Some solid state properties of enantiomers and their racemates

Heats of sublimation and fusion for l and dl-menthol, d and dl-carvoxime and d and dl-dimethyl tartrate were measured. Values of 22.9, 18.8, 21.7, 24.3, 18.5 and 26.9 $\text{kcal}\text{mole}$ and 18.2, 15.7, 24.3, 30.9, 23.3 and 36.2 $\text{cal}\text{g}$ were obtained for heats of sublimation and fusion respectively. Heats of sublimation are believed known to ±5% while heats of fusion are believed qualitatively correct. Generally good agreement was observed with literature values when available. Higher heats of fusion and sublimation are observed for the enantiomer or racemate with the higher fusion temperature. Correlation of these physical properties with the phase diagram for fusion and solubility suggest that differences in the temperature dependence of vapor pressure and solubility might be useful diagnostic tests for discontinuities in molecular packing and phase behavior in the solid state. Prediction of the temperature where racemic mixture to conglomerate transitions occur would be extremely useful for resolving racemates.     

Heats of formation of organic molecules by Ab Initio calculations: Carboxylic acids and esters

A bond and group equivalent scheme that allows the calculation of heats of formation for carboxylic acids and esters from ab initio 6-31G* energies has been developed. For a group of 16 compounds, the rms error for the calculated heats of formation was 0.64 kcal/mol. Heats of formation have been predicted for an additional seven compounds for which the experimental values are either unknown or suspect. © 1992 by John Wiley & Sons, Inc.     

Thermochemische Untersuchungen an cyclischen Ketonen

The enthalpies of formation of the nine cyclic ketones C_nH_2n?2O (n = 4–12) were determined by combustion calorimetry. The heats of vaporization were determined by measuring the vapor pressure as a function of temperature and the heats of fusion of the three compounds with n = 8, 9, 12 which are solids at room temperature, by DSC.     

Solvation of bivalent transition metal cations. I. Heats of transfer of the metal perchlorates between water,acetonitrile, and dimethylsulfoxide

Heats of solution of Mn(ClO₄)₂·6AN, Fe(ClO₄)₂·6AN, Co(ClO₄)₂·6AN, as well as Ni(ClO₄)₂·6AN, Cu(ClO₄)₂·4AN, and Zn(ClO₄)₂·6AN (AN=acetonitrile) have been determined in water, acetonitrile, and dimethylsulfoxide (DMSO). Coupled with the heats of solution of AN in the respective solvents they give the heats of transfer of the metal perchlorates. Heats of transfer of the cations as well as their heats of solvation are calculated using literature data for the heat of transfer of the perchlorate anion and the heats of hydration of the cations, respectively. The results are discussed in terms of the different contributions to the overall heat effects.     

Side-chain crystallinity. I. Heats of fusion and melting transitions on selected homopolymers having long side chains

Heats of fusion, melting transitions, and the derived entropies of fusion were obtained by differential scanning calorimetry for examples from three homologous series of homopolymers having long side chains. Homopolymers having side-chain lengths between 12 and 22 carbon atoms were chosen from the poly(n-alkyl acrylates), the poly(N-n-alkyl-acrylamides) and the poly(vinyl esters). The data demonstrated that only the outer paraffinic methylene groups were present in the crystal lattice. This was concluded because phase diagrams obtained for mixtures of structurally different monomers and homopolymers, as well as for selected copolymers, showed only isomorphism in the polymeric examples. In addition, scanning curves, reflecting the distribution of crystallite sizes, became narrower as the side chains became longer. The critical chain length required to maintain a stable nucleus in the bulk homopolymers was a constant value for each homologous series. It varied between 9 to 12 carbon atoms. When heats of fusion were determined in the presence of methanol, main-chain restraints were freed, thus permitting more methylene groups to enter the crystal lattice. Hence, the heats of fusion, the crystallinity, and melting points increased above that of the bulk state. The magnitude of the contribution to the heats of fusion by each methylene group indicated that the hexagonal paraffin crystal modification prevailed in these homopolymers, in agreement with x-ray data from the literature.     

Deuterium isotope effect on the heats of hydration of barium chloride

Heats of solution of barium chloride, its mono- and dihydrates, and the deuterated hydrates at 25°C have been measured and combined to calculate the deuterium isotope effect on the heats of hydration of barium chloride. Substitution of D₂O for H₂O in the hydrates enhances the heats of dehydration by 1–2%.     

Heats of mixing of atactic polybutene-1 and of a model molecule with normal alkanes-order in long alkanes

Heats of mixing at infinite dilution of atactic polybutene-1 in the alkanes have been measured at 25° with a Calvet microcalorimeter. Heats for a model molecule, the 3,5-dimethylheptane, in the same alkanes were obtained also. The enthalpy parameter X_H was found to be very close for the two sets of systems. Fitting the experimental data with the calculated free volume term of the heats, the parameter X₁₂ (as expressed in the Flory theory) is found to go through a minimum with chain length. This feature is explained thus: for longer alkanes, the large heat is thought to be due to loss of order in the chain solvent. For the short alkanes, X₁₂ is found large to compensate an overestimation of the free volume term.     

Détermination des chaleurs de formation des orthovanadates de certains éléments des terres rares par la méthode d'analyse thermique différentielle

The heats of formation of LnVO₄-type salts (whereLn=Y, La, Sm, Nd, Gd and Dy) were determined by thermal analysis in the reactions of melts of stable lanthanides and V₂O₅ in a 1 ∶ 1 mole ratio. Heats of formation of LnVO₄-type salts can be determined using non-stoichiometric mixtures of the substrates as well. With the aid of DTA the apparent activation energies of the syntheses of the orthovanadates of these lanthanides were calculated.     

Thermochemical study of processes of complex formation of Cu<Superscript>2+</Superscript> ions with L-glutamine in aqueous solutions

Heats of the interaction of Cu(NO₃)₂ solutions with L-glutamine solutions were measured directly by calorimetry at a temperature of 298.15 K and ionic strength values of 0.5, 1.0, and 1.5 (KNO₃). Using RRSU universal software, the experimental data were subjected to rigorous mathematical treatment with allowances made for several concurrent processes in the system. The heats of formation of the CuL⁺ and CuL₂ complexes were calculated from the calorimetric measurements. The standard heats of the complex formation of Cu²⁺ with L-glutamine were obtained by extrapolation to zero ionic strength. The complete thermodynamic characteristic (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o) of the complex formation processes in a Cu²⁺—L-glutamine system was obtained.     

Enthalpies of solution of polyvinylalcohol in water and polyvinylacetate in dioxane

Heats of solution of Polyvinylalcohol (PVA) and Polyvinyl-acetate (PVAc) in Water and Dioxane have been measured by using an isothermal titration calorimeter. The χ_H- parameter of the Flory-Huggins-theory has been evaluated from the calorimetric data. The system PVA + H₂O shows negative χ_H values and the system PVAc + Dioxane shows positive χ_H - values corresponding to exothermic and endothermic heats of dilution repectively.     

Thermal study of decomposition of selected biomass samples

Fears of climate change and increasing concern over the global warming have prompted a search for new, cleaner methods for electricity power generation. Technologies based on utilising biomass are attracting much attention because biomass is considered to be CO₂ neutral. Co-firing of biomass fuels with coal, for example, is presently being considered as a mean for reducing the global CO₂ emissions. Biomass is also applied in thermal conversion processes to produce fuels with higher calorific values and adsorbents. In any case, thermal decomposition is essential stage where volatiles and tars are evolved followed by consequent heats of reactions. In this work sawdust biomass samples were selected in order to study their thermal conversion behaviour. Heats of decomposition for each sample were measured during continuous heating at a prescribed heating rate under inert atmospheric conditions. The decomposition generally commenced in all samples at 150°C and was completed at 460°C in a series of endo and exothermic reactions influenced by its lignin and cellulosic content. Single biomass sample was subjected to heating rates ranging from 10 to 1000°C min^-1 and the effect of heating rate on decomposition was studied. The origin of reactions for each thermal sequence is herein discussed in detail. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heats of formation of organic molecules calculated from ab initio theory and a group equivalent scheme: Alkenes

A bond and group equivalent scheme that allows the calculation of heats of formation of alkenes from ab initio 6-31G* energies has been developed. For a group of 26 compounds, the root mean square (rms) error for the calculated heat of formation was 0.78 kcal/mol. Heats of formation have been predicted for an additional nine compounds for which the experimental values are either unknown or suspect. The heats of hydrogenation of barrelene and related compounds are discussed. © 1994 by John Wiley & Sons, Inc.     

Substituenteneffekte auf die C–C-Bindungsstärke, 8. Standardbildungsenthalpien und Spannungsenthalpien von 1,1,2,2-Tetraethylethylenglykol-dimethylether und D,L-1,2-Dimethyl-1,2-diphenylethylenglykol-dimethylether

Effects of Substituents on the Strength of C - C Bonds, 8¹. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol ^-1, s. d. in parentheses) ΔH°_c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔH^v = 14.3 (± 0.3) and the heat of sublimation of 2 ΔH^sub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°_t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (_s) of 15.9 and 8.0 kcal mol^-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group.     

Chain dynamics of high molecular weight polyethylene as observed from heats of dissolution in slow calorimetry

The Setaram C80 calorimeter with large cells permits accurate measurements of heats in dilute solution. Slow calorimetry is well suited to follow slow phase changes unnoticeable by fast calorimetry. Heats of solution of nascent high molecular weight polyethylene (HMWPE) have been obtained in a slow T-ramp (v=1–12 K-h^–1) in a variety of solvents. A new contribution to the enthalpy of fusion of polyolefins has been found. The total heat of dissolution thus contains two contributions due (1) to the melting of orthorhombic crystals and (2) the disordering of a network of entangled chains. An investigation is made of the new contribution (2), which can evolve at low temperature since strain does not build up due to the solvent. Conditions to change the range of temperature at which the second contribution evolves (nature of the solvent, rate of heating, concentration, cycles of dissolution/crystallization) are investigated.Presented at the Symposium, 76^th CSC Congress, Sherbrooke, Quebec, May 30-June 3, 1993, honoring Professor Donald Patterson on the occasion of his 65^th birthday.     

Solvation of bivalent transition metalcations. II. Heats of transfer of metal perchlorates between water and several amides

Heats of solution of acetonitrile, solvated or hydrated perchlorates, Mn(ClO₄)₂·6AN, Co(ClO₄)₂·6AN, Ni(ClO₄)₂·6AN, Cu(ClO₄)₂·4AN, Cu(ClO₄)₂·6H₂O, Zn(ClO₄)₂·6AN, and Zn(ClO₄)₂·6H₂O have been determined in NMF, DMF, DMA, and water. Complete or almost complete exchange of AN and water molecules in amides is inferred from the visible spectra determinations. The heats of transfer of perchlorate anion from water to DMA and NMF have been obtained from separately determined heats of solution of NaBPh₄, AsPH₄Cl·H₂O, NaClO₄, NaCl, and SiPh₄ in the respective solvents. The heats of transfer of cations from water to amides have been determined from the above data and the heats of solvation of cations using literature data for the heats of hydration.     

A New Generalized Bond Energy/Group Contribution Scheme for Calculating the Standard Heat of Formation of Monomers and Polymers. Part IV. Halocarbons

A self-consistent set of bond energy terms permitting estimation of heats of formation of halogen compounds is evolved. The overall precision attained is ±0.7 kcal/mole, which is considerably better than that of the experimental data. Heats of formation of halogen containing monomers and polymers, and heats of polymerization are calculated.     

Bond Energy Scheme for Estimating Heats of Formation of Monomers and Polymers. VI. Sulfur Compounds

The bond energy scheme is extended to sulfur compounds and heats of formation and atomization energy terms derived from thermochemical data reviewed to 1977, for bonds of sulfur with carbon, hydrogen, halogens, and oxygen atoms. A precision of ± 1 kcal/mole was attainable for the covalent bonds of divalent sulfur in the lowest oxidation state S(± II). The higher valency states: S(IV) and S(VI) involve polar contributions depending upon the electrouegativity of the combining atom as well as (d^π -p^π) orbital promotion energies which are specific to the compound and transferable to other molecules only with a limited precision, no better than about ± 3 kcal/mole. The atomization energy terms (E_a ^25°C) of various bonds of sulfur a are found consistent with the experimental bond dissociation energies and bear a relationship with bond lengths and force constants as observed in the previous work. Heats of polymer-forming reactions and heats of formation of sulfur-containing monomers and polymers are estimated from the newly derived bond energy terms.     

Effect of temperature on thermodynamic characteristics of the dissociation of glycylglycine in aqueous solutions of electrolytes

Heats of reaction of glycylglycine with nitric acid and potassium hydroxide solutions are determined by two calorimetric procedures at 288.15, 298.15, 308.15 K and an ionic strength of solution of 0.25, 0.50, and 0.75 in the presence of KNO₃. Standard thermodynamic characteristics (Δ_r H°, Δ_r G°, Δ_r S°, and Δ_p C°) are calculated for the acid-base reactions in aqueous peptide solutions. The effects of the concentration of background electrolyte and temperature on the heats of dissociation of glycylglycine are considered.     

Thermometric titration of some monoprotic and diprotic acids in aqueous and non-aqueous media

Some mono- and diprotic acids have been titrated thermometrically with strong alkalis in aqueous and non-aqueous media. Thermograms with sharp arrest points were obtained, from which heats of neutralization were measured. Heats of neutralization in the media used were compared and an effect attributable to hydrogen bonding was found.     

Evaluation of hydrophilic-hydrophobic character of zeolites by measurements of their immersional heats in water

Na-type faujasite was modified by SiCl₄ treatment under dealumination-silicon-exchange reaction. Their crystallinities and lattice constants were analyzed by x-ray diffraction, and their surface areas and pore structures were measured by nitrogen adsorption at 77 K. Their hydrophobic characteristics were evaluated by measurements of water adsorption and heats of immersion into weter, and compared with those of ZSM-5 zeolites which were highly silicious at the synthesis process.Adsorption amount ofn-hexane on modified faujasite did not change compared to that on original faujasite, however, the adsorbed amount of water greatly decreased. Heats of immersion in water decreased by the modification and were closely related with the Si/Al ratio of zeolites. Hydrophilic-hydrophobic character is discussed from the viewpoint of water contact angle.