Donor-acceptor-interactions, enthalpies

At low temperature, this equilibrium shifts to the right, and only a small fraction of free peroxyl radicals participate in the reaction, whereas at a high temperature the equilibrium is shifted to the left, and the fraction of bound radicals is small. If we assume that at 210-260 K almost all R02 radicals are bound in complexes with amines, then it can be shown that the difference of low- and high-temperature Ee0 values is close to the enthalpy of this donor-acceptor interaction in the CTC complex AEe0 AHDA = 66.7 - 59.1 = 7.6 kJ mol-1. 

We have dwelled very long on the design of the experiment, but since the approach to be described is an empirical one, the parameters we obtain and the significance of them are only as good as the enthalpy data utilized. The literature is filled with thermodynamic data that are meaningless from the standpoint of providing quantitative data about the magnitude of donor-acceptor interactions because the above described criteria are not met. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

As can be seen, the enthalpies of different apoxy-amine systems, according to different authors, lie in a rather narrow range (100-118 kJ per mole of epoxy groups, i.e. close to the heat of the epoxy ring opening). These data confirm the above conclusion as to the small total contribution of the donor-acceptor interactions in the epoxyamine systems to the observed integrated value of the heat release and the possibility of the application of the isothermal calorimetry method to the reaction kinetic studies. 

Use of the complexation shift as a measure of donor-acceptor interaction is especially treacherous with nuclei other than protons, because chemical shifts of these nuclei are more dependent on the paramagnetic than on the diamagnetic term of the screening tensor (52, 53). Both 19F and 11B resonances of the boron trihalides do shift to high field on complexation, as expected if the complexation shift were due to the increase in electron density on the boron trihalide, and early work indicated that 19F complexation shifts of BF3 could be correlated with enthalpies of formation of the complexes. Although this is true for BF3 adducts of some series of closely related donors (42, 91, 151), such correlations do not occur in other series (169). Table II illustrates that, although there is a tendency for the strongest... 

Riddle and Fowkes [21] have shown that dispersion-only liquids, such as hexane, produce a significant P shift in (C2H,s)3PO. Hence, AN values should be corrected for this dispersion effect. In many cases, this correction is quite substantial. Thus, 13.7 of the original 14.2 AN units assigned to pyridine appear to be due to dispersion rather than to specific electron-pair donor-acceptor interactions, lowering its measure of true Lewis acidity from 14.2 to 0..5. Riddle and Fowkes have found that these dispersion corrected AN values correlate well with the enthalpies of formation of the adducts formed between (C2H5)3P0 and the examined acid. They proposed to use this enthalpy as the true measure of Lewis acidity for a species, which allows to express both the DN and AN (modified AN parameter) numbers in the same units ... 

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

For bis[2,6-bis(dimethylaminoethyl)phenylsilane, the second selected hypercoordinate organosilicon compound, its structure determination proves a formally eight-coordinate silicon center with averaged intramolecular distances of 289 and 310 pm for the two different N Si interactions in the differently overcrowded donor-acceptor complex halves (Fig. 4a). They vary by 21 pm in length and their bond enthalpies have to be each separately approximated, analogously to the ones in the seven-coordinate compound (Figs. 3 and 4). Altogether four rotations around the... 

Surface area around a molecule where optimum enthalpy of interactions of acceptor atoms with H-bond donor probe is realized [38, 39]... 

Surface area around a molecule where optimum enthalpy of interactions of donor atoms with H-bond acceptor probe is realized [38, 39[ Surface area around a molecule where optimum free energy of interactions of donor atoms with H-bond acceptor probe is realized [38, 39[ Sum of enthalpy values (kcalmof interactions between the acceptor atoms in a molecule and donor probe on OEASA [38, 39[ Sum of enthalpy values (kcalmof interactions between the donor atoms in a molecule and an acceptor probe on OEDSA [38, 39[... 

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