Drago-Wayland parameters

An alternative set of factors to HSAB theory that takes into account both electrostatic and covalent contributions is the Drago-Wayland parameters. The enthalpy of formation for the generic acid-base reaction in Equation (14.18) can be calculated from Equation (14.19) using the empirical E and C factors listed in Table 14.9. The subscripts A and B refer to acid and base, respectively ... 

TABLE 14.9 Drago-Wayland parameters using Equation (14.19) for selected acids (A) and bases (B). 

Use the Drago-Wayland parameters in Table 14.9 to calculate AH for the following... 

Softness of neutral acid (from Drago-Wayland parameters by rotation of coordinates)... 

One could, of course, attempt to use Equation (1.4) by taking one set of data to assign values of 5a, 5b, [Pg.5]

Drago and Wayland proposed a quantitative systan of acid-base parameters to account for reactivity by explicitly including electrostatic and covalent factors. This approach uses the equation — A// = + QCg where is the enthalpy of the reaction A + B AB... 

Several individuals have worked to quantify these trends, making scales of Lewis acidity and basicity. Gutmann has created a series of donor numbers (DN) and acceptor numbers (AN) for various solvents, while Drago and Wayland have assigned parameters E and C, which measure electrostatic interactions and covalent bonding potential, respectively. Lastly, Pearson treats each Lewis acid and base with two parameters, relating what is called the strength of the acid/base and the softness/hardness of the acid/base. In fact, the HSAB trend discussed above is primarily a concept developed by Pearson. 

As a starting point, Drago and Wayland chose an indirect method to simplify the calculation of equation (12.42). Iodine was chosen as a reference acid and assigned the parameters Ea = 1 (kcal/mol) and Ca = 1 (kcal/mol). The Eb term was then related to the lone-pair dipole moment and the Cb term to the lone-pair polarizability. This allowed a quantification of the add-base interactions for a number of small molecules. 

Similar to the perturbation theory, Drago and Wayland proposed a four-parameter equation for predicting reaction enthalpies between acid and base species (Drago and Wayland 1965). Both species are each characterized by two independent parameters an E value which measures their ability to partidpate in electrostatic bonding, and a C value which measures their ability to participate in covalent bonding, leading to ... 

Drago and Wayland (1965) proposed an empirical equation which expresses the formation enthalpy of an adduct AB in terms of two parameters S and characterizing the electrostatic and covalent strengths of its constituents ... 

This discussion also shows that neither the covalent nor the electrostatic energies can be written as simple products of parameters characterizing the acid A and the base B, as postulated in the Drago and Wayland (1965) equation, (6.1.15). The properties of the coupled system -for example, the number of formed bonds between the adsorbate and the substrate or the substrate decohesion which results from the charge transfer - are also important. The same remarks apply to the model of adhesion proposed by Bolger (1983) which expresses the adhesion energy as the difference between two energies characteristic of the reactants in the absence of each others. 

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