Chem3D 8.0 Manual - CambridgeSoft

AdministratorThe Total Steric Energy for the conformation is0.8180 kcal/mol. The 1,4 VDW term of 0.6768dominates the steric energy. This term is due to theH-H repulsion contribution.To view the value of one of the dihedral angles thatcontributes to the 1,4 VDW contribution:1. Select the atoms making up the dihedral angleas shown below by Shift-clicking H(7), C(2),C(1), and H(4) in that order.To force a minimization to converge on thetransition conformation to determine the barrier torotation:2. From the Object menu, choose Set DihedralAngle.The following measurement appears.1. In the Measurements table, type 0 in theOptimal column for the selected dihedral angle.2. From the MM2 menu, choose Minimize.The Minimize Energy dialog box appears.3. Click Run.The 60 degree dihedral represents the lowestenergy conformation for the ethane model.If the Rotation Bars are not visible, display them asfollows:1. From the Tools menu, choose Show RotationBars.2. Reorient the model by dragging the Y-axisrotation bar until you have an end-on view.Entering a value in the Optimal column imposes aconstraint on the minimization routine. You areincreasing the force constant for the torsional termin the steric energy calculation so that you canoptimize to the transition state.When the minimization is complete, the reportedenergy values are as follows. The energy for thiseclipsed conformation is higher relative to the142 • Chapter 10: MM2 Computations CambridgeSoftMinimize Energy