Strainless bond enthalpy

BOND ENTHALPY (BE) AND STRAINLESS BOND ENTHALPY (SBE) CONSTANTS AND SUMS... 

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. ... 

At the top of File Segment 5-1 is a heat of fomiation information block. Two sums are listed One is a sum of nomial bond enthalpies for ethylene, and the other is a sum selected from a parameter set of stiainless bonds. Both sets of bond enthalpies have been empirically chosen. A group of molecules selected as nomial generates one parameter set, and a group supposed to be strainless is selected to generate a second set of str ainless bond enthalpies designated SBE in Eile Segment 5-1. The subject of parameterization has been treated in detail in Chapter 4. See Computer Projects 3-6 and 3-7 for the specific problem of bond enthalpies. 

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. 

There are available measured enthalpies of formation of surprisingly few species and so attempts to find patterns are even more difficult here than before. Even if we consider the simpler quantity of the enthalpy of hydrogenation, we find that few polycyclic species qualify. To provide some semblance of conceptual unity, let us limit our attention to those polycyclic olefins in which the double bond being hydrogenated is found in a 5- or 6-membered ring. As such, we will compare the enthalpies of their hydrogenation with that of the normal, relatively strainless cyclopentene and cyclohexene, with 613 equalling 110.3 1.6 and 118.4 1.1 kJmol-1, respectively. 

---