Thermochemical data, estimation

To become familiar with the estimation of thermochemical data... 

All three schemes, the Benson, the Laidler, and the Allen scheme, use four structure contributions for the estimation of thermochemical data of alkanes. As might be guessed, they are numerically equivalent all three schemes provide the same accuracy. This is shown below by Eqs. (7)-(10) for the interconversion of the various contributions. 

Any one of these additivity schemes can be used for the estimation of a variety of thermochemical molecular data, most prominently for heats of formation, with high accuracy [13]. A variety of compilations of thermochemical data are available [14-16]. A computer program based on Allen s scheme has been developed [17, 18] and is included in the PETRA package of programs [19]. 

Because these various quantities are characteristics of the reactants and products but are independent of the reaction path, they cannot provide insight into mechanisms. Information about AG, AH, and AS does, however, indicate the feasibility of any specific reaction. The enthalpy change of a given reaction can be estimated from tabulated thermochemical data or from bond-energy data such as those in Table 1.3 (p. 14) The exan le below illustrates the use of bond-energy data for estimating the enthalpy of a reaction. 

Whether AH for a projected reaction is based on bond-energy data, tabulated thermochemical data, or MO computations, there remain some fundamental problems which prevent reaching a final conclusion about a reaction s feasibility. In the first place, most reactions of interest occur in solution, and the enthalpy, entropy, and fiee energy associated with any reaction depend strongly on the solvent medium. There is only a limited amount of tabulated thermochemical data that are directly suitable for treatment of reactions in organic solvents. Thermodynamic data usually pertain to the pure compound. MO calculations usually refer to the isolated (gas phase) molecule. Estimates of solvation effects must be made in order to apply either experimental or computational data to reactions occurring in solution. 

The activation energy for rotation about a typical carbon-carbon double bond is very high—on the order of 250 kj/mol (about 60 kcal/ mol). This quantity may be taken as a measure of the tt bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 191. 

It is interesting to note that the simple Morse potential model, when employed with appropriate values for the parameters a and D (a = 2.3 x 1010 m 1, D = 5.6 x 10 19 J as derived from spectroscopic and thermochemical data), gives fb = 6.4 nN and eb = 20%, which are quite comparable to the results obtained with the more sophisticated theoretical techniques [89]. The best experimental data determined on highly oriented UHMWPE fibers give values which are significantly lower than the theoretical estimates (fb 2 nN, b = 4%), the differences are generally explained by the presence of faults in the bulk sample [72, 90] or by the phonon concept of thermomechanical strength [15]. 

The needed thermochemistry for many thousands of molecules is available from standard sources such as the JANAF tables. " Polynomial fits of this data in the form required by our kinetics software are also available. However, experimental thermochemical data is often lacking for many of the intermediate species that should be included in a detailed kinetics mechanism. Standard methods have been developed for estimating these properties, discussed in detail by Benson. ... 

Assuming chemical equilibrium between Kuroko-forming minerals and Kuroko ore fluids, the chemical composition of Kuroko ore fluids can be estimated, using thermochemical data. Calculations for the purposes of estimating the chemistry of Kuroko... 

The range of temperatures for each alteration zone can be estimated from the following chemical reactions and thermochemical data available for these reactions... 

Based on the analytical data of K-mica, epidote and K-feldspar and using thermochemical data on these minerals (Helgeson and Kirkham, 1974 Helgeson et al., 1978 Bird and Helgeson, 1981), the /coz range for the propylitic alteration was estimated (Fig. 1.78). 

The following references contain techniques for estimating thermochemical data. 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

The chemistry of carbenes in solution hits been extensively studied over the past few decades.1-5 Although our understanding of their chemistry is often derived from product analyses, mechanistic details are often dependent on thermodynamic and kinetic data. Kinetic data can often be obtained either directly or indirectly from time-resolved spectroscopic methods however, thermochemical data is much less readily obtained. Reaction enthalpies are most commonly estimated from calculations, Benson group additivities,6 or other indirect methods. 

To be consistent the value of E2 should be corrected to constant pressure so that it represents AH for the process involved (flow system studies and static system work with excess inhibitor are essentially constant pressure experiments). Then D < E < D+RT. In the present work a reasonable estimate gives D — E—0J = 57.0 kcal.mole-1. Similarly, D2+D2 should be corrected to 0 °K, giving an estimated value of 59.0 kcal.mole-1. This gives D2 = 2.0 kcal.mole-1. Such corrections are normally within the limits of experimental error, so that experimental values of E are associated directly with dissociation energies, and thermochemical data at 25 °C are used. 

Examples of estimations of the standard potential from thermochemical data can be found in the literature for alkyl halides in water and in nonaqueous solvents.1... 

A significant contribution to the uncertainty interval assigned to the O-H bond dissociation enthalpy in benzoic acid comes from the estimate of the activation enthalpy for the radical recombination. The experimental determination of this quantity is not easy because diffusion-controlled recombination rate constants are very high (109 mol-1 dm3 s 1 or larger) [180]. Therefore, most thermochemical data derived from kinetic experiments in solution rely on some similar assumptions. 

This useful and simple-to-use software package relies on Benson s group additivity scheme [47] to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics [32] and JANAF Tables [22]. The first version of... 

This book contains a small database for organic and inorganic compounds. Its main value, however, is that it describes a group additivity scheme to estimate thermochemical data. An updated and extended list of group parameters is given in NIST Therm [28]. The first edition of this classic work is from 1968. 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

The specific ion interaction approach is simple to use and gives a fairly good estimate of activity factors. By using size/charge correlations, it seems possible to estimate unknown ion interaction coefficients. The specific ion interaction model has therefore been adopted as a standard procedure in the NEA Thermochemical Data Base review for the extrapolation and correction of equilibrium data to the infinite dilution standard state. For more details on methods for calculating activity coefficients and the ionic medium/ ionic strength dependence of equilibrium constants, the reader is referred to Ref. 40, Chapter IX. 

S. W. Benson (1976) Thermochemical Kinetics Methods for Estimation of Thermochemical Data and Rate Parameters, 2nd ed., Wiley, New York. 

Thus, the appropriate C—H bond strengths can be determined. These are given with the relevant thermochemical data in Table III. Also shown in Table III for comparison are alternative estimates of these bond energies (28) which appeared during the course of this work. The two sets of data agree fairly well. 

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