Donor numbers

The solvent donor number, first defined by Gutman [4] and abbreviated to DN, is a system specific, but quantitative measure of the ability of a substance to act as an electron pair donor. In its original form it is defined as the negative of the heat of reaction for the formation of the 1 1 adduct between the test substance and antimony pentachloride, measured in 1,2-dichloroethane as solvent (equation 12.2). 

Marcus [5] has represented the donor numbers in a normalised form DN relative to the strong donor solvent hexamethylphosphortriamide, for which DN = 38.8 kcal mol A selection of DN values is given in Table 12.3. 

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Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Using a method suggested by Saint-Flour and Papirer [100], Schultz and Lavielle obtained A// -values for the interaction of several vapors of differing donor numbers and acceptor numbers with various treated and untreated carbon fibers used in the preparation of carbon fiber-epoxy matrix composites. was expressed as ... 

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... 

Gomberg-Bachmann reaction 62, 116, 189, 206, 253ff., 261, 273, 301 Gutmann donor numbers 375... 

Dodecametallic complexes, 1,167 Dodecamolybdometalates, 3,1045 Dodecatantalates, 3,1029 Dolerophane, 6,855 Donor numbers, 2,74 Dopamine (3-hydroxylase, 6, 711 copper, 6,654... 

Properties of the solvent. Reaction rates will differ with the solvent s polarity, viscosity, donor number etc. Added electrolytes may lower or raise the rates ( salt effects ), and buffer components may do so as well. 

On the basis of the results in acetonitrile, it might be reasonable to assume that the values for A//het(R-R ) and AG°het(R-R ) are apparently close to each other also in sulfolane, since the dielectric constant (43.3) and the donor number (14.8) of this solvent are close to those of acetonitrile (37.5 and 14.1, respectively). On the basis of this assumption, Arnett s equation (28) was examined for reactions of type (23). For these reactions, except for [3-2], only the AGhet(R-R ) values are avtiilable. As shown in Fig. 3, the values for this system are about 10 kcal moP less than predicted from (28). The negative deviation can also be ascribed to steric congestion in these hydrocarbon molecules. The large negative deviations, similar to those observed in sulfolane, are also seen in Fig. 3 for the values of AGSet(R-R ) in DMSO. 

The best-known solvent parameters are the donor number [21] and acceptor number [22] proposed by Gutmann and coworkers. The donor number (DN) for a donor solvent D is defined as the positive value of the enthalpy difference AH (kcalmol ) for the reaction of D with an acceptor-halide SbCls (D + SbCls D SbCls) in an inert medium such as 1,2-dichloroethane. DN is a fair measure for the donor properties of a solvent. The correlations of DN with the solvation energies are known to be good particularly for solvation of cations. A typical example [19] is shown in Fig. 3. 

Generally the magnitudes of solvent exchange rate constants increase in the sequence NH3 > H20 > DMF > MeCN > MeOH, which is largely independent of the nature of the metal ion (108,109). This sequence cannot be readily identified with specific characteristics such as dielectric constant, donor number, electric dipole moment, or stereochemistry, and it appears to reflect the overall solvent characteristic. There may be a correlation between the AH for solvent exchange on [M(solvent)6]2+ and the heat of dissociation of solvent from this species (110). 

From experiments as well as from the Gutmann donor number for acetonitrile (only 50% of that for water), it is well known that addition of water to solutions of lithium ions in acetonitrile leads to the formation of a water coordinated Li+ ion. This can be reproduced for HCN as modeled by quantum chemical calculations (RB3LYP/6-311+G ). 

The increase of the exchange rate constants for solvent exchange on [MS6]2+/3+ in the sequence MeOH < MeCN < DMF < H20 is independent of the metal ion and cannot be linked to specific properties like dielectric constant, donor number or size. It appears to reflect the overall characteristics of the solvent. 

In polar solvents with high donor numbers, the complex is solvated poorly relative to its separate components. This difference in solvation energies nullifies the hydrogen bonding interactions present in the complex, which, as a result, dissociates into its host and guest components. 

Solvent hydrogen-bonding strength using Donor Number parameter (see Reference 32)... 

The donor number, DN, of a solvent, proposed by Gutmann, is a measure of the Lewis basicity of the solvent, i.e. its ability to donate a pair of electrons [16]. The DN is determined by measuring the negative enthalpy for the reaction of equimolar quantities of the solvent with the standard Lewis acid, SbCls, at room temperature in 1,2-dichloroethane (Scheme 1.1), and reflects the ability of the solvent to solvate Lewis acids. SbCls reacts with protic solvents such as alcohols... 

In fact, tris(pyrrolidino)phosphane oxide has a donor number of 1.22. 

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