EQUILIBRIUM HEAT OF FUSION OF POLYTETRAFLUOROETHYLENE

The reliability of the values of equilibrium heat of fusion, △H_m, of polytetrafluoroethylene (PTFE)reported in the literature and the inadequacy of the generally accepted value of 13 .7 or 14.6 cal/gwere discussed. Through extrapolating linear plot of △H_m vs. V_(sp) of the thermally aged PTFE virginas-polymerized samples prepared in accord with ASTM D 1457-81a, △H_m~o=79.6J/g (19.0 cal/g)and V_(sp) (specific volume of amorphous PTFE at 23℃)=0.480 cm~3/g (or ρ_a=2 .08g/cm~3) were ob-tained. The heats of fusion, △H_m, of the thermally aged and γ-irradiated samples are different irrespec-tive of the same V_(sp) value for both samples. A qualitative explanation of this phenomenon was givenbased on the H_v patterns of SALS and Kawai's model.     

Synthesis and X-ray diffraction study of ferrites ErM<Superscript>I</Superscript>Fe<Subscript>2</Subscript>O<Subscript>5</Subscript> (M<Superscript>I</Superscript> = Li,Na, K,Cs)

Ferrites of composition ErM^IFe₂O₅ (M^I = Li, Na, K, Cs) were synthesized by a solid-phase method. The structure of the ferrites was for the first time studied by X-ray powder diffraction. Crystal systems, unit cell parameters, and X-ray and pycnometric densities were determined. For ErLiFe₂O₅, a = 10.510 Å, c = 14.270 Å, V°= 1616.16 ų, Z = 16, V _subcell ^° = 101.01 ų, ρ_x = 6.01 g/cm³, ρ_pyc = 5.97 ± 0.04 g/cm³; for ErNaFe₂O₅, a = 10.519 Å, c = 15.510 Å, V° = 1759.56 ų, Z = 16, V _subcell ^° = 109.90 ų, ρ_x = 5.77 g/cm³, ρ_pyc = 5.72 ± 0.08 g/cm³; for ErKFe₂O₅, a = 11.050 Å, c = 15.480 Å, V° = 1937.33 ų, Z = 16, V _subcell ^° = 121.08 ų, ρ_x = 5.46 g/cm³, ρ_pyc = 5.41 ± 0.04 g/cm³; and for ErCsFe₂O₅, a = 10.78 Å, c = 16.01 Å, V° = 1905.37 ų, Z = 16, V _subcell ^° = 119.09 ų, ρ_x = 6.86 g/cm³, ρ_pyc = 6.61 ± 0.01 g/cm³.     

X-Ray diffraction data for new ferrites ErMFe<Subscript>2</Subscript>O<Subscript>5</Subscript> (M = Li,Na, K)

New ferrites ErMFe₂O₅ (M = Li, Na, K) were synthesized from erbium and iron(III) oxides and lithium, sodium, and potassium carbonates by solid-state annealing. According to X-ray powder diffraction, these compounds crystallize in the orthorhombic system with the following unit cell parameters: ErLiFe₂O₅, a = 10.510 Å, b = 10.776 Å, c = 14.270 Å, V ⁰ = 1616.16 ų; Z = 16, V _subcell ⁰ = 101.01 ų, ρ_X = 6.01 g/cm³, ρ_pycn = 5.97 ± 0.05 g/cm³; ErNaFe₂O₅, a = 10.519 Å, b = 10.785 Å, c = 15.510 Å, V ⁰ = 1759.56 ų, Z = 16, V _subcell ⁰ = 109.90 ų, ρ_X = 5.77 g/cm³, ρ_pycn = 5.72 ± 0.08 g/cm³; ErKFe₂O₅, a = 10.050 Å, b = 11.320 Å, c = 15.480 Å, V ⁰ = 1937.33 ų, Z = 16, V _subcell ⁰ = 121.08 ų, ρ_X = 5.46 g/cm³, ρ_pycn = 5.41 ± 0.04 g/cm³.     

Synthesis and X-ray diffraction study of nanostructured particles of cuprate manganites LaM 2 II CuMnO6 (MII = Mg,Ca, Sr,Ba)

Cuprate manganites of the composition LaM ₂ ^II CuMnO₆ (M^II = Mg, Ca, Sr, Ba) were synthesized from lanthanum, copper(II), and manganese(III) oxides and alkaline-earth metal carbonates by high-temperature solid-phase synthesis. By grinding the produced substances in a ball mill, their nanostructured particles were obtained, the sizes of which were determined with an electron microscope. Indexing the X-ray powder diffraction patterns of the cuprate manganites established that all of them crystallize in the cubic system with the following unit cell parameters: LaMg₂CuMnO₆: a = 15.523 ± 0.033 Å, Z = 6, V ⁰ = 3740.48 ± 0.10 ų, V _el.cell ⁰ = 623.41 ± 0.03ų, ρ_X-ray = 5.81 g/cm³, and ρ_pycn = 5.75 ± 0.06 g/cm³; LaCa₂CuMnO₆: a = 15.422 ± 0.058 Å, Z = 4, V ⁰ = 3667.94 ± 0.174 ų, V _el.cell ⁰ = 916.48 ± 0.04 ų, ρ_X-ray = 3.77 g/cm³, and ρ_pycn = 3.72 ± 0.05 g/cm³; LaSr₂CuMnO₆: a = 15.275 ± 0.049 Å, Z = 4, V ⁰ = 3564.05 ± 0.27 ų, V _el.cell ⁰ = 891.01 ± 0.07 ų, ρ_X-ray = 4.31 g/cm³, and ρ_pycn = 4.25 ± 0.05 g/cm³; and LaBa₂CuMnO₆: a = 15.589 ± 0.029 Å, Z = 4, V ⁰ = 3788.39 ± 0.09 ų, V _el.cell ⁰ = 947.10 ± 0.02 ų, ρ_X-ray = 4.74 g/cm³, and ρ_pycn = 4.70 ± 0.05 g/cm³. The data of an IR spectroscopic study of the cuprate manganites were presented.     

Chromites YbMCr<Subscript>2</Subscript>O<Subscript>5</Subscript> (M = Li,Na, K,Cs): X-ray diffraction study

Ytterbium alkali-metal chromites YbMCr₂O₅ (M = Li, Na, K, Cs) were synthesized by a ceramic procedure from the corresponding oxides and carbonates. Their crystal systems and unit cell parameters were determined by the homology method: for YbLiCr₂O₅, a = 10.34 Å, b = 10.62 Å, c = 15.05 Å, Z = 16, V ^o = 1653.74 ų, ρ_X-ray = 5.85 g/cm³, ρ_pycn = 5.81 ± 0.03 g/cm³; for YbNaCr₂O₅, a = 10.30 Å, b = 10.56 Å, c = 16.46 Å, Z = 16, V ^o = 1790.32 ų, ρ_X-ray = 5.64 g/cm³, ρ_pycn = 5.59 ± 0.07 g/cm³; for YbKCr₂O₅, a = 10.33 Å, b = 10.63 Å, c = 19.93 Å, Z = 16, V ^o = 2188.47 ų, ρ_X-ray = 5.95 g/cm³, ρ_pycn = 5.91 ± 0.03 g/cm³; and for YbCsCr₂O₅, a = 10.34 Å, b = 10.63 Å, c = 18.43 Å, Z = 16, V ^o = 2025.72 ų, ρ_X-ray = 5.19 g/cm³, ρ_pycn = 5.16 ± 0.05 g/cm³.     

Temperature and pressure effects on the kinetics of the Bromate ion-iodide ion-L-ascorbic acid clock reaction

The kinetics of the bromate ion-iodide ion-L-ascorbic acid clock reaction was investigated as a function of temperature and pressure using stopped-flow techniques. Kinetic results were obtained for the uncatalyzed as well as for the Mo(VI) and V(V) catalyzed reactions. While molybdenum catalyzes the BrO-I^? reaction, vanadium catalyzes the direct oxidation of ascorbic acid by bromate ion. The corresponding rate laws and kinetic parameters are as follows. Uncatalyzed reaction: r₂ = k₂[BrO] [I^?][H⁺]², k₂ = 38.6 ± 2.0 dm⁹ mol^?3 s^?1, ΔH? = 41.3 ± 4.2 kJmol^?1, ΔS? = ?75.9 ± 11.4 Jmol^?1 K^?1, ΔV? = ?14.2 ± 2.9 cm³ mol^?1. Molybdenum-catalyzed reaction: r′₂ = k₂[BrO] [I^?] [H⁺]² + k_Mo[BrO] [I^?] [ H⁺]²[M₀(VI)], k_Mo = (2.9 ± 0.3)10⁶ dm¹² mol^?4 s^?1, ΔH? = 27.2 ± 2.5 kJmol^?1, ΔS? = ?30.1 ± 4.5 Jmol^?1K^?1, ΔV? = 14.2 ± 2.1 cm³ mol^?1. Vanadium-catalyzed reaction: r′₁ = k_V[BrO] [V(V)], k_V = 9.1 ± 0.6 dm³ mol^?1 s^?1, ΔH? = 61.4 ± 5.4 kJmol^?1, ΔS? = ?20.7 ± 3.1 Jmol^?1K^?1, ΔV? = 5.2 ± 1.5 cm³ mol^?1. On the basis of the results, mechanistic details of the BrO-I^? reaction and the catalytic oxidation of ascorbic acid by BrO are elaborated. © 1995 John Wiley & Sons, Inc.     

Differential scanning calorimetric analysis of irradiated polytetrafluoroethylene films

Crystalline content of polytetrafluoroethylene (PTFE) can be substantially increased by electron beam irradiation. These changes as a function of radiation dose were examined in PTFE films by differential scanning calorimetry (DSC). Surprisingly small radiation doses (<0.002 Mrad) were found to cause a, fairly substantial increase (25%) in their heat of fusion. Variations in the heat of fusion and the peak melting temperature of PTFE films with radiation dose, in the range of 0.0017 to 16 Mrad, are examined.     

Synthesis and X-ray diffraction study of LaM 1.5 II MnFeO6 manganitoferrites (MII = Mg,Ca, Sr,Ba)

LaM _1.5 ^II MnFeO₆ manganitoferrites (M^II = Mg, Ca, Sr, Ba) have been synthesized by ceramic technology from lanthanum oxide, manganese(III) oxide, iron(III) oxide, and alkali-earth carbonates. X-ray powder diffraction shows that these compounds crystallize in cubic crystal system with the following unit cell parameters: for LaMg_1.5MnFeO₆: a = 20.232 ± 0.032 Å, V ⁰ = 8281.642 ± 0.096 ų, Z = 10, ρ_X = 7.38 g/cm³, ρ_pycn = 7.29 ± 0.06 g/cm³; for LaCa_1.5MnFeO₆: a = 20.056 ± 0.017 Å, V ₀ = 8067.388 ± 0.051 ų, Z = 8, ρ_X = 5.89 g/cm³, ρ_pycn = 5.78 ± 0.06 g/cm³; for LaSr_1.5MnFeO₆: a = 20.117 ± 0.021 Å, V ₀ = 8141.223 ± 0.063 ų, Z = 8, V _u.c. ⁰ = 1017.653 ± 0.008 ų, ρ_X = 6.64 g/cm³, ρ_pycn = 6.56 Å 0.08 g/cm³; for LaBa_1.5MnFeO₆: a = 20.361 ± 0.025 Å, V ₀ = 8441.066 ± 0.075 ų, Z = 8, ρ_X = 7.31 g/cm³, ρ_pycn = 7.25 ± 0.07 g/cm³.     

Synthesis and X-ray diffraction study of ternary ferrites LaNaMnFeO5 and LaKMnFeO5

Ferrites LaNaMnFeO₅ and LaKMnFeO₅ have been synthesized by the ceramic method from lanthanum(III) and iron(III) oxides and sodium and potassium carbonates. The ferrites crystallize in the orthorhombic crystal system with the following lattice parameters: LaNaMnFeO₅: a = 10.943 ± 0.002 Å, b = 11.022 ± 0.001 Å, c = 16.165 ± 0.028 Å, V ⁰ = 1949.72 ų, Z = 16, V _{unit cell} ⁰ = 12.1.85 ų, ρ_X = 4.83, ρ_pycn = 4.76 ± 0.09 g/cm³; LaKMnFeO₅: a = 10.959 ± 0.011 Å, b = 11.036 ± 0.005 Å, c = 17.825 ± 0.074 Å, V ⁰ = 2155.81 ų, Z = 16, V _{unit cell} ⁰ = 134.74 ų, ρ_X = 4.54, ρ_pycn = 4.46 ± 0.07 g/cm³.     

X-ray diffraction characteristics of new chromitomanganites LaM 3 I CrMnO6 and LaM 3 II CrMnO7.5 (MI = Li, Na; MII = Mg, Ca)

LaM ₃ ^I CrMnO₆ (M^I = Li, Na) and LaM ₃ ^II CrMnO_7.5 (M^II = Mg, Ca) chromitomanganites were synthesized by ceramic technology from lanthanum oxide, chromium(III) oxide, manganese(III) oxide, lithium carbonate, sodium carbonate, magnesium carbonate, and calcium carbonate. X-ray powder diffraction shows that these compounds crystallize in cubic or tetragonal systems with the following unit cell parameters: for LaLi₃CrMnO₆ (cubic): a = 10.98 Å, V ^○ = 1323.75 ų, Z = 8, V _u.c ^○ = 165.47ų, ρ_X = 3.64, ρ_pycn= 3.60 ± 0.04 g/cm³; for LaNa₃CrMnO₆ (tetragonal): a = 10.96 Å, c = 15.73 Å, V ^○ = 1889.51 ų, Z = 16, V _u.c ^○ = 118.09 ų, ρ_X = 5.77 g/cm³, ρ_pycn = 5.70 ± 0.07 g/cm³; LaMg₃CrMnO_7.5 (cubic), a = 10.98 Å, V ^○ = 1322.31 ų, Z = 8, V _u.c ^○ = 165.29 ų, ρ_X = 4.41 g/cm³, ρ_pycn = 4.35 ± 0.07 g/cm³; and for LaCa₃CrMnO_7.5 (cubic): a = 10.97 Å, V ^○ = 1319.78 ų, Z = 8, V _u.c ^pO = 164.97 ų, ρ_X = 4.89 g/cm³, ρ_pycn = 4.85 ± 0.05 g/cm³.     

Ferrites YbSrFe2O5.5 and YbBaFe2O5.5: Synthesis and X-ray diffraction, thermodynamic, and electrophysical properties

Ferrites YbSrFe₂O_5.5 and YbBaFe₂O_5.5 are prepared by reacting ytterbium(III) oxide and iron(III) oxide with strontium or barium carbonate in the solid phase. The ferrites crystallize in the orthorhombic system as shown by indexing of their X-ray diffraction patterns with homology modeling: for YbSrFe₂O_5.5, a = 10.74 ± 0.006 Å, b = 10.93 ± 0.006 Å, c = 16.64 ± 0.046 Å, V ⁰ = 1953.3 ų, Z = 16, V _subcell ⁰ = 122.08 ų, ρ_X-ray = 6.26 g/cm³, ρ_pycn = 6.18 ± 0.9 g/cm³; for YbaBaFe₂O_5.5, a = 10.74 ± 0.013 Å, b = 10.99 ± 0.004 Å, c = 17.16 ± 0.017 Å, V ⁰ = 2025.4 ų, Z = 16, V _subcell ⁰ = 126.59 ų, ρ_X-ray = 6.69 g/cm³, ρ_pycn = 6.40 ± 0.32 g/cm³. The calorimetric heat capacities of the ferrites are studied from 298.15 to 673 K. The C _p ^o ～ f(T) curves show λ peaks at 448 K for YbSrFe₂O_5.5 and at 373 K for YbBaFe₂O_5.5, likely, due to second-order phase transitions. The dielectric constants and electrical resistances of the ferrites are studied as functions of temperature from 293 to 493 K.     

X-ray powder diffraction study of nanostructured particles of manganite ferrites NdMIMnFeO5 (MI = Li, Na, K)

Manganite ferrites NdM^IMnFeO₅ (M^I = Li, Na, K) were synthesized from neodymium(III), manganese(III), and iron(III) oxides and lithium, sodium, and potassium carbonates by a ceramic technology. By grinding the obtained compounds in a ball mill, their nanostructured particles were produced, the sizes of which were determined with an electron microscope. X-ray powder diffraction study and indexing established that the nanostructured compounds NdM^IMnFeO₅ (M^I = Li, Na, K) crystallize in the cubic system with the following lattice parameters: NdLiMnFeO₅: a = 20.100 ± 0.034 Å, V ⁰ = 8120.60 ų, Z = 10, V _un.cell ⁰ = 812.06 ų, ρ_X-ray = 7.14 g/cm³, and ρ_pycn = 7.09 ± 0.06 g/cm³; NdNaMnFeO₅: a = 20.102 ± 0.032 Å, V ⁰ = 8123.03 ų, Z = 10, V _un.cell ⁰ = 812.30 ų, ρ_X-ray = 7.11 g/cm³, and ρ_pycn = 7.04 ± 0.06 g/cm³; and NdKMnFeO₅: a = 20.107 ± 0.011 Å, V ⁰ = 8129.09 ų, Z = 10, V _un.cell ⁰ = 812.91 ų, ρ_X-ray = 7.03 g/cm³, and ρ_pycn = 6.95 ± 0.07 g/cm³.     

Synthesis and x-ray diffraction study of new nanostructured manganite ferrites NdM 1.5 II MnFeO6 (MII = Mg, Ca, Sr, Ba)

Manganite ferrites NdM _1.5 ^II MnFeO₆ (M^II = Mg, Ca, Sr, Ba) were synthesized from neodymium(III), manganese(III), and iron(III) oxides and alkaline-earth metal carbonates by a ceramic technology. By grinding the obtained compounds in a ball mill, their nanostructured particles were produced, the sizes of which were determined with an electron microscope. X-ray diffraction study established that the nanostructured compounds crystallize in the cubic and tetragonal systems with the following lattice parameters: NdMg_1.5MnFeO₆ (tetragonal): a = 10.955 Å, c = 17.848 Å, V ⁰ = 2141.975 ų, Z = 16, V _e1.cel1 ⁰ = 133.873 ų, ρ_X-ray = 4.80 g/cm³, and ρ_pycn = 4.76 ± 0.05 g/cm³; NdCa_1.5MnFeO₆ (cubic): a= 10.809 Å, V ⁰ = 1262.864 ų, Z = 8, V _e1.cel1 ⁰ = 157.858 ų, ρ_X-ray = 4.32 g/cm³, and ρ_pycn = 4.27 ± 0.03 g/cm³; NdSr_1.5MnFeO₆ (cubic): a = 10.911 Å, V ⁰ = 1298.953 ų, Z = 8, V _e1.cel1 ⁰ = 162.369 ų, ρ_X-ray = 4.93 g/cm³, and ρ_pycn= 4.88 ± 0.05 g/cm³; and NdBa_1.5MnFeO₆ (tetragonal): a = 11.011 Å, c = 18.001 Å, V ₀ = 2182.479 ų, Z = 16, V _e1.cel1 ⁰ = 136.405 ų, ρ_X-ray = 6.78 g/cm³, and ρ_pycn= 6.75 ± 0.07 g/cm³.     

Study on the influence of PTFE particle size on the polymer thermal behavior: I. Melting

The influence of the size of poly(tetrafluoroethylene) (PTFE) particles, obtained from the formed polymer (bar) by cutting on the fusion was studied in order to get information on the polymer breaking consequence.Three fractions of particle diameters between 1.62–0.42 mm were submitted to DSC in nitrogen, DTA in air and DRX analyses and the results compared to those obtained with the non-processed formed polymer and PTFE mentioned in literature.The study on the heating rate influence on the melting temperature range (T) and the specific heat of fusion (Q/J g^–1) carried out with the fraction of 0.82 mm diameter afforded a method for estimating the polymer specific caloric capacity by means of DSC data.This revised version was published online in November 2005 with corrections to the Cover Date.     

Heat capacity of poly(trimethylene terephthalate)

The heat capacity of poly(trimethylene terephthalate) (PTT) has been measured using adiabatic calorimetry, standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 to 570 K. The semicrystalline PTT has a glass transition temperature of 331 K. Between 340 and 480 K, PTT can show exothermic ordering depending on the prior degree of crystallization. The melting endotherm of semicrystalline samples occurs between 480 and 505 K, with a typical onset temperature of 489 K (216°C). The heat of fusion of the semicrystalline samples is about 15 kJ mol⁻¹. For 100% crystalline PTT the heat of fusion is estimated to be 30 ± 2 kJ mol⁻¹. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum and the Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations (θ₁ = 550.5 K and θ₂ = θ₃ = 51 K, N_skeletal = 19). The calculated and experimental heat capacities agree to better than ±3% between 5 and 300 K. The experimental heat capacities of liquid PTT can be expressed by: $ C^L_p(exp) $ = 211.6 + 0.434 T J K⁻¹ mol⁻¹ and compare to ±0.5% with estimates from the ATHAS data bank using contributions of other polymers with the same constituent groups. The glass transition temperature of the completely amorphous polymer is estimated to be 310–315 K with a ΔC_p of about 94 J K⁻¹ mol⁻¹. Knowing C_p of the solid, liquid, and the transition parameters, the thermodynamic functions enthalpy, entropy, and Gibbs function were obtained. With these data one can compute for semicrystalline samples crystallinity changes with temperature, mobile amorphous fractions, and resolve the question of rigid-amorphous fractions.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2499–2511, 1998     

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na₃FeF₆ and for the Na₃FeF₆ melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δ_trans H(Na₃FeF₆, 920 K) = (19 ± 3) kJ mol⁻¹ and enthalpy of fusion at the temperature of fusion 1255 K, δ_fusH(Na₃FeF₆, 1255 K) = (89 ± 3) kJ mol⁻¹ have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: C _p(Na₃FeF₆, cr, α) = (294 ± 14) J (mol K)⁻¹, for 723 = T/K ≤ 920, C _p(Na₃FeF₆, cr, β) = (300 ± 11) J (mol K)⁻¹ for 920 ≤ T/K = 1233 and C _p(Na₃FeF₆, melt) = (275 ± 22) J (mol K)⁻¹ for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.      

A calorimetric study of the thermodynamic properties of potassium molybdate

The low-temperature heat capacity of K₂MoO₄ was measured by adiabatic calorimetry. The smoothed heat capacity values, entropies, reduced Gibbs energies, and enthalpies were calculated over the temperature range 0–330 K. The standard thermodynamic functions determined at 298.15 K were C _p ^° (298.15 K) = 143.1 ± 0.2 J/(mol K), S°(298.15 K) = 199.3 ± 0.4 J/(mol K), H°(298.15 K)-H°(0) = 28.41 ± 0.03 kJ/mol, and Φ°(298.15 K) = 104.0 ± 0.4 J/(mol K). The thermal behavior of potassium molybdate at elevated temperatures was studied by differential scanning calorimetry. The parameters of polymorphic transitions and fusion of potassium molybdate were determined.     

Use of model compounds to determine equilibrium melting points of polymers

Evidence is given for the relationship describing the approximate dependence of the observed melting points T_m of oligomers on their degree of polymerization n, and its use in determining the equilibrium melting point T^°_m of the extended chain crystal and the heat of fusion Δh. Polyethylene and the n-alkanes, poly(ethylene oxide) and poly(methylene oxide), polyphenylene and other systems are considered.     

The structure and thermochemistry of 3:4,5:6-dibenzo-2-hydroxymethylene-cyclohepta-3,5-dienenone (1) and some related compounds

Condensed and gas phase enthalpies of formation of 3:4,5:6-dibenzo-2-hydroxymethylene-cyclohepta-3,5-dienenone (1, (−199.1 ± 16.4), (−70.5 ± 20.5) kJ mol⁻¹, respectively) and 3,4,6,7-dibenzobicyclo[3.2.1]nona-3,6-dien-2-one (2, (−79.7 ± 22.9), (20.1 ± 23.1) kJ mol⁻¹) are reported. Sublimation enthalpies at T=298.15 K for these compounds were evaluated by combining the fusion enthalpies at T = 298.15 K (1, (12.5 ± 1.8); 2, (5.3 ± 1.7) kJ mol⁻¹) adjusted from DSC measurements at the melting temperature (1, (T _fus, 357.7 K, 16.9 ± 1.3 kJ mol⁻¹)); 2, (T _fus, 383.3 K, 10.9 ± 0.1) kJ mol⁻¹) with the vaporization enthalpies at T = 298.15 K (1, (116.1 ± 12.1); 2, (94.5 ± 2.2) kJ mol⁻¹) measured by correlation-gas chromatography. The vaporization enthalpies of benzoin ((98.5 ± 12.5) kJ mol⁻¹) and 7-heptadecanone ((94.5 ± 1.8) kJ mol⁻¹) at T = 298.15 K and the fusion enthalpy of phenyl salicylate (T _fus, 312.7 K, 18.4 ± 0.5) kJ mol⁻¹) were also determined for the correlations. The crystal structure of 1 was determined by X-ray crystallography. Compound 1 exists entirely in the enol form and resembles the crystal structure found for benzoylacetone.     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.