Group additivity rules

It should be recognized, however, that although bond and group additivity rules represent useful tools when applicable, they are restricted to molecules built from standard bonds and groups. Consequently, when new molecules are encountered, for which available bond and group values do not apply, their thermochemistry must be determined either experimentally or via computational quantum mechanics. 

Estimation of Thermochemistry by the Group Additivity Rules I-C HbCCHjCHjCHCHj]... 

In summary, bond and group additivity rules, as well as the model compound approach, in conjunction with statistical mechanics, represent useful tools for the estimation of thermochemical properties. However, their utility for the determination of thermochemistry of new classes of compounds is limited, especially with regard to the determination of Aiff. For new classes of compounds, we must resort to experiments, as well as to computational quantum mechanical methods. 

Alternatively, for many classes of compounds the heats of formation can be estimated through additivity of bond properties or group additivity rules [32], Let s take a simple example using the additivity of bond properties to estimate the heat of formation of some species A. Suppose we know (1) the heat of formation of a related compound ABR, where B is the atom to which A is bonded and R is the rest of the molecule, (2) the heat of formation of BR, and (3) that for a series of other molecules in which a A-B bond occurs the A-B bond-dissociation-energy is nearly constant, and we assign it the value B.D.E.(A-B) J. Now consider the endothermic reaction... 

As examples we can estimate the AH/ for the following radicals CH3CH2CH2 = 21, CICH2CH2 = 20, HOCH2CH2CH2 = —15, etc. Because of the lack of sufficient experimental data, it is not yet possible to estimate the reliability of these group additivity rules for free radicals. 

AH° and AS° may be calculated from values of H[ and S° listed in various books , or calculated from Atom or Group Additivity Rules . ... 

The molybdenum-alkyl mean bond dissociation enthalpies are in the expected order and in good agreement e.g. with the trend reported for Th(Cp ) R complexes (Cp = rj -C Me ) (9a). On the other hand, it is interesting to note that the group additivity rule seems to apply to the Mo-alkyl family, as shown by the excellent linear relationships 7 and 8, where N is the number of carbon atoms in the alkyl chain. 

Establishing values for bond strengths in the alkanes is equivalent to determining the heats of formation of the various alkyl radicals for this reason we consider also radicals in a chapter that should be dealing exclusively with alkanes. In order to apply group additivity rules to alkyl radicals, seven new groups have to be defined and assigned enthalpy values ... 

Empirical group additivity rules have proved to be effective predictors of chemical shifts for many classes of compounds. Extensive spectral tables are available in many texts and are very useful for structure identification provided representative model compounds are available (e.g., tripeptides for proteins, additivity rules for in organics). The ad hoc explanations that accompany such assignments often have little basis in reality. For accurate interpretation of chemical shift trends, the individual components of the chemical shift tensor must be considered. Molecular orbital calculations on systems of up to... 

These results thus show that whereas the flashpoint was only moderately influenced by the compound structure (their chemical functionality but especially their atomic composition and vapour), autoignition temperatures seem to be closely linked to the structural factors that affect the chain. So additivity rules for estimation of AIT should be sought. Every time a chemical or physical property is highly influenced by the structure, chemists tried to establish rules that enable one to reduce a molecule to characteristic groups for which the contribution to the value of this property is known. This was done for instance by Kinney for boiling points and Benson2 for thermochemical properties. 

By default, Permute would generate all possible combinations of protonation states at all of these functional groups when contained in a molecule. A number of additional rules are used to prevent unreasonable combinations and combinatorial explosion ... 

Table 13.3 Additional rules for discovery (nonGLP) PK assays requiring QC samples (level III). Adapted from [6], with permission from Taylor and Francis Group.

The purpose of the present work is to show how global optimization of a wavefunction of group-function form can be achieved and to discuss some of the general implications of separability, particularly those that relate to so-caUed additivity rules . A second aim is to sketch a generalization of the... 

Values based on rule of group additivity of thermochemical properties, using an assumed value, Hf0, for O—(6)2 group of 19 2 kcal., and further assuming that in the molecular series H—OnH, DH°(H—OnH) = 90 2kcal. for n > 2. 

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