In this section, you will learn how to obtain rotation barrier energy about Carbon-Carbon Bonds.

1. Use the guest account to log in the WebMO demo site.
Click New Job -> Open Editor. A small window opens where you build molecules.

2. Build C2H6 using the above procedure. Be sure to do the CleanUp as before.
Choose Tools > Zmatrix to open the Zmatrix Editor window. In this window, each atom is numbered. The corresponding number appears on the molecule sketch in the WebMOEditor window. Identify the two H atoms that form a dihedral angle(HCCH) of ~180 degree (or 180 degree). It may help to rotate the molecule. In the pulldown menu to the right of this listed dihedral angle, choose S (See next page for H7 and H3):

3. As shown above, enter the Start (180) and Stop (180) scan values, along with 72 steps, then click OK to close the ZMatrix Editor window.

4. Click Close Editor, then click the blue continue arrow. Choose Mopac.
Use the following Job Options: Job Name: C2H6PM3Angle, Calculation: Coordinate Scan, Theory: PM3, Charge: 0, Multiplicity: Singlet
Click the blue "continue" arrow. You should see the job in the queue. When the job is complete, click on the linked name. Scroll down to the Coordinate Scan table and click on the magnifying glass on the table title bar.

5. A Coordinate Scan window opens showing the plot of Energy vs. dihedral angle.
Choose Options -> Axes and in the Options window change the xaxis settings to Start (180), Stop (180), and Interval (60). Click OK.

6. Does the graph make sense? Describe in detail the relationship between the dihedral angle and energy you observe.

7. View a movie of the dihedral angle rotation by scrolling to the Geometry Sequence box. Choose Loop in the pull down menu, then click the "film" in the upper right corner of the box. You can use the controls to the left of the molecule to control things.