Group additivity scheme

Enthalpic increments for a group additivity scheme in thermochemistry of pyridine, quinoline, and their substitution products 99PAC1257. 

This useful and simple-to-use software package relies on Benson s group additivity scheme [47] to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics [32] and JANAF Tables [22]. The first version of... 

This book contains a small database for organic and inorganic compounds. Its main value, however, is that it describes a group additivity scheme to estimate thermochemical data. An updated and extended list of group parameters is given in NIST Therm [28]. The first edition of this classic work is from 1968. 

Estimated by Benson s group additivity scheme as the difference between the heats of formation of ethylene and of two —CH2—groups. See Table 2.6 p. 75. 

It is possible to compare direct measurements of relative stabilities of isomeric ions with comparisons of nonisomeric ions by use of a group additivity scheme. Group additivity schemes have been developed by Benson for heats of formation (and other thermodynamic properties) of organic molecules in the gas phase,40 and by Guthrie to represent free energies of formation in aqueous solution.38 In both cases, energies of unstrained hydrocarbons accurately correspond to a sum of contributions from primary, secondary, tertiary, and quaternary carbons CH3, CH2, CH, and C. 

Now, there is no satisfactory theory of steric effects, although attempts to rationalize the barrier of ethane by quantum-mechanical calculations are appearing more frequently (dementi and Davis, 1966). Furthermore, simple group-additivity schemes of various kinds have had limited success, at best, e.g. for estimating rotational barriers in ethanes (Tang and Chen, 1962), correlating relative reactivities with Taft EB values (Wells, 1963), or evaluating asymmetric induction (Ugi, 1965 Ruch and Ugi, 1966). Semi-empirical calculations by equation (197) have... 

Atomic and group additivity schemes, derived from solvent accessibility calculations and measurements on model systems, have been used to estimate the thermodynamics of hydration of proteins and peptides (Eisenberg and McLachlan, 1986 Ooi and Oobatake, 1988a Ooi et al., 1987). 

The study of the molar volumes of the normal hydrocarbons provided the earliest example of the additivity of group properties (Kopp 1855). The experimentally determined heats of formation for the same homologous series of molecules, CH3(CH2)mCH3, also obey a group additivity scheme (Franklin 1949 Prosen 1946 Benson 1968 Pittam and Pilcher 1972). It is possible to fit the experimental heats of formation for this series, beginning with m = 0, with the expression,... 

Group additivity schemes are based on the idea that molecular properties are the aggregate of contributions from identifiable groups, such as -CH3. 

A group of 32 alcohols was investigated by Spivakovskii et al. using a group additive scheme they reported a 14-variable relation for a QSAR equation. 

A group additivity scheme would thus provide the basis for differentiating between these two isomers. However, the groups of some octane isomers—e.g. 2-methylheptane,... 

In the remainder of this section we discuss the derivation of group values for heat capacities, enthalpies and entropies and provide several examples to illustrate their application. In Section V we will discuss refinements to this simple group additivity scheme. There, the aim is to see if, by a simple expansion beyond the four parameters required for group additivity, we can improve significantly upon its accuracy in predicting thermochemical properties. 

If a reaction is sufficiently well balanced, its enthalpy change will be close to zero. For reactions (7)-(10), the experimental enthalpies of formation imply gas-phase reaction enthalpies (298.15 K) of 5674 + 7, —518 + 7, 52 + 7, and 30 + 7kJmol respectively. All quantum calculations can be avoided by assuming the reaction enthalpy to be zero for a well-balanced reaction. This is the basis of empirical methods such as Benson s group-additivity scheme [5,6]. Where adequate auxiliary data are available, a group-balanced reaction is an excellent choice for quantum chemistry calculations [121]. 

In principle, the various methods used for crystal density prediction of neutral molecular crystals should be applicable to ionic systems. Indeed, group additivity schemes have been developed for ionic crystals [53,54]. Unfortunately, the ionic partners that make up the formula imit in the ionic crystal introduce complications in implementing some of the other methods. The main challenge is properly defining the arrangement of the ionic partners... 

Group inaanents for silicon compounds based on the data reviewed hae, and derived in accordance with the group additivity scheme , are shown in Table 27. Radical and atomic heats of formation for non-silicon-containing species used in this chapter to derive bond dissociation energies are shown in Table 28. Miscellaneous inorganic silane heats of formation are included in Table 29. 

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