Chem3D Users Manual - CambridgeSoft

Keyword
Description
7. On the Property tab, Ctrl+click Heat of
Formation and COSMO Solvation.
Administrator
GEO-OK
MMOK
Automatically sent to MOPAC
to override checking of the
Z-matrix.
Automatically sent to MOPAC
to specify Molecular Mechanics
correction for amide bonds. Use
the additional keyword NOMM
to turn this keyword off.
8. Click Run.
The results appear in the Messages window.
9. From the MOPAC Interface submenu of the
Calculations menu, choose Minimize
Energy.
10. On the Property tab, deselect COSMO
Solvation.
11. Click Run.
The results appear in the Messages window.
Example 5
Comparing the Stability of
Glycine Zwitterion in Water
and Gas Phase
To compare stabilities:
1. From the File menu, choose New Model.
2. Click the Text Building tool.
3. Click in the model window.
A text box appears.
4. Type HGlyOH and press the Enter key.
A model of glycine appears.
5. From the MOPAC Interface submenu of the
Calculations menu, choose Minimize
Energy.
6. On the Theory tab, choose PM3.
To create the zwitterionic form:
1. Click the Text Building tool.
2. Click the nitrogen, type “+”, then press the
Enter key.
3. Click the oxygen atom, type “-”, then press the
Enter key.
The glycine zwitterion is formed.
4. Perform a minimization with and without the
COSMO solvation property selected as
performed for the glycine model.
The following table summarizes the results of the
four analyses.
Form of
glycine
∆H
(kcal/mole)
Solvent
Accessible
Surface Å 2
neutral (H2O) -108.32861 52.36067
zwitterion (H2O) -126.93974 52.37133
194•MOPAC Computations
CambridgeSoft
Computing Properties