The Laidler Terms

The so-called Laidler scheme was developed as a tool to estimate standard enthalpies of formation of organic compounds [90], It relies on the bond-additivity concept, that is, it assumes that the standard enthalpy of atomization of a given molecule in the gas phase (Aat//°, defined as the standard enthalpy of the reaction where all the chemical bonds are cleaved, yielding the gaseous ground-state atoms) can be evaluated by adding the relevant bond enthalpy terms. For instance, in the case of phenol, its standard enthalpy of atomization, or simply its enthalpy of atomization, refers to reaction 5.28 at 298.15 K  

The Laidler terms needed to estimate Aat//0(PhOH, g) are given in the equation  

To atomize the phenol molecule, we have to cleave six carbon-carbon bonds in the aromatic ring (Cb-Cb), five carbon-hydrogen bonds (Cb-H), one carbon-oxygen bond (Cb—O), and one oxygen-hydrogen bond (O-H). The symbol El has been adopted is this equation (instead of the more used symbol E) to avoid confusion with the quantities discussed in the previous section. 

The most reliable values of Laidler terms that can applied to a wide variety of compounds are those recommended by Cox and Pilcher [89]. They were derived from a consistent database that includes experimentally determined standard enthalpies of formation for hundreds of organic compounds. This lengthy but simple exercise involves the choice of a set of bond enthalpy terms that affords the best agreement between experimental and calculated standard enthalpies of 

Despite some lack of physical meaning, the empirical Laidler terms have a rather successful predictive power. That justified their use in the past as replacements for Es values [85,86,88], when computational methods were not readily available. It also justifies recent efforts to develop the Laidler scheme by using a larger number of parameters (accounting, e.g., for intramolecular repulsions) [92], 

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The above analysis reveals that some of the thermochemical data for organotin compounds may not be as accurate as one could hope. Although the information is in general of much better quality than in the case of germanium and lead analogues, we believe that some values in Table 3 should be redetermined. Other examples could have been used to illustrate this point (see also the next section), but once again we wish to resist the temptation of recommending data that in some cases conflict with the available experimental results. By a judicious use of the Laidler terms in Table 4 and/or correlations similar to those in equation 2, it is rather simple to assess other values from Table 3 and predict new data. 

If it is further assumed that there is an exponential dependence of nt on Eu and the distribution of site energies is such that the activation energy values extend between the limits El to E2, it may be shown that the preexponential term includes a factor exp( + 1/y) and, in consequence, there is compensatory behavior. More complete treatments of this model, including further references, are given by Laidler [(33), pp. 119 and 195] and by Bond [(3), p. 143],... 

In accordance with Laidler it follows from this expression that for large values of t the exponential term approaches e ", which is almost zero. Thus, the remaining term responds to the amount of intermediate [EO] available during the steady-state phase. It becomes apparent that the ratios of substrates together with the ratio of rate constants are responsible for the percentage of [EO] proportional to the total enzyme concentration deployed. Due to the fact that the maximum product rate can only be reached when [EO] equals [E]o it can happen that a reaction system with k. - has a slower overall rate than the opposite case. However, the rate of product formation (or the equal rate of substrate consumption) for this system is now defined... 

Laidler, K. J. A Glossary of Terms Used in Chemical Kinetics IUPAC (Draft version, compliments of the author). 

Thus, it is only in an intermediate pH range that both functional groups can coexist in active form. This type of pH dependence has also been observed for other enzyme reactions and explained in similar terms (Laidler, 34). In this connection, the pH dependence of the catalytic activity of cupric glycinate complexes, discussed earlier (Fig. 2) should also be recalled. 

It should be noted that Kirkwood s formula does not appear in his first publication [240] in the form of Eq. (5-86). Nevertheless, it is this form of Kirkwood s formula which is widely known, representing only one of the terms of a more complex equation given in reference [240] with n= Kirkwood s theory was further developed in papers by Kirkwood, Westheimer, and Tanford [241], Laidler and Landskroener [242], and Hiromi [243]. 

The term kTth is the order of magnitude typically found for the frequency factor A. At 300 K, kTIh = 6.25 X 10 s . The task now is to evaluate the partition functions, and techniques for doing this evaluation can be found in Laidler and are beyond the scope of this discussion. 

Various definitions of the term catalyst can be categorized into those that require the regeneration of the promoting species, and those that do not. Usage of definitions that fall into either category can be found [e.g., Leuthardt (27) Laidler (28)]. Some of the actions of thermal polyamino acids have been proven, by various criteria, to be catalytic in the... 

Extension of the Gurney-Butler treatments of the kinetics of electrochemical charge-transfer was made in terms of the "transition-state" theory of Eyring, Glasstone and Laidler in a paper (27) by... 

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