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• The Stretch-Bend term represents the energy<br />

required to stretch the two bonds involved in a<br />

bond angle when that bond angle is severely<br />

compressed.<br />

• The Torsion term represents the energy<br />

associated with deforming torsional angles in<br />

the molecule from their ideal values.<br />

• The Non-1,4 van der Waals term represents<br />

the energy for the through-space interaction<br />

between pairs of atoms that are separated by<br />

more than three atoms.<br />

For example, in trans-2-butene, the Non-1,4 van der<br />

Waals energy term includes the energy for the<br />

interaction of a hydrogen atom bonded to C(1) with<br />

a hydrogen atom bonded to C(4).<br />

The 1,4 van der Waals term represents the energy<br />

for the through-space interaction of atoms<br />

separated by two atoms.<br />

For example, in trans-2-butene, the 1,4 van der<br />

Waals energy term includes the energy for the<br />

interaction of a hydrogen atom bonded to C(1) with<br />

a hydrogen atom bonded to C(2).<br />

The dipole/dipole steric energy represents the<br />

energy associated with the interaction of bond<br />

dipoles.<br />

For example, in trans-2-butene, the Dipole/Dipole<br />

term includes the energy for the interaction of the<br />

two C Alkane/C Alkene bond dipoles.<br />

To build a cis-2-butene and compute properties:<br />

1. From the Edit menu, choose Clear to delete<br />

the model.<br />

2. Double-click in the model window.<br />

A text box appears.<br />

3. Type cis-2-butene and press the Enter key.<br />

A molecule of cis-2-butene appears in the<br />

model window.<br />

4. From the MM2 submenu of the Calculations<br />

menu, choose <strong>Com</strong>pute Properties.<br />

The steric energy terms for cis-2-butene<br />

appears in the Output window.<br />

Below is a comparison of the steric energy<br />

components for cis-2-butene and trans-2-butene.<br />

NOTE: The values of the energy terms shown here are<br />

approximate and can vary slightly based on the type of<br />

processor used to calculate them.<br />

Energy Term<br />

trans-2-<br />

butene<br />

cis-2-<br />

butene<br />

Stretch: 0.0627 0.0839<br />

Bend: 0.2638 1.3235<br />

Stretch-Bend: 0.0163 0.0435<br />

Torsion: -1.4369 -1.5366<br />

Non-1,4 van der Waals: -0.0193 0.3794<br />

1,4 van der Waals: 1.1742 1.1621<br />

Dipole/Dipole: 0.0767 0.1032<br />

Total: 0.137 1.5512<br />

The significant differences between the steric<br />

energy terms for cis and trans-2-butene are in the<br />

Bend and Non-1,4 van der Waals steric energy<br />

terms. The Bend term is much higher in cis-2-<br />

butene because the C(1)-C(2)-C(3) and the C(2)-<br />

C(3)-C(4) bond angles had to be deformed from<br />

their optimal value of 122.0° to 127.4° to relieve<br />

some of the steric crowding from the interaction of<br />

hydrogens on C(1) and C(4). The interaction of<br />

hydrogens on C(1) and C(4) of trans-2-butene is<br />

much less intense, thus the C(1)-C(2)-C(3) and the<br />

<strong>ChemOffice</strong> 2005/Chem3D MM2 and MM3 <strong>Com</strong>putations • 159<br />

<strong>Com</strong>pute Properties

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