Enthalpy of formation of triphenylphosphine sulfide

The first measurements of the enthalpies of combustion, sublimation, and fusion of an organo-phosphorus sulfide, triphenylphosphine sulfide, are reported: _c H _m ^o (C₁₈H₁₅PS, cr)=–(10752.58 ±2.90), _sub H _m ^o (C₁₈H₁₅PS, 403 K)=(136.80±6.09), and _fus H _m ^o (C₁₈H₁₅PS, T_m=435.92 K) =(30.53±0.21) kJ·mol^–1. Correction of the phase change enthalpies toT=298.15K and p^o =0.1 MPa results in the standard phase change enthalpy values of _sub H _m ^o (298.15 K)=(142.8 ±6.8) and _fus H _m ^o (298.15 K)=(19.28±0.21) kj·mol^–1. Accordingly, the enthalpies of formation of solid, liquid, and gaseous triphenylphosphine sulfide are derived: _f H _m ^o (C₁₈H₁₅PS, cr) =(63.20±2.56), _fH _m ^o (C₁₈H₁₅PS, l)=(82.48±2.57), and _fH _m ^o (C₁₈H₁₅PS, g)=(206.0±7.3) kJ·mol^–1. From these ancillary data, the P=S double-bond enthalpy is 394 kJ-mol^–1 and in good agreement with earlier reaction calorimetry results. These phosphorus sulfide values are compared with those for the arsenic sulfides. Plausibility arguments are given for our results.     

What are the relative steric demands of carboxyl and methyl groups?

The relative steric demands of carboxyl and methyl groups are compared by contrasting the difference quantity H _f ^o (g, ArCOOH) — H _f ^o (g, ArCH₃) for a collection of alkylated benzoic acids and toluenes with the value for Ar=C₆H₅, the archetypical (i.e., unsubstituted) benzoic acid and toluene. We conclude that carboxyl and methyl groups are nearly the same size.     

Kinetics of enzymatic hydrolysis of sodium carboxymethylcellulose with different degrees of polymerization by cellulase

The reaction cellulase (EC 3.2.1.4)—sodium carboxymethylcellulose (Na-CMC) with different degrees of polymerization (n=140, 640 and 900) was investigated by the use of a modifiedMichaelis-Menten equation, valid for enzymatic hydrolysis of linear homopolymers. TheMichaelis-Menten constant [Km (M)=6.31·10^–2mol/dm³] and the reaction rate constant (k ₊₂=4.07·10^–6s^–1), which correspond to the enzymatic hydrolysis of a single bond in the homopolymer substrates are determined. The free energy ( =101 kJ/mol), which corresponds to the degradation and formation of a single bond in the enzyme—polymer substrate is also estimated. This energy expressed in electronvolt units is =1.39 eV. The ratio between the effective cross section of the reactive substrate bond and the active enzyme center is =1.22.

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Kinetik der enzymkatalysierten Hydrolyse von Natriumcarboxymethylcellulose mit verschiedenem Polymerisationsgrad durch Cellulase

Zusammenfassung Die modifizierte Gleichung nachMichaelis-Menten wird bei der durch Cellulase (EC 3.2.1.4) katalysierten hydrolytischen Spaltung von Natriumcarboxy-methylcellulose (Na-CMC) verschiedenen Polymerisationsgrades (n=140, 640 und 900) angewandt. Es wurde dieMichaelis-Menten-Konstante [Km (M)=6.31·10^–2mol/dm³] und die Reaktionsgeschwindigkeitskonstante (k ₊₂=4.07·10^–6s^–1), die der enzymatischen Hydrolyse einer Einfachbindung im homopolymeren Substrat entspricht, berechnet. Die freie Energie ( =101 kJ/mol), die dem Abbau und der Bildung einer Einfachbindung im Enzym—Polymer-Substrat entspricht, wurde bestimmt. Diese Energie — ausgedrückt in Elektronvolt-Einheiten — beträgt =1.39 eV. Das Verhältnis zwischen den effektiven Querschnitten der reaktiven Substratbindung ( _S ) und des aktiven Enzym-Zentrums ( _E ) beträgt =1.22.

     

Thermodynamics of ionization of aqueous sulfamic acid,an almost-strong electrolyte

We have measured enthalpies of dilution of aqueous sulfamic acid solutions at 25°C and used results of these measurements to calculate the standard enthalpy of ionization of sulfamic acid. The average H _1on ⁰ obtained in this work was 735±200 J-mol^–1. We have also measured enthalpies of solution of crystalline sulfamic acid in water at 25°C. The results from both measurements are combined with some earlier results from Wu and Hepler to obtain a best value for the standard enthalpy of solution, H _S ⁰ =19.2±0.2 kJ-mol^–1.     

Theoretical studies of [n]paracyclophanes and their valence isomers

Summary The valence isomerisations of benzene, [6]- and [7]paracyclophane to their Dewar benzene and prismane isomers are studied with the MNDO method using the unrestricted Hartree-Fock (UHF) and the configuration interaction (C.I.) approximations. The enthalpy of the reaction Dewar benzene benzene is H° _r =–68.9 kcal/mol and the activation enthalpy is H°=27.9 kcal/mol (with C.I.). The reaction path hasC _2v symmetry.The determination of several points of the lowest potential energy surface of [6]- and [7]paracyclophanes leads to a minimum reaction path having the same topology as for the potential energy surface of the nonbridged benzene. The only difference is a quantitative change in the energy values of the aromatic isomers due to the deformation introduced by the alkyl chain. For [6]paracyclophane, the activation enthalpy is H°=24.6 kcal/mol and the activation entropy is S ⁰=0.6 cal K^–1 mol^–1 calculated with C.I.The enthalpy of the reaction prismane Dewar benzene is H° _r –32 kcal/mol and the activation enthalpy is H°19 kcal/mol. The highest molecular symmetry group common to both molecules isC _2v , whereas the symmetry group of the reaction path is lowered toC _s . Along this reaction path is located a biradicaloid intermediate, separated by low activation barriers from the products. No significant changes of the potential energy surfaces are found for the bridged [n]prismanes and the [n]Dewar benzenes.All the calculated values, reaction enthalpies, activation enthalpies and entropies, are in a good agreement with literature experimental data.This article is dedicated to Professor A. Pullman     

Thermochemistry of aqueous solutions of alkylated nucleic acid bases V. Enthalpies of hydration of N-methylated adenines

Enthalpies of solution in water, H _sol ^o , and of sublimation, H _subl ^o , were determined experimentally for a number of crystalline N-methyl adenines: m⁶Ade, m ₂ ^6,6 Ade, m⁹Ade, m ₂ ^6,9 Ade, and m ₃ ^6,6,9 Ade. Derived standard enthalpies of hydration H _hydr ^o , were corrected for the calculated cavity terms H _cav ^o to yield enthalpies of interaction H _int ^o of the solutes with their hydration shells. The increments of H _int ^o per unit area of the water-accessible molecular surface S _B , H _int ^o (CH₃)/S _B (CH₃), for the particular methyl groups: is considered to be the net effect of the gain in the energy resulting from van der Waals' interactions and of the loss in the energy due to polar interactions upon methyl substitution. It proved to vary somewhat numerically in agreement with the theoretically predicted hydration schemes of adenine. Comparison of H _int ^o /S _B value for adenine with those previously determined for uracil and thymine indicates that the aminopurine moiety is less hydrated than the diketopyrimidine ring.     

Thermodynamic study of the three isomers of bromo and iodobenzoic acids. Part II

Using our technique of combustion of small amount of a substance, we determined by calorimetry the standard molar enthalpy of formation in the condensed state and atT=298.15 K of the three isomers of bromo and iodobenzoic acids. Associating to these values their standard molar enthalpies of sublimation previously measured, it was possible to determine their standard molar enthalpies of formation in the gaseous state and atT=298.15 K. The experimental values of the thermodynamic properties _f H _m ^o (cr, 298.15 K), _f H _m ^o (cr, 298.15 K), _sub H _m ^o (298.15 K), and _f H _m ^o (g, 298.15 K) are given for the two series. From the experimental value of the standard molar enthalpy of atomization, it was possible to determine an enthalpy value for the C_b-Br and C_b-I bonds. The experimental and theoretical values of the resonance energy of bromo and iodobenzoic acids are compatible. The relative stability of some monosubstituted derivatives of benzoic acid studied in our laboratory is also discussed.Part I is concerned with Ref. 22 (for bromobenzoic acids) and with Ref. 23 (for iodobenzoic acids).     

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. IX. Enthalpies of hydration of 9-methyl-8-alkyladenines and 6,9-dimethyl-8-alkyladenines

Enthalpies of solution in water, H _sol ^o , and enthalpies of sublimation, H _sub ^o , were determined experimentally for a number of crystalline derivatives of adenine: 6,8,9-trimethyladenine; 6,9-dimethyl-8-ethyladenine; 6,9-dimethyl-8-propyladenine; 6,9-dimethyl-8-butyladenine; 8,9-dimethyl-adenine and 9-methyl-8-ethyladenine. Standard enthalpies of hydration, H _hydr ^o , derived from these data were calculated. The latter were discussed together with the values for variously alkylated adenines, determined previously. The data obtained show that the dependence of enthalpy of hydration on the number of methylene groups added upon substitution with 8-n-alkyl groups of 9-methyladenine and 6,9-dimethyladenine is nonlinear.     

The use of DSC to characterize structural relaxation in thermosetting polymers

Structural relaxation in different epoxy-anhydride and epoxy-diamine resins has been investigated by differential scanning calorimetry using annealing and cooling rate experiments. The annealing experiments lead to the determination of enthalpy loss,H, at an equivalent annealing temperatureT _a=T _g-20, and for periods of annealing time, t_a, between 1 h and 4 months. The variation ofH with logt_a, defines a relaxation rate per decade,rrpd, which is very sensitive to changes of the epoxy network. The cooling rate experiments allow the determination of the apparent activation energy,h ^*. The effect of the degree of crosslinking, the addition of a reactive diluent, which acts as flexibilizer, and the length of cross-link onrrpd and h^* was studied.Financial support has been provided by DGICYT (Project no.PB93/1241). The authors are grateful to CIBA-GEIGY for supplying the epoxies and hardeners, and to HUNTSMAN CORPORATION EUROPE for supplying the JEFFAMINE*, J.M.H. wishes to acknowledge assistance for a sabbatical period from theMinisterio de Education y Ciencia.     

Isothermal titration calorimetric studies of surfactant interactions with negatively charged, ‘hairy’ latex nanoparticles

Isothermal titration calorimetry (ITC) measurements of the mixture of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) with negatively charged, hairy copolymer latices (poly-(2,3-epoxypropylmethacrylate-co-methacrylic acid) in different ratio) at high water excess indicate a monomer adsorption mechanism of CTAB by the polymer particles. The number of adsorbed CTAB molecules at saturation corresponds approximately to the number of negative elementary charges bound at the surface of the latices. The mixing enthalpy is the sum of demicellization and sorption enthalpies. At 25 °C for CTAB the demicellization enthalpy amounts to 10 kJ/mol, whereas the adsorption enthalpy varies from –7 kJ/mol (surface charge density of the latices =–0.37 C/m²) to +3 kJ/mol (=–0.085 C/m²). The hydrodynamic radius R_H of the latex particles upon titration of cationic detergent and salt (NaBr) decreases by about 2 nm until the onset of aggregation near the isoelectric point. Titration of nonionic or anionic detergents has much less influence on the hydrodynamic radius and produces no measurable adsorption heat. The results are consistent within a model of latex particles with extended negatively charged polymer chains interacting predominantly via Coulombic forces with detergents.     

Quadrupole polarizability and hyperpolarizability of carbon monoxide

Summary We report a study of the electric dipole-quadrupole (A _,,), quadrupole-quadrupole (C _,,), dipole-octopole (E _,) polarizability and the dipole-dipole-quadrupole (B _,,) hyperpolarizability of carbon monoxide. All values are obtained from finite-field self-consistent field (SCF) and fourth-order manybody perturbation theory (MP4) calculations. Our best values for the dipole-octopole polarizability areE _z,zzz=60.19 andE _x,xxx=–38.06e ² a ₀ ⁴ E _h ^–1 . For the dipole-dipole-quadrupole hyperpolarizability we reportB _zz,zz=–296,B _xz,xz=–170,B _xx,zz=88 andB _xx,xx=–178e ³ a ₀ ⁴ E _h ^–2 .     

Enthalpy-entropy compensation in ionic micelle formation

The enthalpy-entropy compensation in ionic surfactant micellization process over a large temperature range is examined. The surfactants SDS and C₁₆TAB are investigated experimentally, and the enthalpy and entropy changes are evaluated based on phase separation or mass action models together with the other three surfactant systems. The relationship between compensation temperature and the reference temperatures is discussed.Notations C _p heat capacity change, J/mol-K - CMC critical micelle concentration,M - CMC₀ critical micelle concentration atT=T ₀,M - G Gibbs free energy change, kJ/mol - H enthalpy chang, kJ/mol - h _c enthalpy change for transfer of a methylene group to water, kJ/mol - R gas constant, 8.314 J/mol-K - S entropy change, J/mol-K - S _c entropy change for transfer of a methylene group to water, J/mol-K - S ^* entropy change atT=T ^*, J/mol-K - T temperature,K - T _c compensation temperature, K - T _H temperature at which H=0, K - T ₀ temperature at the minimum point, K - T ^* 112°C Greek Letters degree of dissociation     

Charge control of phenol complexation with unsaturated compounds containing group IVB organometallic substituents. Interpretation of NMR spectra of acetylenic derivatives

²⁹Si and ¹¹⁹Sn chemical shifts in the NMR spectra of acetylenic derivatives Me₃ECCX (E = Si or Sn; X — organic substituents) and shifts of frequencies () of stretching vibrations of OH groups in the IR spectra of phenol when hydrogen-bonded to these compounds have been analyzed. In each of two series (E = Si or Sn), the and values are connected by a linear relationship; this indicates that there is virtually no effect of the magnetic anisotropy of the X substituents on the chemical shifts. It has been established that the shifts of the frequencies and the ²⁹Si and ¹¹⁹Sn chemical shifts are the relative characteristics of the effective negative charge on the carbon atoms of the triple bond in Me₃ECCX compounds.For the previous communication of this series, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2069–2073, December, 1993.     

Enthalpies of dilution for 1∶3 and 3∶1 electrolytes aqueous solutions

The enthalpies of dilution for aqueous solutions of [Co(cn)₃)]Cl₃, [Co(pn)₃)]Cl₃ and [Co(tn)₃]Cl₃, (whereen=1,2-diaminoethane,pn=1,2-diaminopropane, andtn=1,3-diaminopropane) have been measured at 298.15 K, and up to 1 mol kg^–1+, using a new large isoperibol calorimeter by the long-jump method. Relative apparent molar enthalpies,_291-01, have been extracted as an empirical equation relatingL andm. We have compared our complex salts with the 31 and 13 aqueous systems found in the literature. Theoretical predictions for 13 and 31 electrolytes in the Restricted Primitive Model are also shown.This work was supported in part by CICYT (M.E.C., Spain) under Research Project No.: PB92-0553.     

Microcalorimetric determination of enthalpies of solution of methyl alkanes in glacial acetic acid

An empirical equation H _{s, m}=0.84n gives the enthalpy of solution of 30 branched, linear and cyclic alkanes in glacial acetic acid. The parametern is an effective carbon number which expresses the number of carbon atoms in the longest chain plus one half of the pendant methyl groups.     

Magnetic properties of transition metal ion cubic 5T2 terms in axial ligand fields

The theory of magnetic anisotropy and susceptibility is worked out for cubic ⁵T₂ terms, the degeneracy of which is partially lifted by a ligand field component of axial symmetry as well as by spin-orbit coupling. Matrix elements are calculated by application of the method of Abragam and Pryce to a set of M.O. based wave-functions. The anisotropy in covalency of the metal-ligand bond and in spin-orbit coupling is taken into account. Numerical values of principal magnetic moments, P and P , are calculated as function of kT/, /, and .The theoretical results are employed in a rigorius analysis of existing single crystal magnetic data on high-spin iron(II) compounds. For (NH₄)₂Fe(SO₄)₂·6 H₂O, =–100 cm^–1, =1070 cm^–1 and =0.8 to 0.6 is obtained. For FeSiF₆·6 H₂O, =–80 cm^–1, =–760cm^–1 at 77.3K and –580 cm^–1 between 20.4 and 1.57K, and 0.7 is derived. No unique fit is possible for K₂Fe(SO₄)₂·6 H₂O. The data are reproduced to better than ±1% in most cases. The limitations of the approach are stressed.

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Zusammenfassung Die Theorie der magnetischen Anisotropie und der magnetischen SuszeptibilitÄt eines ⁵ T ₂ Grundterms im oktaedrischen Ligandenfeld wird für den Fall entwickelt, da\ die Entartung unter dem Einflu\ einer axial-symmetrischen Feldkomponente sowie der Spin-Bahn-Wechselwirkung teilweise aufgehoben ist. Eigenfunktionen werden auf der Grundlage der M.O.-Theorie aufgestellt, Matrixelemente mittels der Theorie von Abragam und Pryce ermittelt. Der Anisotropie der Metall-Ligand-Bindung sowie der Spin-Bahn-Kopplung wird Rechnung getragen. Numerische Werte für die magnetischen Hauptmomente P und P werden in AbhÄngigkeit von kT/, / und berechnet.Die Ergebnisse der Theorie werden für eine genaue Analyse der verfügbaren magnetischen Daten aus Messungen an Einkristallen magnetisch normaler Eisen(II)-Verbindungen verwendet. Für (NH₄)₂Fe(SO₄)₂·6 H₂O werden =–100 cm^–1, = 1070 cm^–1 und = 0,8 bis 0,6 erhalten. Für FeSiF₆·6 H₂O ergeben sich =–80 cm^–1, =–760 cm^–1 bei 77,3K und =–580 cm^–1 zwischen 20,4 und 1,57K sowie 0,7. Die experimentellen Daten können in den meisten FÄllen auf ±1% genau oder besser wiedergegeben werden. Bei K₂Fe(SO₄)₂·6 H₂O gelang keine eindeutige Bestimmung der theoretischen Parameter. Die Grenzen der vorliegenden Behandlung werden kritisch diskutiert.

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Résumé La théorie de l'anisotropie magnétique et de la susceptibilité magnétique est développée pour les termes cubiques ⁵ T ₂, dont la dégénérescence est partiellement levée par une composante à symétrie axiale du champ de ligandes ainsi que par le couplage spin-orbite. Les éléments de matrice sont calculés à l'aide de la méthode d'Abragam et Pryce employée sur un système M.O. des fonctions d'onde de base. On a pris en consideration l'anisotropie de la liaison entre le metal et le ligand et aussi du couplage spin-orbite. Les valeurs numériques des moments magnétiques principaux P et P sont calculées en fonction de kT/, /, et .Les résultats théoriques sont employés dans une analyse détaillée des données magnétiques disponibles sur monocristaux des composés ferreux spin-élevés. Pour (NH₄)₂Fe(SO₄)₂·6 H₂O on a obtenu =–100 cm^–1, =1070 cm^–1 et =0.8 à 0,6. Pour FeSiF₆·6 H₂O on a eu comme résultat =–80 cm^–1, =–760 cm^–1 à 77,3 K et =–580 cm^–1 entre 20,4 et 1,57K avec 0,7. Les valeurs expérimentales sont réprésentées dans la plupart de cas plus précises que ±1%. Au cas de K₂Fe(SO₄)₂·6 H₂O une détermination unique était impossible. Les limites de cette approximation sont discutées.

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Postdoctoral Fellow, Theoretical Chemistry, 1963–1964.     

Frozen-liquid PMR spectra and interactions of water with spherical carbon adsorbents

The PMR spectrum from a spherical carbon adsorbent shows separate signals from the water in the pores ( 2–0 ppm) and outside the granules ( 5 ppm) at temperatures above 273 K because of the short spin relaxation times in the presence of numerous paramagnetic centers. Below 273 K, the water bound to the carbon after heat treatment above 1273 K may be in two states: A ( _A 2–0 ppm) and B ( _B –12 to –13 ppm). The enthalpy of (H₂O) _A (H₂O) _B is –50 kJ/mole. State B is associated with the graphitization occurring above 1273 K. It is suggested that the anomalous chemical shifts in states A and B are due to axial coordination in relation to the condensed benzene ring systems at the negatively charged surfaces.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 1, pp. 111–115, January–February, 1990.     

Dampfdruckmessungen und Protonenresonanzuntersuchung an Hydriden der intermetallischen Phasen Ti2(Ni,Co) und Ti2(Ni,Fe)

NMR and hydrogen equilibrium pressure measurements were performed on hydrides of the intermetallic compounds Ti₂(Ni, Co) and Ti₂(Ni, Fe). The following values of enthalpy H and entropy S for the formation of the hydrides of the intermetallic phases Ti₂Co and Ti₂Ni were found: H(Ti₂CoH _y )=–47.6 kJ/mol H₂, H(Ti₂NiH _y )=–53.7 kJ/mol H₂; S(Ti₂CoH _y )=–119.8 J/(K·mol H₂), S(Ti₂NiH _y )=–127.5 J/(K·mol H₂). By substitution of Ni or Co by Fe, the values of H and S of the corresponding quaternary hydrides become less negative. An interpretation of the experimental results is tried by the model ofShaltiel and coworkers.Proton diffusion was investigated in a series of the intermetallic hydrides Ti₂(Ni, Co)H _x and Ti₂(Ni, Fe)H _x . The diffusion rate is lowered by increased Ni/Fe substitution. Substitution of Ni by Co scarcely effects the hopping process. The activation energies were found to be smaller for the Ti₂Ni-hydrides compared with the Ti₂Co-hydrides.

Herrn Prof. Dr.H. Nowotny zum 70. Geburtstag gewidmet.     

An experimental test of the double solubility rule

Vapor pressures, fusion (H_fus), and sublimation (H_s) enthalpies are reported ford, dl-2,3-dibromobutane-1,4-diol, andd,dl-dimethyl 2,3-diacetyltartrate. Values of 8.1, 7.04, 6.9, and 6.6 kcal/mol for H_fus were measured by differential scanning calorimetry and values of 28.4, 27.3, 26.2, and 25.5 kcal/mol (not corrected to 25) were calculated for H_s from the temperature dependence of vapor pressure, respectively. The vapor pressure ofdl-2,3-dibromobutane-1,4-diol, which crystallizes as a conglomerate, exceeds the vapor pressure of thed form by more than a factor of 2 over the temperature range studied. The ratio of vapor pressures ofdl- andd-dimethyl 2,3-diacetyltartrate, which crystallizes as a racemic compound, is approximately 1.5.     

A simplified View of δ → δ* Transition Energies in Compounds with multiple metal-metal bonds: The isolated δ — δ* manifold model

The energetics of the * transition in quadruply bonded complexes are investigated using a very simple valence-bond formalism, called the isolated * manifold (IDDM) model. In this model all electrons except for those that occupy the or * molecular orbitals are ignored, as are explicit metal-ligand interactions. The resulting equations allow the calculation of transition energies very inexpensively, albeit with poor quantitative agreement: the * transition in prototypical quadruple-bond systems is predicted to occur at energies greater than 70,000 cm¹. The model incorporates configuration interaction between the two¹ A _1g configurations (|| and |**|) to roughly the same extent as do correlated all-electron calculations. The application of the method to systems that involve relative changes in * transition energies, such as the torsional twisting of quadruple bonds, is more successful quantitatively.