M. A. V. Ribeiro da Silva

Saturation Vapor Pressures and Transition Enthalpies of Low-Volatility Organic Molecules of Atmospheric Relevance: From Dicarboxylic Acids to Complex Mixtures

There are a number of techniques that can be used that differ in terms of whether they fundamentally probe the equilibrium and the temperature range over which they can be applied. The series of homologous, straight-chain dicarboxylic acids have received much attention over the past decade given their atmospheric relevance, commercial availability, and low saturation vapor pressures, thus making them ideal test compounds. Uncertainties in the solid-state saturation vapor pressures obtained from individual methodologies are typically on the order of 50-100%, but the differences between saturation vapor pressures obtained with different methods are approximately 1-4 orders of magnitude, with the spread tending to increase as the saturation vapor pressure decreases. Some of the dicarboxylic acids can exist with multiple solid-state structures that have distinct saturation vapor pressures. Furthermore, the samples on which measurements are performed may actually exist as amorphous subcooled liquids rather than solid crystalline compounds, again with consequences for the measured saturation vapor pressures, since the subcooled liquid phase will have a higher saturation vapor pressure than the crystalline solid phase. Compounds with equilibrium vapor pressures in this range will exhibit the greatest sensitivities in terms of their gas to particle partitioning to uncertainties in their saturation vapor pressures, with consequent impacts on the ability of explicit and semiexplicit chemical models to simulate secondary organic aerosol formation.

Energetics of metal-oxygen bonds in metal complexes of β-diketones

The thermochemical data available for standard enthalpies of formation, both in the crystalline and gaseous phases, of metal B-diketonates is reviewed, and the calculation of enthalpies of gas-phase decomposition of these complexes is presented. The homolytic metal-oxygen bond-dissociation enthalpies for several complexes of &diketones with different metals are calculated, and the results discussed in terms of structure and bonding and, whenever appropriate, trends in the data are suggested and discussed.

Enthalpies of combustion of 1,4-naphthoquinone, 9,10-anthraquinone, 9,10-phenanthraquinone, 1,4,9,10-anthradiquinone, 5,8-dihydroxy-1,4-naphthoquinone, and 1,4-dihydroxy-9,10-anthraquinone

Abstract The standard ( p o = 0.1 MPa) molar enthalpies of combustion in oxygen at 298.15 K were measured by static-bomb calorimetry for some quinones and dihydroxyquinones. The standard molar enthalpies of sublimation at 298.15 K were measured by microcalorimetry for 1,4-naphthoquinone and 9,10-phenanthraquinone; values were selected from the literature for the remaining compounds in order to derive the standard molar enthalpies of formation in the gaseous state. −Δ c H m o (cr) ( kJ·mo l −1 ) Δ f H m o (g) ( kJ·mol −1 ) 1,4-Naphthoquinone 4604.1 ± 1.1 −97.5 ± 1.9 9,10-Anthraquinone 6464.0 ± 2.1 −75.7 ± 2.9 9,10-Phenanthraquinone 6497.8 ± 1.6 −49.3 ± 2.8 1,4,9,10-Anthradiquinone 6033.6 ± 2.4 −209.0 ± 4.0 5,8-Dihydroxy-1,4-naphthoquinone 4196.8 ± 0.5 −499.1 ± 3.2 1,4-Dihydroxy-9,10-anthraquinone 6057.4 ± 1.0 −471.7 ± 2.3 The energies of the intramolecular hydrogen bonds in the dihydroxyquinones were assessed as (25 ± 3) kJ·mol −1 . 1,4,9,10-Anthradiquinone is apparently considerably strained, and although its reaction with water produces 1,4-dihydroxy-9,10-anthraquinone, a concomitant formation of hydrogen peroxide is shown to be thermodynamically improbable.

Enthalpies of combustion of four methyl-substituted heptane-3,5-diones and benzoylacetone

Abstract The standard enthalpies of combustion in oxygen at 298.15 K of the following β-diketones were measured in a static-bomb calorimeter: The standard enthalpies of formation of these β-diketones were derived for the keto-enol equilibrium mixtures in the condensed state and for the liquid and gaseous enol forms.

Thermochemical and theoretical study of some methyldiazines

The standard (p0 = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.

Recent work on the thermochemistry of some nitrogen heterocycles

Values available in the literature for standard molar enthalpies of formation, both in the condensed and in the gaseous phases, at temperature 298.15 K, for quinolines, methylquinolines, hydroxyquinolines, methylhydroxyquinolines, amino- quinolines, cyanoquinolines, halohydroxyquinolines and nitroquinolines are reviewed, and the results are interpreted in terms of contribuitions of the different groups and of molecular structure.

Mean copper-ligand binding enthalpies in copper(II) complexes of dimethylglyoxime, glycine, acetic acid and 4-phenylamino-3-penten-2-one

Abstract The standard enthalpies of formation of some crystalline copper(II) complexes were obtained from solution-reaction calorimetric measurements and the enthalpies of sublimation of Cu 2 (acetate) 4 , Cu(dmg) 2 and Cu(PhNacac) 2 were obtained from torsion-Knudsen vapour pressure measurements. The differences between the mean Cu-ligand and H-ligand dissociation enthalpies were derived and are given below. a . Complex a Δ f H m XXX (cr)(kJmol −1 ) Δ H m XXX (sub)(kJmol −1 ) t D (Cu-L)- D (H-L)(kJmol −1 ) Cu 2 (acetate) 4 −1765.8 ± 2.7 106.1 ± 0.9 −67.1 ± 1.8 Cu(dmg) 2 −171.5 ± 2.3 93.1 ± 0.8 −63.7 ± 1.8 Cu(PhNacac) 2 −174.3 ± 2.7 128.1 ± 0.8 −92.3 ± 3.7 Cu(gly) 2 −940.8 ± 1.7 111 ± 15 −26.3 ± 7.7 a Hdmg, Dimethylglyoxime; Hgly, glycine; PhNacacH, 4-phenylamino-3-penten-2-one.

THE ROLE OF BOUND WATER ON THE ENERGETICS OF DNA DUPLEX MELTING

A combination of common and low-temperature differential scanning calorimetry (DSC) techniques was used to detect the thermodynamic parameters of heat denaturation and of ice-water phase transitions for native and denaturated DNA, at different low water contents. We suggest that the main contribution to the enthalpy of the process of the heat denaturation of DNA duplex (35±5 kJ/mol bp) is the enthalpy of disruption of the ordered water structure in the hydration shell of the double helix (26±1 kJ/mol bp). It is possible that this part of the energy composes the non-specific general contribution (70%) of the enthalpy of transition of all type of duplexes. For DNA in the condensed state the ratioα=ΔCp/ΔS ~2 is smaller than for DNA in diluted aqueous solutions (α≅2–4). This means that there are other sources for the large heat capacity change in diluted solutions of DNA – for example the hydrophobic effects and unstacking(unfolding) of single polynucleotide chains.

Enthalpy of formation of 4-hydroxypyridine

In this note, we report the re-determination of the enthalpy of combustion of 4-hydroxypyridine in Manchester and of its enthalpy of sublimation in Porto.

Standard enthalpies of formation and sublimation of some crystalline bis(8-hydroxyquinolate) metal complexes

Abstract Resulting from a collaboration among three departments, the standard enthalpies of formation of some crystalline bis(8-hydroxyquinolate) metal complexes were determined by reaction-solution calorimetry. Enthalpies of hydrolysis were measured for several samples of each complex using various acidic solvents. 8-hydroxyquinoline is written as Hox. At 298.15 K, Δ f H θ m (M(ox) 2 ,cr)/(kJ mol −1 ) values are: Mg(ox) 2 , −516.0 ± 4.0; Ca(ox) 2 , −607.7 ± 4.4; Sr(ox) 2 , −585.9 ± 4.1; Ba(ox) 2 , −573.4 ± 4.5; Cu(ox) 2 , −103.3 ± 3.3; Zn(ox) 2 , −271.5 ± 3.6; Cd(ox) 2 , −179.6 ± 4.3; Pb(ox) 2 , −145.3 ± 3.8; Mn(ox) 2 , −307.1 ± 3.7; Fe(ox) 2 , −219.7 ± 4.5; Co(ox) 2 , −160.4 ± 4.2; Ni(ox) 2 , −165.5 ± 5.6. Where feasible, standard enthalpies of sublimation at 298.15 K were determined by measuring vapour pressures as functions of temperature. Δ g cr H θ m (M(ox) 2 )/(kJ mol −1 ) values are: Mg(ox) 2 , 230.2 ± 4.0; Cu(ox) 2 , 168.7 ± 7.3; Zn(ox) 2 , 183.2 ± 6.3; Cd(ox) 2 , 201.7 ± 7.5; Pb(ox) 2 , 187.1 ± 6.2; Mn(ox) 2 , 194.6 ± 10.4; Co(ox) 2 , 205.3 ± 4.0; Ni(ox) 2 , 175.4 ± 6.7. From the standard enthalpies of formation in the gaseous state, the mean metal-ligand dissociation enthalpies were derived and compared with corresponding values for other complexes of these metals.