Gutmann concept

The Gutmann concept has been criticized in a number of respects [Dr 62,65b, 81, Bo 66]. Since the donicity is the — dH value of a solvation reaction, it does not take into account the following ... 

The above considerations also reveal the limitations of the Gutmann donicity conception. To illustrate the more general nature of the Gutmann concept, the following reaction was subjected to a quantitative thermodynamic treatment [Gu 74] ... 

The impact of the donor-acceptor concept. V. Gutmann and R. Resch, Comments Inorg. Chem., 1982,1, 265-278 (44). 

Gutmann introduced3 the concepts of donor number (donicity) and acceptor number (acceptivity), as dimensionless numbers, for the characterization of donor properties of bases independently of the solvent. 

The enthalpy change during donor-acceptor adduct formation has been related by Drago to the sum of two terms (1) the product of the electrostatic properties of the acid and the base, Ef. and E and (2) the product of their tendency toward covalent bonding, Ca and Cb [10]. For the particular case, where the acceptor is specified to be SbCb (and the inert solvent is 1,2-dichloroethane), the negative of this enthalpy change (in kcal mol", 1 cal = 4.184 J) is the Gutmann donor number, DN [2,11]. These concepts are further discussed in Chapter 3. 

Indeed, the mathematical form of the mixed-potential concept (Bockris, 1954) has been applied to a number of chemical processes which, it has been shown, in fact, consist of two partnered surface electrochemical processes (Spiro, 1984). Thus, energy conversion processes at the surface of mitochondrial cells may involve the electrochemical oxidation of glucose as the anodic reaction and the electrochemical reduction of oxygen as the cathodic (Gutmann, 1985). 

R. Boyer, Concepts in Biochemistry, Brooks/ Cole, Monterey, CA, 1999. See also F. Gutmann, C. Johnson, H. Keyzer, J. Molnar, Charge-Transfer Complexes in Biological Systems, Dekker, New York, 1977. 

Many physical chemists have embraced the concepts of donicity (donor numbers, DN) and acceptor numbers (AN) as developed by Gutmann and his co-workers [12], The DN is measured by the heat of reaction of the donor solvent and antimony pentachloride in a 1 1 ratio as a dilute solution in 1,2-dichloro-ethane. It is taken to be a measure of the strength of the Lewis base. The AN is measured as the relative shift of the 31P NMR peak in triethylphosphine oxide dissolved in the sample solvent. Hexane is given the value of zero on the scale, and antimony pentachloride is given the value of 100. The AN is taken to be a measure of the strength of the Lewis acid. The applications of the concepts have... 

As a cautionary reminder, the SL and Fuoss treatments rely on electrostatic principles as a basis and thus suffer the same limitations noted above in the discussion of ion solvation. Just as including donor-acceptor interactions was needed in understanding ion transfer in Sec. Ill, it has been found that the same concepts help to elucidate ion pairing. In particular, the electron-pair donor ability of the solvent, as manifested by the Gutmann donor number DN [147, was shown to play a key role in the understanding of ion pairing of alkali metal salts [230—232]. 

The Lewis acid-base reaction leading to complex formation has been recently considered in relation to the role of solvation effects. Many scales of thermodynamic parameters have been suggested. The concept of donor numba- (DN) was proposed by Gutmann, and defined as the AH (kcalmoD ) for the intoaction of a basic solvent with SbCls in 1,2-dichloromethane at room temperature ... 

The practical applicability of the Gutmann-Mayer concept was proved by calculation of the standard free energies of dissolution AG values) of sodium chloride and potassium chloride in various solvents (Tables 4.14 and 4.15), of the AG values of the coordination reaction C0CI3-hCP- CoClJ" in various organic solvents (T able 4.16) and of the rate constant of the reaction of p-nitrofluorobenzene with piperidine (Table 4.17). 

The success of the Gutmann donicity concept can be attributed primarily to the fortunate choice of antimony(V) chloride as reference acceptor. The limitations of the empirical solvent strength scales presented in Chapter 4 are due, in almost every system, to various physical or chemical properties of the model. Also in examinations aimed at learning more about special phenomena of less general validity, the primary step is the selection of the appropriate model. 

On the basis of a study of the solvent dependence of the chemical shift of Taft el al, [Ta 63] suggested that the solvent effect can be attributed to donor-acceptor chemical interactions. The first really convincing confirmation of this concept was provided by the Na NMR studies by Erlich, Popov et al [Er 70, Er 71, Gr 73, Po 79]. In this work a linear correlation was demonstrated between the Gutmann donicities of the solvents and the chemical shift values for solutions of sodium perchlorate and sodium tetraphenylborate in various organic solvents. This clearly showed that the variation of the chemical shift of the Na in the systems investigated was caused predominantly by the donor-acceptor interaction between the donor solvent and the sodium ion. 

A special type of interaction, the acid-base interaction, is a fairly recent discovery. It is based on the chemical concept of a Lewis acid and base, which is briefly described. The acid/base definition was proposed separately by J. N. Bronsted and G. N. Lewis. Restatement of these definitions by Lewis in 1938 led to their popularity and acceptance. The Lewis definitions are an acid is a substance which can accept an electron pair from a base a base is a substance which can donate an electron pair. By this definition, every cation is an acid in addition to chemical compounds such as BF3 and Si02. Conversely, anions and compounds like NHj, PH3, and CgHjCH2NH2 are bases. According to the acid-base theory, adhesion results from the polar attraction of Lewis acids and bases (i.e., electron-poor and electron-rich elements) at the interface. This theory is attributed to the work by Fowkes et Gutmann, and Bolger and Michaels . ... 

The ability to express quantitatively the acid/ base interaction potential of individual polymers and of other constituents in polymer systems, raises the possibility of also obtaining the acid/base interaction between pairs of components. Among the first to consider the possibility were Papirer and coworkers, whose adoption of the Gutmann acid/base concept led to eqn. 12, and the definition of acid/base interaction constants and for a solid. Values of these parameters for two solids (1,2) could then be us to define a pair interaction value, A, as for example in ref 29 ... 

More recently, it has been shown, in particular by Fowkes and co-workers [2,6,7], that electron acceptor and donor interactions, according to the generalized Lewis acid-base concept, could be a major type of interfacial forces between two materials. This approach is able to take into account hydrogen bonds which are often involved in adhesive joints. Inverse gas chromatography at infinite dilution for example is a well adapted technique [8-10] for determining the acid-base characteristics of fibres and matrices. Retention data of probes of known properties, in particular their electron acceptor (AN) and donor (DN) numbers according to Gutmann s semi-empirical scale [11], allow the determination of acid-base parameters, and Kj), of fibre and matrix surfaces. It becomes then possible to define a "specific interactions parameter" A at the fibre-matrix interface, as the cross-product of the coefficients and Kq of both materials [10,11] ... 

Lewis [5] was the first to describe acids and bases in terms of their electron accepting and electron donating properties. Mulliken [6] further refined the understanding of the acid base interactions for which he was awarded the Nobel Prize for Chemistry. His quantum mechanical approach introduced the concept of two contributions, an electrostatic and a covalent, to the total acid-base interaction. Pearson [7] introduced the concept of hard and soft acids and bases, the HSAB principle, based on the relative contributions from the covalent (soft) interaction and the electrostatic (hard) interaction. In his mathematical treatment he defined the absolute hardness of any acid or base in terms of its ionisation potential and electron affinity. Pearson s is probably the most robust approach, but the approaches in most common use are those developed by Gutmann [8] and Drago [9], who separately developed equations and methods to quantify the acid or basic strength of compounds, from which their heats of interaction could be calculated. 

---