Unformatted text preview: ation agree with the
experimental result of 45 degrees?
If four methyl groups are placed in the ortho positions on each of the phenyl rings, how do you expect the
equilibrium torsional angle to be affected? Explain, and then verify with a calculation: build the tetramethyl
substituted compound and perform a calculation of its equilibrium geometry using Avogadro and the MMFF94 force
field (check for convergence in the same way you did for biphenyl). Report the equilibrium torsional angle and
compare with your expectations. PART C ( 11 p oints) - D etermination o f F orce F ield P arameters In this part of the assignment, you will vary the bond length and bond angle of a simple molecule and use the
energies to extract the force constants for bond stretching and angle bending for the MMFF94 force field.
Determination o f S tretching F orce C onstant
1. Build and determine the equilibrium geometry of the water molecule using the MMFF94 force field. Measure
and record the total energy in kcal/mol and the equilibrium bond length in Å.
2. The next step is to vary the O-H bond distance for one of the bonds from 0.85 to 1.15 Å in 7 steps (0.05 Å
increments). To do this, follow the instructions in Appendix 4 to use the Bond Manipulation tool,
adjust the bond length of one of the O-H bonds to each of the suggested values (the other O-H bond distance and
bond angle should not be changed). Then, compute single point energies for each bond length and record these
values in kJ/mol (see Appendix 3 for instructions on computing single point energies); these energies must be
converted into kcal/mol in step 3. 3. Make two graphs of your results using Microsoft Excel or another spreadsheet package. The first graph should
include the energy on the y-axis in kcal/mol and the bond displacement on the x-axis. The bond displacement
is defined as the difference between the bond length and its equilibrium value; the bond displacement is negative
when the bond is compressed and positive when the bond is stretched.
To determine the stretching force constant for the O-H bond, a second graph should be created with the energy in
kcal/mol again placed on the y-axis, but this time the s...
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