Chem3D 8.0 Manual - CambridgeSoft

dForceThe dForce field contains a constant used todecrease the bond stretching force constant of aparticular conjugated double bond. The forceconstant Kx for a bond with a calculated pi bondorder x is:K x = K 2 - (1 - x) * dForcewhere K 2 is the force constant for a nonconjugateddouble bond, taken from the BondStretching table.The higher the value of K x for the bond between twopi atoms, the more difficult it is to compress orstretch that bond.dLengthThe dLength field contains a constant used toincrease the bond length of any conjugated doublebond. The bond length lx for a bond with acalculated pi bond order x is:l x = l 2 + (1 - x) * dLengthwhere l 2 is the bond length of a non-conjugateddouble bond, taken from the Bond Stretchingtable. The higher the value of l x for the bondbetween two pi atoms, the longer that bond is.Record OrderWhen sorted for Bond Type, the order of the recordsin the Conjugated Pisystem Bonds table is asfollows:1. Records are sorted by the first atom typenumber in the Bond Type field. For example,the record for bond type 2-2 is listed before therecord for bond type 3-4.2. For records where the first atom type number isthe same, the records are sorted by the secondatom type number in the Bond Type field. Forexample, the record for bond type 2-2 is listedbefore the record for bond type 2-3.ElectronegativityAdjustmentsThe parameters contained in the ElectronegativityAdjustments table (ElectronegativityAdjustments.TBL) are used to adjust the optimalbond length between two atoms when one of theatoms is attached to a third atom which iselectronegative.For example, the carbon-carbon single bond lengthin ethane is different than that in ethanol. The MM2parameter set has only a single parameter forcarbon-carbon single bond lengths (1.523Å). Theuse of electronegativity correction parametersallows the C-C bond in ethanol to be corrected. Theelectronegativity parameter used in theElectronegativity Corrections table is the 1-1-6angle type, where atom type 1 is a C Alkane andatom type 6 is an O Alcohol. The value of thisparameter is -0.009Å. Thus the C-C bond length inethanol is 0.009Å shorter than the standard C-Cbond length.MM2 ConstantsThe MM2 Constants table (MM2 Constants.TBL)contains parameters which Chem3D uses tocompute the MM2 force field.AppendixChem3D 8Electronegativity Adjustments• A25