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ChemOffice.Com - CambridgeSoft

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• The energy required to keep two electrons,<br />

each on separate pi atoms, from moving apart<br />

and<br />

• The energy required to keep two electrons,<br />

occupying the same orbital on the same pi<br />

atom, from moving apart.<br />

The units of the repulsion energy are electron<br />

volts (eV). The repulsion energy is more<br />

positive the more electronegative the atom.<br />

For example, an alkene carbon has an repulsion<br />

energy of 11.134 eV, and the more electronegative<br />

pyrrole nitrogen has an repulsion energy of 17.210<br />

eV.<br />

Pi Bonds<br />

The Pi Bonds table (Conjugated PI System<br />

Bonds.xml) contains parameters used to correct<br />

bond lengths and bond angles for bonds that are<br />

part of a pi system. In Chem3D, additional<br />

information is used to compute the pi system<br />

portions of the MM2 force field for the pi bonds in<br />

a model.<br />

There are five fields in records in the Pi Bonds<br />

table: Bond Type, dForce, dLength, Quality, and<br />

Reference.<br />

Bond Type<br />

The Bond Type field is described by the atom type<br />

numbers of the two bonded atoms.<br />

For example, bond type 2-2 is a bond between two<br />

alkene carbons.<br />

dForce<br />

The dForce field contains a constant used to<br />

decrease the bond stretching force constant of a<br />

particular conjugated double bond. The force<br />

constant Kx for a bond with a calculated pi bond<br />

order x is:<br />

K x = K 2 - (1 - x) * dForce<br />

where K 2 is the force constant for a nonconjugated<br />

double bond, taken from the Bond<br />

Stretching table.<br />

The higher the value of K x for the bond between<br />

two pi atoms, the more difficult it is to compress or<br />

stretch that bond.<br />

dLength<br />

The dLength field contains a constant used to<br />

increase the bond length of any conjugated double<br />

bond. The bond length lx for a bond with a<br />

calculated pi bond order x is:<br />

l x = l 2 + (1 - x) * dLength<br />

where l 2 is the bond length of a non-conjugated<br />

double bond, taken from the Bond Stretching<br />

table. The higher the value of l x for the bond<br />

between two pi atoms, the longer that bond is.<br />

Record Order<br />

When sorted for Bond Type, the order of the<br />

records in the Conjugated Pisystem Bonds table is<br />

as follows:<br />

1. Records are sorted by the first atom type<br />

number in the Bond Type field. For example,<br />

the record for bond type 2-2 is listed before the<br />

record for bond type 3-4.<br />

2. For records where the first atom type number<br />

is the same, the records are sorted by the<br />

second atom type number in the Bond Type<br />

field. For example, the record for bond type 2-<br />

2 is listed before the record for bond type 2-3.<br />

Electronegativity<br />

Adjustments<br />

The parameters contained in the Electronegativity<br />

Adjustments table (Electronegativity<br />

Adjustments.xml) are used to adjust the optimal<br />

Appendices<br />

<strong>ChemOffice</strong> 2005/Appendix Parameter Tables • 659<br />

Pi Bonds

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