Empirical donor number

Although theories of solution (this chapter) and formation of extractable complexes (see Chapters 3 and 4) now are well advanced, predictions of distribution ratios are mainly done by comparison with known similar systems. Sol-vatochromic parameters, solubility parameters, and donor numbers, as discussed in Chapters 2-4, are so far mainly empirical factors. Continuous efforts are made to predict such numbers, often resulting in good values for systems within limited ranges of conditions. It is likely that these efforts will successively encompass greater ranges of conditions for more systems, but much still has to be done. 

Donor number (or donicity), DN — is an empirical semiquantitative measure of nucleophilic properties (-> acid-base theories, subentry Lewis acid-base theory) of a solvent defined as the negative of the standard molar heat of reaction (expressed in kcalmol-1) of the solvent D with antimony pentachloride to give the 1 1 adduct, when both are in dilute solution in the inert diluent 1,2-dichloroethane, according to the reaction scheme ... 

An empirical semiquantitative measure of the nucleophilic properties of EPD solvents is provided by the so-called donor number DN (or donicity) of Gutmann [53, 67] [cf. also Section 7.2). This donor number has been defined as the negative A// values for 1 1 adduct formation between antimony pentachloride and electron-pair donor solvents (D) in dilute solution in the non-coordinating solvent 1,2-dichloroethane, according to Eq. (2-10) f... 

The donor number approach has been criticized for both conceptual [141] and experimental reasons [28, 133, 138, 265], Therefore, the search for other empirical Lewis-basicity parameters has continued. 

A more simplified but likewise sueeessful empirical two-parameter approach for the deseription of solvent effects has been proposed by Krygowski and Fawcett [113]. They assume that only specific solute/solvent interactions need to be eonsidered. These authors postulated that the solvent effeet on a solute property A can be represented as a linear funetion of only two independent but eomplementary parameters describing the Lewis aeidity and Lewis basicity of a given solvent. Again, for reasons already mentioned, the t(30) values were chosen as a measure of Lewis acidity. In addition, Gut-mann s donor numbers DN [26, 27] were chosen as a measure of solvent basicity cf. Table 2-3 and Eq. (7-10) in Sections 2.2.6 and 12, respectively). Thus, it is assumed that the solvent effect on A can be described in terms of Eq. (7-62) . 

These correlations, especially with the donor number, mostly reflect the enthalpic effect. There was no correlation of the entropic effect with DN in the case of metal cations [86], since this term reflects more the extent of solvent orientation by the component of the redox system and disordering of the original solvent structure, than the strength of the complex-solvent interactions. Also, Svaan and Parker [96] did not find any correlation between the entropy of formation of different ion radicals and the empirical solvent parameter. 

The Gutmann s Donor Number (DN) was proposed [Gutmann, 1978] as a quantitative empirical parameter for solvent nueleophilicity. For most solvents it was found to correlate well with the p scale. 

Empirical donor-acceptor parameter Quantum chemical index A B Mean square deviation Number of compounds... 

Various attempts have been made to classify solvents, e.g. according to bulk and molecular properties empirical solvent parameter scales hydrogen-bonding ability and miscibility >. In table I solvents are divided into classes on the basis of their acid-base properties which can be used as a general chemical measure of their ability to interact with other species. Detailed information on these and other solvents, their symbols, fusion and boiling pointe and Gg), bulk properties (6,Ti, q), and currently-used correlation parameters DN (donor number), Ej-value, and AN (acceptor number) is given in Appendix A-1. 

Theoretical and semiphenomenological approaches use the bulk properties or molecular quantities, dipole and quadrupole moments, polarizability, and so on, or mean force potentials in order to take these effects into account. Empirical methods take account of them with the help of empirical scales such as donor numbers, acceptor numbers, or correlation parameters. 

The approximately linear relationship between the donor number and the —ZIHa d values for the interactions between an acceptor A and the donor solvent D allows immediately an empirical approach to predict —zIHa d values. Calorimetric data must be available for the interactions of the given acceptor A with at least two different donors the donor numbers of which must be known. These values may be plotted against the donor numbers of the donors. From this plot —JHa.D for any D with given DNsbCU can be readily derived. 

The positive charge of the cationic solvation centre is partly delocalized over the pyridinium ring and shielded by the phenyl substituents. Therefore, the E,p values depend strongly on the electrophilic solvation power of the solvents, i.e. on their hydrogen-bond donor (HBD) ability or Lewis acidity, rather than on their nucleophilic solvation capability, for which the Donor Numbers of Gutmann (19) are the better empirical solvent parameters. 

MICHEAU - My comments deals with azeotropic binary mixtures. We have recently made some experimental measurements of the thermodynamic parameters of a thermochronic equilibrium in azeotropic liquid mixtures. Our thermochronic equilibrium (Nickel complexes NiR + 2S N1R2S2) is sensitive to the donor number of the solvent S which is an empirical measure of the availability of the electronic doublet of S. What we have found in azeotropic mixtures (alcohol + halogenated hydrocarbons) is that near the room temperature there is a kind of natural compensation of the alcohol doublet availability by the presence of halogenated hydrocarbons molecules this compensation shifts the equilibrium position near 50/50 (solvated vs non solvated complex). This property is spontaneous with the azeotropes we have studied, but must to be adjusted accurately by varying the molar ratio with similar binary mixtures not giving azeotropes. So, it appears that azeotropes exhibit from this point of view some singular propertie. My question is Do you have or do you know some results about reactivity studies in azeotropic mixtures Could an azeotrope be considered as a model of a particular supermolecule or cluster ... 

Beeause of the often-observed inadequacies of the dielectric approach, that is, using the dielectric constant to order reactivity changes, the problem of correlating solvent effects was next tackled by the use of empirical solvent parameters measuring some solvent-sensitive physical property of a solute chosen as the model compound. Of these, spectral properties such as solvatochromic andNMR shifts have made a spectacular contribution. Other important scales are based on enthalpy data, with the best-known example being the donor number (DN) measuring solvent s Lewis basicity. 

The first reference in Sec. 10.5.3 correlates the absorption bands of the first complex with the solvent Et parameter. The second reference reports an empirical linear correlation between (V1-V2) and aDN+PAN(V and V2) are the wave numbers of the two long wavelength bands of the complex and a and P are empirical constants related to the donor number DA and acceptor number AN of the solvent. The reference also correlates these numbers with the hyperfme splitting constant of V esr spectra. However, as Selbin (CAe ./fev.65(1965)153) pointed out, meaningful theoretical interpretations are complicated by [Pg.296]

Remember that Donor Numbers (DN) are used as an empirical semiquantitative measure of the nucleophilic properties of solvents and are particularly used when discussing the influence of solvent polarity in reactions involving cations. They range from dichloromethane (DN —0.0 kcal moT, reference solvent) to HMPA... 

Using the empirical parameter of the solvent polarity Z based on the molar energy of the transition in E, in kilocalorie per mole (kcal/mol) for the CT band, the Ep values of sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate are shown in Table 6.3. These data illustrate the analogy between the Z values of the sodium salt of l-methyl-4-[2-(4-hydroxyphenyl)ethenyl)]pyridinium] hydrogensquarate with Ep(30), which is the empirical solvent polarity parameter, based on the intramolecular charge transfer absorption of a pyridinium-N-phenolate betaine dye. The Z values are practically equal in the solvents acetone, pyridine, and cyclohexane, which means that for a difference in the values of 8, solvent donor number (DN) and solvent acceptor number (AN) of 35.3, 33.1, and 18.9 (kcal/mol), the... 

The solvent triangle classification method of Snyder Is the most cosDBon approach to solvent characterization used by chromatographers (510,517). The solvent polarity index, P, and solvent selectivity factors, X), which characterize the relative importemce of orientation and proton donor/acceptor interactions to the total polarity, were based on Rohrscbneider s compilation of experimental gas-liquid distribution constants for a number of test solutes in 75 common, volatile solvents. Snyder chose the solutes nitromethane, ethanol and dloxane as probes for a solvent s capacity for orientation, proton acceptor and proton donor capacity, respectively. The influence of solute molecular size, solute/solvent dispersion interactions, and solute/solvent induction interactions as a result of solvent polarizability were subtracted from the experimental distribution constants first multiplying the experimental distribution constant by the solvent molar volume and thm referencing this quantity to the value calculated for a hypothetical n-alkane with a molar volume identical to the test solute. Each value was then corrected empirically to give a value of zero for the polar distribution constant of the test solutes for saturated hydrocarbon solvents. These residual, values were supposed to arise from inductive and... 

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