Thermochemical studies

The energies of protonation of the complete series of methyl and ethyl phosphines have been calculated. pATa values for the hydroxyphenyl- 

Drowart, Chem. Phys. Letters, 1971, 8, 10. 

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Thermochemical studies of organo-transition metal carbonyls and related compounds. J. A. Connor, Top. Curr. Chem., 1977, 71, 72-110 (122). 

It is always important in thermochemical studies to be aware of the temperature at which the thermochemical properties are determined, and to combine only those properties at the same temperature. Temperature corrections can be made by using integrated heat capacities over the temperature ranges in question. However, it is often assumed that the temperature corrections for ionization energies and electron affinities are small (<1 kJ/mol) and therefore can be neglected. 

Equilibrium measurements measure the relative AG, and thermochemical studies generally are interested in enthalpy values, AH. The enthalpy can be obtained from AG by using the relation AG = AH — TAS. The entropy of proton transfer can either be estimated, reliably calculated using electronic structure calculations, or can be measured directly by using a Van t Hoff approach. Measuring the quantity AS requires a variable temperature study. 

Connor, J. A. Thermochemical Studies of Organo-Transition Metal Carbonyls and Related Compounds. 71, 71-110 (1977). 

Wagner pioneered the use of solid electrolytes for thermochemical studies of solids [62], Electrochemical methods for the determination of the Gibbs energy of solids utilize the measurement of the electromotive force set up across an electrolyte in a chemical potential gradient. The electrochemical potential of an electrochemical cell is given by ... 

Thermochemical Studies of Organo-Transition Metal Carbonyls and Related Compounds... 

Photoelectron and electron transmission spectroscopy indicate that there is appreciable interaction between the acetylene units of [129] (Houk et al., 1985). Both homoconjugation and hyperconjugation are proposed. Dewar and Holloway (1984) suggest that the through-bond interactions dominate. Similar thermochemical studies to those performed with the triquinacene series were carried out on [129] and some acyclic homoconjugated acetylenes (Scott et al., 1988). From these data it was concluded that decamethyl[5]pericyclyne should be classed as a homoaromatic molecule. As already discussed for the triquinacene series, the species used as non-homoaromatic models (and the calculated compensations for strain energies) may be inappropriate and thus this conclusion should be treated with some caution. Using our probes for homoaromaticity we were not able to obtain any evidence in support of the homoaromaticity of [129] (Williams and Kurtz, unpublished results). 

The classical calorimetric methods addressed in chapters 7-9, 11, and 12 were designed to study thermally activated processes involving long-lived species. As discussed in chapter 10, some of those calorimeters were modified to allow the thermochemical study of radiation-activated reactions. However, these photocalorimeters are not suitable when reactants or products are shortlived molecules, such as most free radicals. To study the thermochemistry of those species, the technique of photoacoustic calorimetry was developed (see chapter 13). It may be labeled as a nonclassical calorimetric technique because it relies on concepts that do not fit into the classification schemes just outlined. 

The thermochemical study of photochemical or photochemically activated processes is not amenable to most of the calorimeters described in this book, simply because they do not include a suitable radiation source or the necessary auxiliary equipment to monitor the electromagnetic energy absorbed by the reaction mixture. However, it is not hard to conceive how a calorimeter from any of the classes mentioned in chapter 6 (adiabatic, isoperibol, or heat flow) could be modified to accommodate the necessary hardware and be transformed into a photocalorimeter. 

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