Group increments

Heats of formation can be estimated with reasonable accuracy by additivity of group increments and corrections for ring effects. 

Calculation of group increments for oxygen, sulfur and nitrogen compounds has allowed the estimation of conventional ring-strain energies (CRSE) for saturated heterocycles from enthalpies of formation. For 1,3-dioxolane, CRSE is about 20 kJ mol . In 2,4-dialkyl-l,3-dioxolanes the cis form is always thermodynamically the more stable by approximately 1 kJ mol" . 

TABLE 2-383 Group Increments for the Ambrose Method Concluded)... 

If the thermodynamic data for a compound of interest have not been determined and abulated, it may be possible to estimate AHf or AGj from tabulated data pertaining to dividual structural units. Procedures have been developed for estimating thermodynamic characteristics of hydrocarbons and derivatives by summing the contributions of the constituent groups. The group increments are derived from experimental thermochemical iata and therefore depend on the existence of reliable data for the class of compounds of merest. 

The volume and surface of 3-8 were calculated, and were found to agree with those obtained from sums over group increments within 1%. Thus, it can be stated that the group increments in Table 14 are transferable quantities. 

Schafer, L., C. Van Alsenoy, and J. N. Scarsdale. 1982. Estimates for Systematic Empirical Corrections of Consistent 4-21G Ab Initio Geometries and Their Correlations to Total Energy Group Increments. J. Mol. Struct. (Theochem) 86, 349-364. 

Schmitz et al. <2001JP090> developed two group increment schemes for converting HF/6-31G(d) and B3LYP/6-31G(d) calculated energies of aliphatic amines to estimations of heats of formation. Application to the pyrrolizidine yielded calculated values of —1.09 (HF) and —1.02 (B3LYP) instead of — 0.93 kcal mob1 (experimental). 

By contrast, we do not use Benson group increments , a generally powerful thermochemical technique summarized in the volume by S. W. Benson himself, Thermochemical Kinetics, 2nd edition, Wiley, New York, 1976, and used in many thermochemical chapters throughout the Patai series. For the classes of compounds discussed in the current chapter, we believe the necessary number of parameters (included to reflect electrostatic interactions, proximity effects, steric repulsions and ring corrections) is excessive. 

It is shown that the solubility of fullerenes in vegetable oils can be predicted and justified on the basis of the solubility parameters of C60 and C70 and of the glycerol esters of fatty acids. A detailed procedure for the calculation of the solubility parameters of fullerenes and vegetable oils by group increment is reported. 

Keywords Adducts, C60, C70, esters of fatty acid, excipients, fatty acids, fullerenes, glycerol esters of fatty acids, grafting, group increment method, solubility, solubility parameter, vehicles for drug delivery, vegetable oils, triglycerides... 

It is interesting to note that such molar volumes can be calculated also by group increments using the van der Waals molar volume for a carbon atom, which is 6.95 ml/mol (Van Krevelen, 1990). 

Knowing the fullerene molar volume, either the experimental or the calculated value, it is quite straightforward the calculation of the solubility parameter by substituting the tabulated [Pg.321]

Being a mixture of different fatty acids esters of glycerol, the calculation with group increment of the solubility parameter of vegetable oils presents some... 

A theoretical justification of the surprising solubility of C60 and C70 fullerenes in vegetable oils has been proposed in terms of solubility parameter of fullerenes and vegetable oils. It has been shown that the solubility parameter of C60 and C70 fullerenes can be calculated by a group increment approach following Van Krevelen... 

The acid-catalyzed ester hydrolysis provides a good target for MM treatments. DeTar first used hydrocarbon models in which an ester was approximated by an isoalkane (74) and the intermediate (75) by a neoalkane (76). He assumed that if the rate of reaction truly is not influenced by polar effects but is governed only by steric effects of R, as has been generally postulated, the rate must be proportional to the energy difference (AAH ) between 74 and 76. The AAH f is mainly determined by the van der Waals strain in these branched alkanes. Nonsteric group increment terms were carefully adjusted, and statistical mechanical corrections for conformer populations... 

Carbon-13 shifts of some 2-substituted 1,3-dithianes as protected carbonyl derivatives and nucleophilic carbonyl equivalents are collected in Table 4.63 [413], Methyl group increments for shift predictions according to eq. (4.1) are available, also taking contributions of the twist in addition to the chair conformation into account [413]. 

Group increments for silicon compounds based on the data reviewed here, and derived in accordance with the group additivity scheme29-31, 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. 

TABLE 27. Group increment contributions to standard enthalpies of formation (A//f°/kJ mol-1) for organosilicon compounds... 

An exception is the calculation of formation enthalpies for organic compounds based on group increments and strain energies 131. 

Despite these structural features, the adamantane ring system is not strain-free 142). Comparison of an estimated heat of formation for adamantane based on group increments derived from acyclic alkanes in completely skew-free conformations 142> with the experimentally determined value 143 indicates that adamantane is strained to the extent of 6.48 kcal/mole. 

A major difference with desired reactions is that the stoichiometry is often unknown, that is, the decomposition products are unknown. The reason is that decomposition reactions are often affected by the triggering conditions and thus often run along different reaction paths. This is a major difference compared to a total combustion, for example. The consequence is that the decomposition enthalpy cannot be predicted using standard enthalpies of formation AHjj taken from, for example, tables or estimated by group increment methods, such as Benson groups [3, 4] ... 

A well-known tool for the estimation of reactivity hazards of organic material is called CHETAH [5]. The method is based on pattern recognition techniques, based on experimental data, in order to infer the decomposition products that maximize the decomposition energy, and then performs thermochemical calculations based on the Benson group increments mentioned above. Thus, the calculations are valid for the gas phase, but this may be a drawback, since in fine chemistry most reactions are performed in the condensed phase. Corrections must be made, but in general they remain small and do not significantly affect the results. 

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