Bond-separation reactions

Bond separation reactions such as (I) are examples of chemical changes in which there is a retention of the number of bonds of a given formal type, but with a change in their relationship to one another. Such reactions are often called isodesmic. 

A b initio calculations may be applied to isomerization reactions or "bond separation reactions in which the bond type persists and correlation energies are unlikely to alter. For example,... 

A decrease in the strain energy of (105) relative to (104) is also found in 6-31G //3-21G calculations (86ZSK151) respective values determined from the isodesmic bond separation reactions are 113.3 and 135.0 kcal/ mol. 

As will be discussed in Chapter 13, calculated energies of one particular class of isodesmic reactions, so-called bond separation reactions, may be combined with experimental or high-quality calculated thermochemical data in order to lead directly to accurate heats of formation. These in turn can be used in whatever types of thermochemical comparisons are of interest. We start our assessment of isodesmic processes with bond separation reactions. Following this, we consider description of bond dissociation energies, hydrogenation energies and acid and base strengths in terms of isodesmic processes, that is, not as absolute quantities but expressed relative to standard compounds. 

A bond separation reaction takes any molecule comprising three or more heavy (non-hydrogen) atoms into the set of simplest (two-heavy-atom) molecules containing the same bonds. The only requirement is that bonding must be defined in terms of a single valence (Lewis) structure or set of equivalent valence structures. This in turn guarantees that the bond separation energy is unique. ... 

In this case, it is necessary to (arbitrarily) choose one of the structures, leading to ambiguity in the definition of the bond separation reaction. 

Table 6-14 Mean Absolute Deviations from G3 Calculations in Energies of Bond Separation Reactions... 

BP, BLYP, EDFl and B3LYP density functional models all lead to significant improvements over both Hartree-Fock and local density models, at least in terms of mean absolute deviations. While most reactions are better described, there are exceptions. Most notable among these is the bond separation reaction for tetrachloromethane. All four models show a highly exothermic reaction in contrast with both G3 and experimental results which show a nearly thermoneutral reaction. Similar, but somewhat smaller, effects are seen for isobutane and trimethylamine. As was the case with Hartree-Fock calculations. 

Semi-empirical models are completely unsatisfactory in describing the energetics of bond separation reactions and should not be employed for this purpose. PM3 provides the best account of the three, but individual errors are often in excess of 10 kcal/mol. 

Table 12-5 Effect of Choice Geometry on Energies of Bond Separation Reactions, 6-31G Model... 

As previously described in Chapter 6, a bond separation reaction breaks down any molecule comprising three or more heavy (nonhydrogen) atoms, and which can be represented in terms of a classical valence structure, into the simplest set of two-heavy-atom molecules containing the same component bonds. For example, the bond separation reaction for methylhydrazine breaks the molecule into methylamine and hydrazine, the simplest molecules incorporating CN and NN single bonds, respectively. 

A bond separation reaction is uniquely defined. Therefore, a bond separation energy is a molecular property . Given that a bond separation reaction leads to products, the heats of formation of which are either known experimentally or can be determined from calculations, combining a calculated bond separation energy with experimental (or calculated) heats of formation, gives rise to a unique value for the heat of formation. For example, a heat of formation for methylhydrazine may be obtained from the thermochemical cycle. 

Here, AErx is the calculated energy of the bond separation reaction of methylhydrazine and AHf(NH3), AHf(CH3NH2) and AHf(NH2NH2) are experimental (or calculated) heats of formation. 

More precisely, a bond separation reaction is unique to a particular valence structure. The bond separation reaction for a molecule which can only be represented by multiple (nonidentical) valence stmctures may not be uniquely defined. 

Heats of formation obtained from bond separation reactions from Hartree-Fock, EDFl and B3LYP density functional and MP2 models all with 6-3IG and 6-311+G basis sets are compared to experimental values in Table 13-1. Errors in calculated quantities are exactly the same as those for the underlying bond separation reactions. ... 

Bond separation reactions and heats of formation obtained from bond separation energies suffer from two serious problems. The first is that bond types in reactants and products for some types of processes may not actually be the same or even similar . The bond separation reaction for benzene is an obvious example. Here to reactant (benzene) incorporates six equivalent aromatic carbon-carbon bonds, midway between single and double bonds, while the products (three ethanes and three ethylenes) incorporate three distinct carbon-carbon single bonds and three distinct carbon-carbon double bonds. 

The second problem is even more serious. The number of product molecules in a bond separation reaction increases with the size of the reactant, and (presumably) so too does the overall magnitude of error in the calculated bond separation energy. Whereas errors in bond separation energies (and in heats of formation derived from bond separation reactions) are close to acceptable limits ( 2 kcal/ mol) for small molecules (see discussion in Chapter 6), it is likely that will rapidly move outside of acceptable limits with increasing molecular size. 

There is no obvious best solution to these problems. One direction is to redefine (or generalize) the bond separation reaction such that the products are not restricted to the smallest (two-heavy-atom) molecules, but rather include molecules made up of larger components as well (functional groups, rings, etc.). For example, were the phenyl ring and the carboxylic acid functional group included as fragments , then the bond separation reaction for w-toluic acid could be written. 

This process, unlike the originaT bond separation reaction, )OoH... 

The idea of using calculated energies of bond separation reactions together with limited experimental thermochemical data to supply accurate heats of formation was proposed many years ago, but until recently was not practical. Original reference R. Ditchfield, W.J. Hehre, J.A. Pople and L. Radom, Chem. Phys. Ltrs., 5, 13 (1970). 

Table 13-1 Heats of Formation from Bond Separation Reactions (2)... 

---