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6. Hold down the S key and select the bond
between the C(1) and C(2) atoms.
NOTE: Holding down the S key temporarily
activates the Select tool.
7. Select one of the dihedral rotators, then enter
57 in the text box and press the Enter key.
A nearly eclipsed conformation of ethane is
displayed.
TIP: To view this better, rotate the model on the Y
axis until the carbon atoms are aligned.
Use Mopac to create the precise eclipsed transition
state:
8. Holding down the S and shift keys, click on any
two nearly eclipsed hydrogen atoms, such as
H(4) and H(7), to identify the dihedral to track.
You should have a nearly coplanar four-atom
chain, such as H(4)-C(1)-C(2)-H(7), selected.
9. From the Structure menu, point to
Measurements, and choose Dihedral Angle.
The Measurements table appears and displays
an actual value for the selected dihedral angle
of about 3 degrees (this will vary slightly
between experiments).
10.From the MOPAC Interface submenu of the
Calculations menu, choose Optimize to
Transition State.
11.Click the Copy Measurements to
Messages box on the Job Type tab.
12.Click Run.
The ethane model minimizes so that the
dihedral is 0 degrees, corresponding to the
eclipsed conformation of ethane, a known
transition state between the staggered minima
conformations.
To see the Newman projection of the eclipsed
ethane model:
1. Select both carbon atoms.
2. From View Position submenu of the View
menu, click Align View Z-Axis With Selection.
NOTE: If you perform an Energy Minimization
from the same starting dihedral, your model would
optimize to the staggered conformation of ethane where
the dihedral is 60 degrees, instead of optimizing to the
transition state.
Computing Properties
To perform a single point calculation on the current
conformation of a model:
dihedral = 2.9224
dihedral = 3.1551
1. From the MOPAC Interface submenu of the
Calculations menu, choose Compute
Properties.
The Compute Properties dialog box appears.
2. On the Theory tab, choose a potential energy
function to use for performing the calculation.
NOTE: For more information about the potential energy
functions available in MOPAC see ‘Computation
Concepts”
180•MOPAC Computations
CambridgeSoft
Computing Properties