Physicochemical properties, structure, and conformations of 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4mim]NTf2 ionic liquid

Thermodynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim]NTf2) ionic liquid have been studied by adiabatic calorimetry in the temperature range of 5 to 370 K. This compound has been found to form crystal, liquid, and glass. The temperature and enthalpy of fusion for [C(4)mim]NTf(2) have been determined to be T(fus) = 270.22 +/- 0.02 K and Delta(fus)H = 23.78 +/- 0.04 kJ.mol(-1), respectively. The heat capacity of crystalline [C(4)mim]NTf(2) in the T range of 205 to 255 K may vary by a few percent, subject to the procedure of the crystal preparation. The glass transition temperature for [C(4)mim]NTf(2) has been found to be T(g) = 181.5 +/- 0.1 K. On the basis of the results of DFT quantum chemical calculations, the experimental vibrational spectra, and the available literature data, thermodynamic properties of [C(4)mim]NTf(2) in the ideal-gas state have been calculated by the statistical thermodynamic methods. The entropy values for the gaseous compound obtained from the experimental data and the calculations are in satisfactory agreement.     

Thermodynamic relationship between structural isomers of the thermochromic compound bis(N-Isopropyl-5,6-benzosalicylideneiminato)nickel(II)

A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis( N-isopropyl-5,6-benzosalicylideneiminato)nickel(II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G' and B' phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below approximately 290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G', and then the B' phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S degrees (B) - S degrees (G) = 32.8 J K (-1) mol (-1), H degrees (B) - H degrees (G) = 16.0 kJ mol (-1), and G degrees (B) - G degrees (G) = 6.25 kJ mol (-1), respectively.     

比热和直流磁化率证明N+H…O-氢键的电子自旋翻转在D-和L-丙氨酸单晶中的不对称相变

为了解决D-和L-丙氨酸在约270K相变的分岐和机理,对其单晶、多晶粉末及原料利用微分扫描量热仪测定比热.用三线法以蓝宝石作校正,并与手册的D-和L-丙氨酸标准比热值比较.在单晶中,实验观察到吸热相变峰最高处时的温度及热焓为:D-丙氨酸,Tc=272.02K,△H=1.87J·mol-1;L-丙氨酸,Tc=271.85K,△H=1.46J·mol-1;热焓差为0.41J·mol-1.参比晶体D-缬氨酸,Tc=273.59K,△H=1.75J·mol-1;L-缬氨酸,Tc=273.76K,△H=1.57J·mol-1;热焓差为0.18J·mol-1.实验发现已测量过的单晶磨成多晶粉末后再测,相变峰消失.说明相变与晶格有关.变温中子衍射排除了D→L的构型相变,但发现N+H…O-氢键沿D-和L-丙氨酸单晶的c轴反向变化.变温偏振拉曼散射反映相变机制与N+H…O-中电子的轨道磁偶极矩相关,观察到偏振光的不对称散射.在外加磁场强度H为+1T和-1T下,变温测定D-和L-丙氨酸晶体的直流磁化率,证明在270K有电子自旋翻转的相变.电子自旋的向上或向下,取决于晶格中NH+3的扭曲振动及N+H…O-氢键沿晶体c轴的方向.由于自旋的定轴性,可以解释单晶和多晶粉末比热结果的分岐.     

Cooperativity of dynamics of 18-crown-6 molecule forming one-dimensional chain in Cs2(18-crown-6)3[Ni(dmit)2]2

Heat capacity of Cs2(18-crown-6)3[Ni(dmit)2]2 was measured by adiabatic calorimetry. A broad thermal anomaly was observed around 225 K. The entropy gain (about 52 J K(-1) mol(-1)) is much larger than that expected for twofold disordering of 18-crown-6 assumed in the previous structure analysis. The shape of thermal anomaly was qualitatively explained by a linear Ising model developed for cooperative disordering in polymers. The 18-crown-6 molecules forming a one-dimensional chain in the crystal are orientationally disordered with moderate cooperativity.     

Thermal- and photoinduced spin-state switching in an unprecedented three-dimensional bimetallic coordination polymer

The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) right arrow over left arrow low-spin (LS) transition. The critical temperatures are T(c1)=185 and T(c2)=148 K. Each step involves approximately 50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are DeltaH(1)=3.6+/-0.4 kJ mol(-1) and DeltaS(1)=19.5+/-3 J K(-1) mol(-1); DeltaH(2)=4.8+/-0.4 kJ mol(-1) and DeltaS(2)=33.5+/-3 J K(-1) mol(-1). Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P2(1)/c, Z=16) at all temperatures. There are five [FeN6] pseudo-octahedral sites linked by pmd, [Ag(CN)2]- and [Ag2(CN)3]- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the AgAg distances. Correlations between the variation of the unit-cell volume and the change of AgAg interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred.     

液晶HpBAB的相变和熔化熵的同系规律性的热力学研究

在260～380 K范围内, 用DSC法研究液晶HpBAB的相变。实验查明HpBAB在室温还以玻璃性晶体状态存在。它的玻璃转变、熔化和清亮过程的热力学参数准确地被测到。提出对-正烷氧基亚苄基-对-氨基苯腈同系列中熔化熵和烷基碳原子数的线性关系; 改进Aranow统计模型并估算直线斜率为Rln(3/g)(1相似文献   

The thermodynamic properties of polytetrafluoroethylene

Polytetrafluoroethylenes of different crystallinity were analyzed between 220 and 700 K by differential scanning calorimetry. A new computer coupling of the standard DSC is described. The measured heat capacity data were combined with all literature data into a recommended set of thermodynamic properties for the crystalline polymer and a preliminary set for the amorphous polymer (heat capacity, enthalpy, entropy, and Gibbs energy; range 0–700 K). The crystal heat capacities have been linked to the vibrational spectrum with a θ₃ of 54 K, and θ₁ of 250 K, and a full set of group vibrations. C_v to C_p conversion was possible with a Nernst–Lindemann constant of A = 1.6 × 10^?3 mol K/J. The glass transition was identified as a broad transition between 160 and 240 K with a ΔC_p of 9.4 J/K mol. The room-temperature transitions at 292 and 303 K have a combined heat of transition of 850 J/mol and an entropy of transition of 2.90 J/K mol. The equilibrium melting temperature is 605 K with transition enthalpy and entropy of 4.10 kj/mol and 6.78 J/K mol, respectively. The high-temperature crystal from is shown to be a condis crystal (conformationally disordered), and for the samples discussed, the crystallinity model holds.     

Calorimetric study of correlated disordering in [Hdamel]2[Cu(II)(tdpd)2] x 2 THF crystal

The heat capacity of [Hdamel]2[Cu(II)(tdpd)2] x 2 THF was measured from 6 to 250 K by adiabatic calorimetry. There are four heat anomalies around 150 K associated with disordering in the orientation of the uncoordinated THF molecules and in the conformation of the out-of-plane allyl groups of [Hdamel](+) units. The total entropy of transition was determined to be 19.8 J K(-1) mol(-1), less than the 4R ln 2 (R = gas constant) expected from the crystal structure at room temperature. The smallness of the total entropy change on phase transitions proves the presence of the strong motional correlation between the adjacent allyl groups. The calorimetric conclusion agreed with the crystal structure at 200 K re-examined in this study.     

稀土高氯酸盐-甘氨酸配合物[Sm2(Gly)6(H2O)4](ClO4)6·5H2O单晶 体的低温热 容及标准生成焓

合成了稀土高氯酸盐-甘氨酸配合物晶体。经热重、差热、化学化析及有关文献对比,确定其组成是[Sm2(Gly)6(H2O)4](ClO4)6·5H2O,单晶结构,纯度是99.0%.熔点分析仪分析知其没有固定熔点,在79～370K温区,用高精密全自动绝热量仪对单晶配合物进行了热容测定,发现该配合物在低温段没有反常热容。348.07K附近是该配合物的分解温区,配合物的分解温度、分解熵和分解焓分别是346.89K,44.669kJ/mol和128.77J/K·mol。计算机拟合了热容对温度的多项式方程,在79～318K温区,Cp=1294.56+624.17K-11.893X^2+75.075X^3+23.762X^4.在常压,298.15K下用具有恒温环境的反应热量计测定了配合物的标准生成焓值为-8022.405kJ/mol。     

A molecular dynamics study of the thermodynamic properties of calcium apatites. 1. Hexagonal phases

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, delta(lat)H298(o), of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, C(p,m), in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H(m) = (H(m,298) - 298C(p,m)) + C(p,m)T, yielding C(p,m) = 694 +/- 68 J x mol(-1) x K(-1), C(p,m) = 646 +/- 26 J x mol(-1) x K(-1), C(p,m) = 530 +/- 34 J x mol(-1) x K(-1), and C(p,m) = 811 +/- 42 J x mol(-1) x K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappaT, of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.     

The thermal properties of poly(oxy-1,4-benzoyl), poly(oxy-2,6-naphthoyl), and its copolymers

The thermal properties, i.e., heat capacity, enthalpy, entropy, and Gibbs function, and the transition behavior of the copolymer system of 4-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid have been studied based on differential scanning calorimetry. The heat capacities of the glass, crystal, and anisotropic melt are shown to be largely additive on a molar basis. Additivity is lost in the two transition regions, glass transition and disordering transition. Isothermal crystallization experiments on the copolymers revealed the existence of two types of crystals which melt at high temperature (fast-grown crystals) and low temperature (slowly grown crystals). The ATHAS computation method is used to bring heat capacities of the solid state into agreement with approximate frequency spectra. The changes in heat capacity at the glass transitions occur at 434°K for the poly(oxy-1,4-benzoyl) [33.2 J/(K mol)] and at 420°K for poly(oxy-2,6-naphthoyl) [46.5 J/(K mol)]. The copolymers have a transition range of above 100°K. The anisotropic melt is linked to the well-known condis state of poly(oxy-1,4-benzoyl) by a continuous changes in disorder and mobility without an additional first-order transition.     

Phase transitions in mesophase macromolecules. V. Transitions in poly(oxy-1,4-phenylene carbonyl-co-oxy-2,6-naphthaloyl)

The copolymer system HBA (4-hydroxybenzoic acid) and HNA (2,6-hydroxyhaphthoic acid) has been studied mainly by means of differential scanning calorimetry. Heat capacities for the two homopolymers and three copolymers are reported from 175 to 650 K. The heat capacities are additive with composition. The whole copolymer system represents an example of a mesophase intermediate between the liquid-crystalline and the condis-crystalline state with a small amount of crystallinity. The disordering transitions (520–620 K) have a eutecticlike phase diagram. The transitions to the isotropic states occur above the decomposition temperatures (600–700 K). The glass transitions (377–434 K) show normal changes in heat capacity (31–34 J/K mol), but are broadened to as much as 200 K.     

Differential AC-chip calorimeter for glass transition measurements in ultrathin films

A differential AC-chip calorimeter capable of measuring the step in heat capacity at the glass transition in nanometer-thin films is described. Because of the differential setup, pJ/K sensitivity is achieved. Heat capacity can be measured for sample masses below 1 ng in broad temperature range as needed for the study of the glass transition in nanometer-thin polymeric films. Relative accuracy is sufficient to investigate the changes in heat capacity as the step at the glass transition of polystyrene. The step is about 25% of the total heat capacity of polystyrene. The calorimeter allows for the frequency dependent measurement of complex heat capacity in the frequency range from 1 Hz to 1 kHz. The glass transition in thin polystyrene films (50–4 nm) was determined at well-defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits (±3 K)—neither at constant frequency (40 Hz) nor for the trace in the activation diagram (1 Hz–1 kHz). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2996–3005, 2006     

Thermodynamic Properties of Salsoline Salsolinodithiocarbamate

Heat of solution of salsoline salsolinodithiocarbamate (SS) at 1 : 18 000, 1 : 36 000, and 1 : 72 000 dilution (mol per mol H₂O) was determined on a DAK-I-IA calorimeter, and the standard heat of solution at infinite dilution was estimated. The temperature dependence of the heat capacity of SS was studied over the range 173–348 K. The heats of combustion and fusion, and the standard heat of formation of liquid (molten) and crystalline SS were estimated by indirect methods.     

Liquid-like relaxation in hyperquenched water at < or = 140 K

Micrometre-sized water droplets were hyperquenched on a solid substrate held at selected temperatures between 150 and 77 K. These samples were characterized by differential scanning calorimetry (DSC) and X-ray diffraction. 140 K is the upper temperature limit to obtain mainly amorphous samples on deposition within 16-37 min. DSC scans of glassy water prepared at 140 K exhibit on heating an endothermic step assignable to glass --> liquid transition, with an onset temperature (T(g)) of 136 +/- 2 K on heating at 30 K min(-1). For T(g) of approximately 136 K, water relaxes during deposition at 140 K for 16 min, moving towards metastable equilibrium. The apparent increase in heat capacity (deltaC(p)) depends, for a given rate of heating, on the rate of prior cooling, and a so-called overshoot develops. 140 K deposits cooled at a rate of 5, 2 or 0.2 K min(-1) show on subsequent reheating at a rate of 30 K min(-1) deltaC(p) values of 0.7, 1.1 and 1.7 J K(-1) mol(-1). This is consistent with liquid-like relaxation at 140 K, and it indicates that different limiting structures are obtained. When these 140 K deposits are in addition annealed at 130 K for 90 min, after slow-cooling at 5, 2 or 0.2 K min(-1), their deltaC(p) values on subsequent reheating are similar to those of hyperquenched glassy water (HGW) deposits made at 77 K and annealed at 130 K. Thus, the previous deltaC(p) value of 1.6 J K(-1) mol(-1) obtained with glassy water samples annealed at 130 K (A. Hallbrucker, E. Mayer and G. P. Johari, Philos. Mag. B, 1989, 60, 179) must be an upper-bound limit because it contains a contribution from an overshoot. The T(g) value of 140 K deposits, which had relaxed during deposition towards metastable equilibrium, is within experimental error the same as that of 140 K deposits annealed in addition at 130 K. This contradicts Yue and Angell's (Y. Yue and C. Angell, Nature, 2004, 427, 717) claim for assigning the endothermic step to a sub-T(g) peak or a "shadow" T(g). Our new data further support the proposed fragile-to-strong transition on cooling liquid water from ambient temperature into the deeply supercooled and glassy state. We also describe in detail experimental aspects to obtain HGW specimens, show the ultrastructure of the deposits using electron microscopy, and discuss the mechanism of our hyperquenching method.     

Mechanochemical effect in the iron(III) spin crossover complex [Fe(3-MeO-salenEt2]PF6 as studied by heat capacity calorimetry

Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)(2)]PF(6) are very sensitive to mechanochemical perturbations. Heat capacities for unperturbed and differently perturbed samples were precisely determined by adiabatic calorimetry at temperatures in the 10-300 K range. The unperturbed compound shows a cooperative spin crossover transition at 162.31 K, presenting a hysteresis of 2.8 K. The anomalous enthalpy and entropy contents of the transition were evaluated to be Delta(trs)H = 5.94 kJ mol(-1) and Delta(trs)S = 36.7 J K(-1) mol(-1), respectively. By mechanochemical treatments, (1) the phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy gains at the phase transition due to the spin crossover phenomenon were diminished to Delta(trs)H = 4.94 kJ mol(-1) and Delta(trs)S = 31.1 J K(-1) mol(-1), and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The mechanochemical perturbation exerts its effect more strongly on the low-spin state than on the high-spin state. It shows a substantial increase of the number of iron(III) ions in the high-spin state below the transition temperature. The heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)(2)]PF(6) also were measured. The lattice heat capacity of the iron compounds has been estimated from either the measurements on the cobalt complex using a corresponding states law or the effective frequency distribution method. These estimations have been used for the evaluation of the transition anomaly.     

Magnetostructural study of 2-(4-N-tert-butylaminoxylphenyl)benzimidazole

The heat capacity of the title organic free radical, PhBABI, was measured over 0.3-300 K by adiabatic calorimetry and relaxation methods in the presence of external magnetic fields up to 9 T. A hump in the magnetic heat capacity was observed with a maximum at about 15 K in zero field, which did not shift at fields up to 9 T. The experimental magnetic entropy was in good agreement with the theoretical value of R ln 2 (= 5.76 J K(-1) mol(-1)) for S = 1/2 systems. The higher temperature, field-insensitive feature was fitted to several antiferromagnetic Heisenberg models. The best fits were obtained using spin ladder and coupled spin bilayer models.     

乙醇和甲苯组成的最低共沸混合物热力学性质的研究

The molar heat capacity of the azeotropic mixture composed of ethanol and toluene was measured by a high precision adiabatic calorimeter from 80 to 320 K. The glass transition and phase transitions of the azeotropic mixture were determined based on the heat capacity measurements. A glass transition at 103.350 K was found. A solid-solid phase transition at 127.282 K, two solid-liquid phase transitions at 153.612 and 160.584 K were observed, which correspond to the transition of metastable crystal to stable crystal of ethanol and the melting of ethanol and toluene, respectively. The thermodynamic functions and the excess ones of the mixture relative to the standard temperature 298.15 K were derived based on the relationships of the thermodynamic functions and the function of the measured heat capacity with respect to temperature.     

Using heat capacity and compressibility to choose among two-state models of liquid water

We extend to heat capacity Cp the model of Vedamuthu, Singh, and Robinson (J. Phys. Chem. 1994, 98, 2222). This model and that of Bartell (J. Phys. Chem. B 1997, 101, 7573) fit successfully, even in the supercooled region, the temperature dependence of Cp, volume, and isothermal compressibility kappa(T). The Robinson model is superior for kappa(T). Tanaka's model (J. Chem. Phys. 2000, 112, 799) fails for C(p) even after correction of a derivational error. All three models assume that the liquid consists of low-density component 1 and high-density component 2. We conclude that Robinson's tactics, ignoring the intercomponent equilibrium constant and determining compositions solely from volumes, yield the most reliable compositions and individual-component properties. Our fits of the Robinson model to C(p) yield at 0 degrees C H(2) - H(1) of (135 +/- 35) J/g, H(1) - H(ice) of 0.8DeltaH(fus), and C(2) - C(1) of (0.1 +/- 0.7) J/K.g. The enthalpy difference between the components is largely responsible for the rapid change of C(p) at the lowest supercooled temperatures. We propose an adjustment to Speedy and Angell's (J. Chem. Phys. 1976, 65, 851) experimental values of kappa(T) for supercooled water.     

Heat capacity and chemical equilibria of liquid selenium

Heat capacities of liquid selenium have been measured by computer interfaced differential scanning calorimetry in the metastable region with an accuracy of ± 1% from 330 to 520°K. To avoid crystallization, the measurements were done on cooling. A semiquantitative fitting of the heat capacity to vibrational energy contributions, free volume (hole) effects, and heats of reaction from the changes in the ring-chain and depolymerization equilibria was possible to within ±5% of the newly measured and literature data between the glass transition temperature (ca. 303°K) and 1000°K. It could be established that the shift in the ring-chain equilibrium is not the major reason for the overall decrease in heat capacity above the glass transition temperature. The floor temperature, which was earlier placed at about 356°K, is possibly below the glass transition temperature. The increase in heat capacity beyond 800°K has been linked with the depolymerization reaction.