Strainless bond energies

We can also calculate a strainless heat of formation of anfi-butane, which is the heat of formation of an anti structure made up of hypothetical strainless bonding interactions. As shown, we add the strainless bond increments to obtain the value of -32.92 kcal/mol for the strainless bond enthalpy (SBE). This value must also be corrected for the T / R term, but the POP and TOR terms are again taken to be zero. Thus, the strainless AH°f (hfs) is the sum of —32.92 and + 2.40, which is —30.52 kcal/mol. That differs from the heat of formation calculated with normal bond increments (—30.51 kcal/mol) by 0.01 kcal/mol, which is the inherent strain energy of anti-butane. ... 

I. A property present in a molecular entity (or a transition state) if the energy of that entity or state is enhanced due to unfavorable bond lengths, bond angles, or dihedral angles relative to some appropriate standard. It is the standard enthalpy of a structure relative to a strainless structure (real or hypothetical). 2. The change of volume or shape of a body, or portion of a body, due to the influence of one or more applied forces. 

Strain energies (SE) are obtained as difference of heats of formation between the olefins indicated and the sum of bond increments for their strainless counterparts. 

For details on the calculation of heats of formation from molecular mechanics, molecular orbital theory, or bond/group increments, see the discussion in Allinger, N. L. Schmitz, L. R. Motoc, 1. Bender, C. Labanowski, J. K. /. Am. Chem. Soc. 1992,114,2880. The strain energy of a conformation can be calculated from values of heats of formation from these sources also. The relationships in Figure 3.39 among heat of formation, T/R, strainless increments, and strain energy hold no matter what the source of the heat of formation value. 

The strainless standard for alkanes (MM4 and earlier) is defined by five parameters. These are the C-C and C-H bonds and three branching parameters. Me, Iso, and Neo, where the methylene group is a default quantity (with an energy value of zero). Methane is also given a strain energy of zero. 

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