Thermochemical data, estimation group additivity

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

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. 

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. 

This study reports bond energies, enthalpy, entropy, heat capacity, internal rotation potential, and structure data for a series of unsaturated peroxides. Thermochemical property groups are developed as well for future use in group additivity estimation methods. 

As illustrated in Fig. 1, there are essentially four methods for obtaining thermochemical data for the species in our reaction mechanism. The first choice is to find the needed data in databases or in the literature in general. This includes both published experimental data and published quantum chemical calculations, which can also be a reliable source of thermochemical data. If no information on a substance is available in the literature, one should consider whether it can be treated by group additivity methods. If a well-constructed group additivity method is available for the class of molecules of interest, the results, which can be obtained with minimal effort, will be comparable in accuracy to those from the best quantum chemistry calculations. If group additivity is not applicable to the molecules of interest, then we may want to carry out quantum chemistry calculations for them, as discussed in detail in an earlier chapter. In some cases, the effort required to carry out the quantum chemical calculations may not be warranted, and we may want to make coarser, empirical estimates of thermochemical properties. 

If thermochemical data for a given substance are not available in the compilations cited above or in easily accessible literature, then the next best approach is to use an experimentally based empirical estimation technique. The most common approach of this type is group additivity,... 

Density, refractive index, viscosity, and isothermal vapour-liquid equilibrium data for hexafluoroacetone-water mixtures, i.e. solutions of the gem-diol (CFa)gC(OH)t, are now available, and so are thermochemical parameters for the addition of water and methanol across the C=0 bond of the ketone to give the hydrate, (CF3)2C(OH)a, and the hemiacetal, (CFa)2C(OH)OMe, respectively. The value for the heat of hydration of hexafluoroacetone [Affr°(soln.) = —93.6 kJ mol" cf. MeCO-CHjCI, -8.4 MeCO CHCl -23.0 MeCHO, -21.3 and CCI.-CHO, -58.5 kJ mol ], taken with that estimated for acetone [-4 4 kJ mol" (too low to have been measured reliably)] and the difference in the heats of hydrogenation of formaldehyde (-92 kJ mol- ) and acetone (—54 kJ mol- ), leads to the conclusion that two CFj groups destabili2e a carbonyl bond by ca. 50 kJ mol-, a factor which correlates with the remarkable reactivity of hexafluoroacetone in carbonyl additions. ... 

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