Experiment: Contour Plots for Molecular Orbitals


    The JAVA applet below plots the cross section of the relative electron density cloud for several localized molecular orbitals, ymo . A localized molecular orbital is the linear combination of two atomic orbitals,yn, l, m l . Each of the bonding atoms contributes one atomic orbital to the molecular orbital. An example of a molecular orbital for the hydrogen molecule, HaHb, where the subscripts a and b are labels used to distinguish the hydrogen atoms is

ymo    =     y1, 0, 0 (Ha )     +     y1, 0, 0 (Hb )     =     1sHa     +     1sHb

The electron density, (ymo )2, at any set of coordinates is related to the probably of finding an electron in a unit volume of space at those coordinates. The relative electron density at a set of coordinates is the electron density at those coordinates divided by the maximun electron density calculated for the plot. The contour plot is a two dimensional display of the regions where the values of the relative electron density lie within a specific range. These regions are marked with symbols that indicate the range of relative electon density.

Symbol Range of Relative Electron Density
$ 1.00 - 0.50
+ 0.50 - 0.25
0 0.25 - 0.10
/ 0.10 - 0.02
* 0.02 - 0.01

    To obtain a contour plot of one of the molecular orbitals listed below, enter the effective nuclear charges for atoms a and b. These are the nuclear charges "seen" by electrons in the atomic orbitals that will linearly combine to form the molecular orbital. Enter the internuclear distance and click the button of the molecular orbital that you wish to plot. Use the Effective Nuclear Charge Calculator to calculate the effective nuclear charge "seen" by the electron. Wait for the caption to appear above the contour plot before you move the page or begin a new calculation.

    Use the applet in the design and performance of experiments to answer the following questions.

  1. Construct contour plots for the 2pz + 2pz molecular orbital on N2 at internuclear distances of 2.40, 1.70, and 1.10 Å. How and why does the electron density between the two nitrogen nuclei change with a change in the internuclear distance?
  2. Construct contour plots for the 2pz - 2pz molecular orbital on N2 at internuclear distances of 2.40, 1.70, and 1.10 Å. How and why does the electron density between the two nitrogen nuclei change with a change in the internuclear distance?
  3. Which of the molecular orbitals listed below are sigma (s) bonding molecular orbitals?
  4. Which of the molecular orbitals listed below are sigma star (s*) antibonding molecular orbitals?
  5. Which of the molecular orbitals listed below are pi (p) bonding molecular orbitals?
  6. Which of the molecular orbitals listed below are pi star (p*) antibonding molecular orbitals?
  7. Construct a contour plot for the 2pz + 2pz molecular orbital on CO with r = 1.13 Å. How and why does the contour plot for the 2pz + 2pz molecular orbital on CO differ from the contour plot of the 2pz + 2pz molecular orbital on N2?

K. Jeffrey Johnson 73




Dr. Nutt's CHE 115 Course