J. A. Martinho Simoes

Enthalpies of formation of cis-azobenzene and trans-azobenzene

The standard ( p ° = 0.1 MPa) molar enthalpy of formation of crystalline trans -azobenzene was determined from its enthalpy of combustion in oxygen at 298.15 K measured by static-bomb calorimetry. The enthalpy of isomerization: Δ isom H ° m (cr, cis → trans )/(kJ · mol −1 ) = -(49.1 ± 1.0), was measured by reaction-solution calorimetry: the isomerization was catalysed by dicyclopentadienyltitanium diiodide in toluene solution. The enthalpy of isomerization in heptane solution: Δ isom H ° m ( trans → cis )/(kJ · mol −1 ) = -(48.9 ± 2.3) when coupled with the enthalpies of solution of the two forms in heptane confirmed the reaction-solution calorimetric value. For the crystalline solids:Δ f H ∘ m ( trans -azobenzene, cr)/(k/J · mol -1 ) = (308.6 ± 1.9),Δ f H ∘ m ( cis -azobenzene, cr)/(k/J · mol -1 ) = (357.7 ± 2.1) The enthalpy of sublimation of trans -azobenzene was determined from the variation of vapour pressure with temperature measured by the Knudsen method: Δ g cr H ° m ( trans -azobenzene)/ (kJ · mol −1 ) = (93.6±1.9).

Standard enthalpies of formation of sodium alkoxides

Abstract The standard enthalpies of formation of several crystalline sodium alkoxides, Δ H o f (NaOR, cr), have been determined by reaction-solution calorimetry. A linear correlation has been found between Δ H O f (NaOR, cr) and Δ H o f (ROH, 1/cr) for R = n-alkyl, enabling the prediction of data for other sodium alkoxides. The results were also used to derive the lattice energies and the thermochemical radii of the anions OR − .

Standard molar enthalpies of formation of lithium alkoxides

Abstract The standard ( p 0 = 0.1 MPa) molar enthalpies of formation of several crystalline lithium alkoxides, Δ H f 0 (LiOR, cr), have been determined by reaction-solution calorimetry at 298.15 K. A linear correlation has been found between Δ H f 0 (LiOR, cr) and Δ H f 0 (ROH, 1) for R = n-alkyl, enabling the prediction of data for other lithium alkoxides. The deviations from the linear correlation observed for R ue5fb i Pr and t Bu were tentatively explained in terms of the electronegativities of the OR groups. The experimental data were also used to derive the lattice energies and the thermochemical radii of the anions OR − . The results were compared with those derived from the enthalpies of formation of the analogous sodium alkoxides, reported in a previous publication.

ESTIMATION OF STANDARD ENTHALPIES OF FORMATION OF INORGANIC AND ORGANOMETALLIC COMPOUNDS. III, HOMOLEPTIC METAL HALIDES

Abstract Linear plots of standard enthalpies of formation of homoleptic metal halides, MX m against the standard enthalpies of formation of the gaseous hydrogen halides, HX, were used to assess a large number of literature data and to gain a better understanding of the correlations themselves. The method is extremely simple, and enables the estimation of new data.

Enthalpy of formation of the benzoyl radical by photoacoustic calorimetry

Abstract The energetics of the benzoyl radical have been studied by photoacoustic calorimetry. The value obtained for D[PhC(O)-H], 371 ± 11 kJ mol−1 (89 ± 3 kcal mol−1), agrees with an early result derived from iodination kinetic experiments and does not support a recent suggestion that the radical is resonance stabilized.

Metalbromine bond-enthalpy contributions in M(η-C5H5)2Br2 complexes (M = Mo, W)

Abstract Reaction-solution calorimetric measurements of reactions of M(η-C5H5)2H2 (c) (M = Mo, W) with carbon tetrabromide in toluene led to bond-enthalpy contributions D (Moue5f8Br) = 242.0 kJ mol−1 and D (Wue5f8Br) = 298.9 kJ mol−1

A Short and Illustrated Guide to Metal-Alkyl Bonding Energetics

This volume is called Energetics of Organic Free Radicals. Why then a (short) chapter on metal-alkyl bonding energetics? The subject could have a direct impact on the main topic of the book if very accurate metal-alkyl bond dissociation enthalpy data were available, which, together with the enthalpies of formation of the parent organometallic molecules and of the organometallic fragments (produced upon dissociaxadtion of the metal-alkyl bonds), would lead to the enthalpies of formation of the alkyl radicals. However, very few gas-phase metal-carbon bond dissociation enthalpies are accurately known for coordinatively saturated organometallic complexes and the data bank on the energetics of unsatxadurated (neutral or ionic) metal species is rather limited [1J. The present information on thermochemistry of organometallic compounds is thus almost irrelevant to the energetics of organic free radicals. In reality, things happen the other way around: organometallic thermochemists often need thermochemical data of organic free radicals to derive metal-ligand bond dissociation enthalpy values. In other words, the energetics of organometallic molecules may be regarded as an applicaxadtion example of the energetics of free radicals. This would probably be enough to justify this chapter, but there is another reason, which will become apparent throughout the text: in general, the enthalpies of metal-ligand bonds and the corresponding ligand-hydrogen bonds follow nearly parallel trends.

Estimates of Thermochemical Data for Organometallic Compounds

Literature methods that have been used to estimate the energetics of organometallic reactions are critically surveyed and illustrated with selected examples.