Small Enthalpy Research Articles

NAD(P)+-NAD(P)H Models. 49. Proximity Effect of a Phenyl Group on the Electron Transfer Process

Abstract 1-Propyl-1,4-dihydronicotinamide (PNAH), 1-(3-phenylpropyl)-1,4-dihydronicotinamide (1a), 1-(2-phenylthioethyl)-1,4-dihydronicotinamide (1b), and 1-(4-phenylthiobutyl)-1,4-dihydronicotinamide (1c) and their 4-deuterated analogs are synthesized as NAD(P)H-models and subjected to the reduction of m-nitro-α,α,α-trifluoroacetophenone in the presence or absence of magnesium ion in acetonitrile. Kinetics at 323 K have been studied. Kinetic isotope effect is small (2.33) only when 1a or 1b is employed for the reduction in the presence of magnesium ion, whereas it appears large (≈3.2) under other conditions. The result has been interpreted in terms of steric interference by a proximate phenyl group on the approach of the substrate to form a binary (in the absence of magnesium ion) or ternary (in the presence of magnesium ion) complexes. Thermodynamic parameters for the formation of complexes between the models and magnesium ion have been studied. It has been elucidated that enthalpy of complexation is small for 1a than others. However, the small enthalpy of complexation is compensated by small entropy of complexation affording almost constant free-energy of complexation throughout the models studied.

Polarization optical study of a new orthorhombic-monoclinic phase transition in chromium-chlorine boracite, Cr3B7013Cl

Abstract In a previous study1 an electric field/temperature phase diagram investigation of Cr-Cl boracite suggested the existence of a phase transition in zero electric field between about 143 and 183K. However, the continuous temperature variation of spontaneous birefringence on (100)-cuts perpendicular to the orthorhombic c-axis failed to prove such a transition between 133 and 264K (TC)1. In the present study a first order phase transition, in zero electric field, from the orthorhombic (mm2) to a monoclinic (m) ferroelectric phase has indeed been identified, both by differential scanning calorimetry and polarized ligth studies on (110) and (111)-cuts. The passage from mm2 to m is accompanied by the formation of a domain pattern of extremely fine lamellae, similar to that of analogous transitions found in Fe-Cl, Co-Cl, Zn-Cl1 and Fe-I boracites1–2. The transition has a small enthalpy change (ΔH ≈ 0.12kJmole-1) and is sluggish : large transition range and thermal hysteresis (visual : on upheat 175–187K, ...

A hydrofluoric acid solution calorimetric investigation of glasses in the systems NaAlSi3O8-KAlSi3O8 and NaAlSi3O8-Si4O8

Glasses in the systems NaAlSi3O8-KAlSi3O8 and NaAlSi3O8-Si4O8 have been studied by means of hydrofluoric acid solution calorimetry at 50°C. Results indicate small negative enthalpies of mixing in the former system and small positive departures from ideality in the latter.

Kinetics of the reaction of pentacyanonitrosylferrate(II) with aliphatic amines

The reactions of pentacyanonitrosylferrate(II) with ethyl-, n-propyl-, n-butyl-, cyclohexyl- and benzylamines were studied in dilute aqueous solution at 8.6–9.6 pH and 15–35 °C. Nitrosation, diazotation and deamination processes take place in the reactions resulting in alcohols and N 2 gas as final products. On the basis of spectrophotometric and gasvolumetric experiments the rate law was determined as follows. v = k[RNH 2][Fe(CN) 5NO 2−] The dependence on pH was interpreted by the protonation equilibria of the amines. From the function of the logarithm of rate constants vs. reciprocal temperature, relatively small activation enthalpies (15–70 kJ mol −1) and extremely high negative activation entropies [(−80) − (−240) J K −1 mol −1] were found. The mechanism was interpreted by the analogy with nitrous acid diazotation. A parallel trend was observed between the rate constants at 25 °C and the basicity constants of the amines.

Infrared and Raman spectra, conformational stability, and vibrational assignment of divinylfluoroborane

The infrared spectra of gaseous (3500 to 450 cm −1 and solid (3500 to 50 cm −1) divinylfluoroborane, (C 2H 3) 2BF, and the Raman spectra (3500 to 20 cm −1) of the liquid and solid have been recorded. All of the normal modes have been assigned, based on band contours, depolarization values and group frequencies. From a comparison of the spectra in the fluid and solid states, it is concluded that the molecule exists in at least two conformations in the fluid phase but mainly as one conformer of C s symmetry in the solid state. The failure to obtain a “good” crystalline sample in the solid state is believed to be due to a very small enthalpy difference between the conformers. The results are compared to the corresponding quantities in several similar molecules.

Computer assisted differential thermal analysis at high pressures: Phase studies on liquid crystals

In the present work it is shown that phase transitions exhibiting very small transition enthalpies can be detected by computer assisted DTA especially at high pressures. The method is demonstrated on measurements for the liquid crystal 4-octyloxy-4′-cyanobiphenyl (80CB) up to 2 kbar.

Thermochemistry of (Fe 2+, Mg)SiO 3 orthopyroxene

Enthalpies of solution in eutectic (Li, Na) 2B 2O 4 melts at 1023 K were measured for five synthetic orthopyroxenes on the join MgSiO 3-FeSiO 3. The pyroxenes were synthesized at 1120°C and 20 kbar and thus were presumed to be highly disordered. The measurements indicate a small positive enthalpy of mixing, with W H = 950 cal/ MSiO 3. Enthalpy of solution measurements were made on a natural, well-ordered orthopyroxene near the composition En 52.5Fs 47.5 and on this material after heat-treatment at 1150°C and 20 kbar. Irreversible expansion of the unit-cell constants of the natural pyroxene after heat-treatment at various temperatures was used to characterize the degree of M-site disorder. The observed enthalpy of solution decrement of 0.85 kcal/MSiO 3 between the natural En 52.5 and the same material heated at 1150° corresponds to about half of the maximum possible disordering, or ΔX Fe M1 ≅ 0.25, which leads to a ΔH of 7.5 kcal/M 2Si 2O 6, for the exchange reaction: Fe( M2) + Mg( Ml) = Fe( Ml) + Mg( M2) if M-site interaction energy terms are ignored. This Δ H is larger than inferred from any of the analyses of site-occupancy data except that of Besancon (1981), who gave a very similar value. The measured Δ H of disorder and the W H of mixing together indicate a large Δ H as great as 3.2 kcal for the reciprocal reaction: Fe 2 Si 2 O 6 + Mg 2 Si 2 O 6 = Fe( M2) Mg( M1) Si 2 O 6 + Fe( M1) Mg( M2) Si 2 O 6 as anticipated by Sack (1980). As a consequence of the inferred magnitudes of ΔHof the exchange and reciprocal reactions, departures from ideality of Gibbs energy of mixing of orthopyroxene are very small at 700°–1000°C. Activities of MgSiO 3 and FeSiO 3 may be replaced by their mol fractions at all temperatures in most petrologic calculations.

The interaction between bovine serum albumin and the molybdate ions

The binding of molybdenum(VI) has been studied with BSA using polarographic (pH 5.57) and equilibrium dialysis (pH 5.57, 7.50 and 9.50) techniques. The intrinsic association constants and the number of binding sites have been calculated from Scatchard plots. The effect of temperature on these constants was studied at pH 5.57 by polarographic technique and it was found that these values were decreased with increasing temperature. The effect of pH was found to be similar by equilibrium dialysis technique. The number of sites available for binding with the molybdate ions is much less than the actual number of cationic groups, which are 26, 21 and 18 at pH values 5.57, 7.50 and 9.50 respectively. Statistical effects seem to be more pronounced at lower molybdate ion concentrations and electrostatic at higher concentrations. The values of different thermodynamic parameters have been reported. The large positive entropy change of binding, coupled with the small enthalpy change of binding, have been interpreted to mean that the major contribution to the free energy of binding comes from the release of the solvent molecule from the molybdate-BSA complex.

The Metabolemeter II-Detection of Mesogenic Polymorphism under Pressure Using Small Quantities

Abstract Every first order phase transition for mesogens can be detected by the continuous recording, as a function of temperature, of the pressure of the sample enclosed in a metallic cell. Studies of a compound (octylcyanobiphenyl) exibiting several transformations (crystal-smectic A, smectic A-nematic, nematic-liquid) show that this barometric method is convenient to detect polymorphism from one thermobarogram, and to draw the Clapeyron phase diagram; the smectic A-nematic transition involving small enthalpy and volume changes is clearly observable and this method is very sensitive. Moreover, the enthalpies and volumes of transition can be deduced from the pressure changes. Lastly, pressure and temperature are intensive data, and very little sample (1.3 mm3) may be used.

The interaction between bovine serum albumin and the molybdate ions

The binding of molybdenum(VI) has been studied with BSA using polarographic (pH 5.57) and equilibrium dialysis (pH 5.57, 7.50 and 9.50) techniques. The intrinsic association constants and the number of binding sites have been calculated from Scatchard plots. The effect of temperature on these constants was studied at pH 5.57 by polarographic technique and it was found that these values were decreased with increasing temperature. The effect of pH was found to be similar by equilibrium dialysis technique. The number of sites available for binding with the molybdate ions is much less than the actual number of cationic groups, which are 26, 21 and 18 at pH values 5.57, 7.50 and 9.50 respectively. Statistical effects seem to be more pronounced at lower molybdate ion concentrations and electrostatic at higher concentrations. The values of different thermodynamic parameters have been reported. The large positive entropy change of binding, coupled with the small enthalpy change of binding, have been interpreted to mean that the major contribution to the free energy of binding comes from the release of the solvent molecule from the molybdate-BSA complex.

A study on diffuse antiferroelectric-paraelectric phase transition in pseudobinary Pb(Fe1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 solid solution

An antiferroelectric-paraelectric diffuse phase transition in pseudobinary (1−x)Pb(Fe1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 solid solution for 0.60≤x≤1.00 is investigated by its dielectric property and calorimetric measurements. The dielectric constant shows a diffuse phase transition behavior for 0.60 ≤x≤0.88, but shows a sharp phase transition behavior for 0.90≤x≤1.00. The apparent dielectric transition temperature increases with increasing external frequency for 0.60≤x≤0.88, but remains constant with respect to the change in external frequency for 0.90≤x≤1.00. Dielectric curves as a function of temperature show broadened peaks for 0.60≤x≤0.88, but show relatively sharp peaks for 0.90≤x≤1.00. The transition enthalpy change also increases with increasing x. The diffuse phase transition for 0.60≤x≤0.88 is accompanied by a small enthalpy change. The small enthalpy difference between the antiferroelectric and paraelectric phases seems to lower the barrier between the two phases, and in the process, the diffuse phase transition seems to be induced.

Phase stability investigations of the palladium-cadmium system: part ii. structural studies

The crystal structure and the precise lattice parameters of palladium-cadmium alloys containing 33 to 60 at. pct Cd were determined by X-ray diffraction at 294 K, using samples quenched from 1073 K. The results indicate that at 1073 K the β1(Ll0) phase extends from 33 to 55 at. pct Cd while the β′(B2) phase is stable from 56 to 60 at. pct Cd. No evidence for the existence of a separate high-temperature phase between the β, and β′ phase fields was found. A modification of the currently accepted Pd-Cd phase diagram near the equiatomic composition is proposed. The lattice parameters of the fct β1 - phase have the following values at the stoichiometric composition:a = 0.4 2817 nm,c = 0.3 6310 nm. The molar volume of the β1-phase is a linear function of composition from 33 to 50 at. pct Cd. The partial molar volumes of Pd and Cd have the following values in this range: VPd = 8.87 cm3/g-atom, VCd = 11.18 cm3/g-atom. An analysis of Pd-and Pt-based Ll0 phases indicates that the c/a ratio of these phases is related to their enthalpy of formation. Phases with a small enthalpy of formation have a high c/a ratio while phases with a large enthalpy of formation exhibit a small c/a ratio.

Interaction of Povidone with Aromatic Compounds III: Thermodynamics of the Binding Equilibria and Interaction Forces in Buffer Solutions at Varying pH Values and Varying Dielectric Constant

The complex formation of a series of aromatic compounds with povidone was studied in buffer solutions and organic solvent mixtures by equilibrium dialysis. For all the ligand molecules studied, a linear relationship was found between r, the number of moles of bound ligand per mole of povidone, and the free ligand concentration. The binding constants and the free energies of binding (—∆F), were greater for compounds in the nonionic state and increased with the number of hydroxyl groups which were capable of forming hydrogen bonds. They decreased with temperature elevation. The thermodynamic data showed entropy gains during the binding process accompanied by small negative enthalpy values. The increased ability to form hydrogen bonds and the increase in ionization of the ligand molecule was reflected in more negative ∆H and decreasing ∆S values. (The thermodynamic values were interpreted on the basis of the "iceberg" concept of water structure.) From these entropy and enthalpy changes, hydrogen and hydrophobic bondings appeared to be the most important types of binding. In organic solvent mixtures, the association constants lowered with increasing ethanol or propylene glycol concentration; a line relationship between the free energy and the dielectric constant of the solvent mixtures was observed.

Kinetics and temperature dependence of carboxymyoglobin ligand photodissociation

We have observed the rate of oxymyoglobin (MbO2) photodissociation at room temperature and carboxymyoglobin (MbCO) photodissociation as a function of temperature (260-10 K) by means of picosecond spectroscopy. The Mb + O2 and Mb + CO photodissociated states have also been characterized. Based on the picosecond experimental data, we postulate that the photodissociation of ligated myoglobin is a nonactivitated process, and the mechanism involves either a small enthalpy barrier or none at all.

Thermodynamics of binary mixtures containing cyclic ethers 3. Excess enthalpies of oxolane, 1,3-dioxolane, oxane, and 1,3-dioxane + benzene and + tetrachloromethane

The molar excess enthalpy H E has been measured as a function of mole fraction x at atmospheric pressure and 298.15 K for each of the six binary liquid mixtures: benzene + oxolane, and + 1,3-dioxolane, and tetrachloromethane + oxolane, + 1,3-dioxolane, + oxane, and + 1,3-dioxane. The overall consistency of the flow-calorimetric measurements is characterized by standard deviations from Redlich-Kister type smoothing equations which do not exceed ±1 per cent of the extreme value of H E. The mixtures with cyclic monoethers exhibit relatively large negative excess enthalpies, e.g. for (oxolane + tetrachloromethane) H E = −788.0 J mol −1 at x = 0.5. Mixtures containing cyclic diethers show rather small positive or negative excess enthalpies, e.g. for (1,3-dioxolane + tetrachloromethane), H E = 146.3 J mol −1 at x = 0.5, and for (1,3-dioxane + tetrachloromethane), H E = − 119.0 J mol −1. The latter mixture shows S-shaped dependence of H E on x with the very small positive section located at small mole fractions of cyclic ether.

THERMALLY REVERSIBLE FORMATION OF COBALT(III) TETRAPHENYLPORPHIN π-CATION RADICAL GENERATED FROM CHLOROTETRAPHENYLPORPHINATOCOBALT(III) IN CHLORINATED HYDROCARBONS

Abstract In the solution of chlorotetraphenylporphinatocobalt(III) (CoTPPCl) , a π-cation radical of six-coordinate Co(III)TPP is reversibly formed by thermal or light activation with a small enthalpy change.

Analysis of the dependence on temperature of the acid dissociation constants for mono-carboxylic acids in water in terms of a two-stage mechanism

The dependence on temperature of acid dissociation constants in water are examined for four systems. The final equation is based on a two-stage dissociation where the enthalpy terms are assumed to be independent of temperature. The relatively small overall enthalpy change for dissociation is shown to be a consequence of the enthalpy changes for the individual stages. The dependence of the overall heat-capacity quantity on temperature is marked, showing a minimum at a characteristic temperature. For acetic acid, the calculated dependence is in qualitative agreement with calorimetric data.

Adsorption of surfactants on rocks: Microcalorimetric approach applied to tertiary oil recovery

AbstractResults obtained for the adsorption of surfactants (non‐ionic and ionic) on a ground sand (4.2 m2 g−1) and on a kaolin (19.4 m2 g−1) are described. The very small enthalpy of displacement of water by the surfactant molecules (5–17 mJ m−2 of adsorbing surface) and the slow structuring or rearrangement of adsorbed molecules (over several hours) would be extremely difficult to detect by techniques other than microcalorimetry.

Hydrogen absorption in some A 2B intermetallic compounds with the MoSi 2-type structure (C11 b)

Hydrogen absorption was studied for the MoSi 2-type phases Zr 2Cu, Hf 2Cu, Ti 2Cu, Ti 2Pd, Zr 2Pd, Hf 2Pd, Pd 2Zr and Pd 2Hf. In all but the last two compounds substantial hydrogen absorption was observed. Hydrogen favors the tetrahedral sites in the lattice where it is surrounded by four zirconium, titanium or hafnium metal atoms. At the composition A 2BH 2 all these sites are occupied; on further hydrogen absorption additional sites, possibly octahedral, are occupied. The intermetallic compound hydrides which contained copper decomposed at relatively low temperatures (approximately 200 °C) whilst no decomposition was observed in the palladium compounds at substantially higher temperatures (400–500 °C). The difference in behavior is predominantly due to the difference in the stability of the intermetallic compounds. The copper compounds have low stability, as evidenced by their small enthalpies of formation, whilst the palladium compounds, with estimated enthalpies of formation in excess of 100 kJ (g atom) −1, are among the most stable intermetallic compounds known.

On the occurrence of substitutional and triple defects in intermetallic phases with the B2 structure

Theoretical models for the thermodynamic properties of intermetallic B2 phases are employed to explain a priori the empirical observation that B2 phases with a small enthalpy of formation exhibit substitutional defects, whereas B2 phases with a large enthalpy of formation exhibit triple defects. The following expressions for the concentration of anti-structure atoms ( N β A / N) as a function of the enthalpy of formation ΔH ∝ of the stoichiometric compound, the enthalpy of formation of vacancies ΔH v , and the temperature T, are derived: substitutional defects: ( N β A / N) s = 0.5 exp[( ΔH ∝)(2/ RT)] triple defects: (N β A/N) t = 0.315 exp[( 2 3 ΔH ∝ − 1 3 ΔH v)2/RT)] . These equations indicate that the formation of substitutional defects, which depends solely on ΔH ∝, becomes more difficult with increasing ΔH ∝. On the other hand, the formation of triple defects, which depends on both ΔH ∝ and ΔH v , becomes comparatively easier, assuming that ΔH v has approximately the same value for all B2 phases. It is predicted that some B2 phases might exhibit a hybrid behavior, with both defect types occurring in comparable concentrations.