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four membered rings, the parameters in the<br />

3-Membered Ring Angles.xml and 4-Membered<br />

Ring Angles.xml are used.<br />

The Angle Bending table contains the parameters<br />

used to determine the bond angles in your model.<br />

In Chem3D Pro, additional information is used to<br />

compute the angle bending portions of the MM2<br />

force field for the bond angles in your model.<br />

The 4-membered Ring Angles table contains the<br />

parameters that are needed to determine the bond<br />

angles in your model that are part of 4-membered<br />

rings. In Chem3D, additional information is used to<br />

compute the angle bending portions of the MM2<br />

force field for any bond angles in your model which<br />

occur in 4-membered rings.<br />

The 3-membered Ring Angles table contains the<br />

parameters that are needed to determine the bond<br />

angles in your model that are part of 3-membered<br />

rings. In Chem3D, additional information is used to<br />

compute the angle bending portions of the MM2<br />

force field for any bond angles in your model which<br />

occur in 3-membered rings.<br />

Each of the records in the Angle Bending table, the<br />

4-Membered Ring Angles table and the 3-<br />

Membered Ring Angles table consists of seven<br />

fields: Angle Type, KB, –XR2–, –XRH–, –XH2–,<br />

Quality, and Reference.<br />

Angle Type<br />

The first field, Angle Type, contains the atom type<br />

numbers of the three atoms which describe the<br />

bond angle.<br />

For example, angle type 1-2-1 is a bond angle<br />

formed by an alkane carbon bonded to an alkene<br />

carbon which is bonded to another alkane carbon.<br />

Notice that the alkene carbon is the central atom of<br />

the bond angle.<br />

KB<br />

The KB, or the angle bending constant, contains a<br />

measure of the amount of energy required to<br />

deform a particular bond angle. The larger the value<br />

of KB for a particular bond angle described by three<br />

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

that bond angle.<br />

–XR2–<br />

–XR2–, the third field, contains the optimal value<br />

of a bond angle where the central atom of that bond<br />

angle is not bonded to any hydrogen atoms. In the<br />

–XR2– notation, X represents the central atom of a<br />

bond angle and R represents any non-hydrogen<br />

atom bonded to X.<br />

For example, the optimal value of the 1-1-3 angle<br />

type for 2,2-dichloropropionic acid is the –XR2–<br />

bond angle of 107.8°, since the central carbon (C-2)<br />

has no attached hydrogen atoms.<br />

The optimal value of the 1-8-1 angle type for<br />

N,N,N-triethylamine is the –XR2– bond angle of<br />

107.7°, because the central nitrogen has no attached<br />

hydrogen atoms. Notice that the central nitrogen<br />

has a trigonal pyramidal geometry, thus one of the<br />

attached non-hydrogen atoms is a lone pair, the<br />

other non-hydrogen atom is a carbon.<br />

–XRH–<br />

The –XRH– field contains the optimal value of a<br />

bond angle where the central atom of that bond<br />

angle is also bonded to one hydrogen atom and one<br />

non-hydrogen atom. In the –XRH– notation, X<br />

and R are the same as –XR2–, and H represents a<br />

hydrogen atom bonded to X.<br />

For example, the optimal value of the 1-1-3 angle<br />

type for 2-chloropropionic acid is the –XRH– bond<br />

angle of 109.9°, since the central carbon (C-2) has<br />

one attached hydrogen atom. The optimal value of<br />

the 1-8-1 angle type for N,N-diethylamine is the –<br />

XRH– value of 107.7°, because the central N has<br />

Appendices<br />

<strong>ChemOffice</strong> 2005/Appendix Parameter Tables • 657<br />

Angle Bending, 4-Membered Ring Angle Bending, 3-Membered Ring Angle Bending

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