Clementina Teixeira

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).

Estimation of standard enthalpies of formation of crystalline inorganic and organometallic complexes

Abstract Good linear correlations have been found between standard enthalpies of formation of crystalline inorganic and organometallic complexes MXnLm and enthalpies of formation of ligands L or LH in their standard reference state. These correlations may be used to estimate enthalpies of formation of new complexes.

Organometallic thermochemistry at CQE-IST. An overview

Abstract A brief overview of the organometallic thermochemistry research made at Lisbon, for over 25 years is given in this paper. The examples presented illustrate how those studies motivated the building of new instrumentation, and how the obtained results were used to discuss the ‘strength’ of the transition metalligand bonds, rationalise reactivity trends, and develop empirical methods to estimate thermochemical data.

Estimation of standard enthalpies of formation of inorganic and organometallic compounds. II

Correlations between the standard enthalpies of formation of organometallic and inorganic compounds MXnLm and of ligands LH or LHm, both in their standard reference states, are examined in detail and illustrated with examples. They provide a simple method of predicting new values and assessing experimental data.

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.

Enthalpies of formation of complexes [M(η-C5H5)2(SR)2](M = Mo, W, or Ti; R = C3H7, C4H9, or C6H5); metal–sulphur bond enthalpies

Standard enthalpies of formation (kJ mol–1) at 298.15 K were determined by reaction-solution calorimetry: ΔHf⊖{[Mo(η-C5H5)2(SC6H5)2],c}= 309.4 ± 4.8; ΔHf⊖{[W(η-C5H5)2{SC3H7-n)2],c}= 23.5 ± 5.6; ΔHf⊖{[Ti(η-C5H5)2(SC3H7-n)2],c}=–253.0 ± 9.7; ΔHf⊖{[Ti(η-C5H5)2(SC6H5)2],c}= 65.8 ± 7.9; and ΔHf⊖{[Ti(η-C5H5)2(S2C6H3CH3)]}= 1.9 ± 9.4. From these data the metal–sulphur mean bond-dissociation enthalpies (text-decoration:overlineD) and bond-enthalpy terms (E) have been derived, using extended-Huckel molecular-orbital calculations to take account of changes in structure upon dissociation.

Photomicrocalorimetry: Photosubstitution of carbonyl by phosphites in [Mn(η5-C5H4CH3)(CO)3]

Abstract A photomicrocalorimeter is described for studies of reactions involving oxygen and moisture-sensitive compounds in organic solvents. This calorimeter was used to investigate the thermochemistry of photochemical substitutions of a carbonyl in [Mn(η 5 C 5 H 4 CH 3 )(CO) 3 ] by P(OR) 3 (R  Ph or 1 Pr). The measured reaction enthalpies in heptane (λ = 400 nm), ΔH r = −15.7 ± 0.5 and −18.6 ± 0.9 kJ mol −1 , respectively, were identified with the bond dissociation enthalpy differences D (MnCO)  D [MnP(OPh) 3 ] and D (MnCO)  D [Mn-P(O i Pr) 3 ] in solution. These results are discussed in terms of the basicity of the phosphates, spectroscopic data of the complexes (CO stretching frequencies), and the available thermochemical data for other transition metal-phosphorus(III) ligand interactions.

Enthalpies of formation of [Mo(η-C5H5)2(SR)2] complexes; R = n-C3H7, i-C3H7, n-C4H9, or t-C4H9

The standard enthalpies of formation of the title complexes at 298.15 K have been determined by reaction-solution calorimetry. The results gave ΔHf⊖{[Mo(η-C5H5)2{S(n-C3H7)}2], c}= 4.6 ± 5.3, ΔHf⊖{[Mo(η-C5H5)2{S(i-C3H7)}2], c}= 57.1 ± 5.7, ΔHf⊖{[Mo(η-C5H5)2{S(n-C4H9)}2], c}= 14.0 ± 5.7, and ΔHf⊖{[Mo(η-C5H5)2{S(t-C4H9)}2], c}= 6.9 ± 4.4 kJ mol–1. The metal–sulphur mean bond-enthalpy contributions have been derived as [Mo–S(n-C3H7)]≈ 237, [Mo–S(i-C3H7)]≈ 202, [Mo–S(n-C4H9)]≈ 211, and [Mo–S(t-C4H9)]≈ 193 kJ mol.