiments in Physical Chemistry, 5th ed.; McGraw-Hill: New York, 1988.

    Sinha, H. K.; Yates, K. Journal of the American Chemical Society. 1991, 113, 6062-6067.

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