Quantum mechanical calculations of molecular properties and ideal gas heat capacity of difluoromethane

Ideal gas heat capacities are important for the calculation of caloric properties of real fluids. But as shown in the past, they are not always available with required accuracy or are still lacking. In this paper, we combine quantum mechanical calculations and statistical thermodynamics. In order to find a route to a reliable prediction of ideal gas heat capacity data, we applied various quantum mechanical methods differing by computational effort to difluoromethane (CH₂F₂). Only the structural formula and fundamental physical constants enter into the calculations. It is shown that quantum mechanics leads to accurate molecular data. Reliable experimental heat capacity data reveal that an accuracy of better than 0.5% is obtained for the ideal gas heat capacity of difluoromethane.     

Standard thermodynamic properties of nitrobenzene in the ideal gas state

The standard enthalpy of formation of gaseous nitrobenzene was calculated by the G3X nonempirical quantum-mechanical method. The value obtained (56 kJ/mol) was noticeably lower than the experimental enthalpy, which casts doubt on the accuracy of measurements. The thermodynamic functions (C° _p , S°, ?[G° ? H°(0)]/T, H° ? H°(0), Δ_f H°, and Δ_f G°) of nitrobenzene in the ideal gas state were calculated by the statistical thermodynamics method over the temperature range 150–1500 K (the molecular constants, including the structural parameters, vibrational frequencies, and internal rotation potential used in the calculations were taken from the literature or estimated by the quantum-mechanical method).     

Zeolite 4A: heat capacity and thermodynamic properties

The heat capacity of zeolite 4A (also known as LTA, Linde Type A and sodium zeolite A), in the temperature range from 37 to 311 K, is reported. The heat capacity shows no anomalies in this temperature range. Thermodynamic parameters, H, S and G, relative to their values at T=0 K were derived. From these data, we find that zeolite 4A is stabilized by strong enthalpic interactions. Furthermore, its thermodynamic stability results from the strong Si---O and Al---O bonds in the primary building units, with bond strengths very close to those in other similar materials.     

Low-temperature heat capacity and thermodynamic properties of crystalline lead formate

As one 3-D coordination polymer, lead formate was synthesized; calorimetric study and thermal analysis for this compound were performed. The low-temperature heat capacity of lead formate was measured by a precise automated adiabatic calorimeter over the temperature range from 80 to 380 K. No thermal anomaly or phase transition was observed in this temperature range. A four-step sequential thermal decomposition mechanism for the lead formate was found through the DSC and TG-DTG techniques at the temperature range from 500 to 635 K.     

2-碘-3-硝基甲苯的低温热容和热力学性质

通过精密自动绝热热量计测定了2-碘-3-硝基甲苯(C~7H~6INO~2)在79～373K温区的摩尔热容。实验结果表明,这个化合物在331～340K温度区间有一个固-液熔化相变,其熔化温度、摩尔熔化焓、摩尔熔化熵以及该样品的化学纯度分别为：(339.311±0.13)J·mol^-^1·K^-^1和99.73%。用热容多项式议程进行数值积分获得了该物质在298.15～370K温区每隔5K的热力学函数值。用DSC分析对它的固-液相变过程作了进一步的研究。     

Specific heat capacity and thermodynamic properties of CuTeO3 and HgTeO3

Tellurites of CuTeO₃ and HgTeO₃ are synthesized and their specific molar heat capacities are experimentally determined for the first time. The tellurites discussed in the present paper are used for preparation of optical glasses with special properties for optoelectronics, nuclear and power industries. The tellurites synthesized are prepared for chemical analysis, differential thermal analysis and X-ray analysis. The use of the tellurites studied is related to knowing their thermodynamic properties like specific molar heat capacity (C _p,m), enthalpy \( \left( {\Delta_{{{\text {T}}^{\prime}}}^{\text{T}} H_{\text{m}}^{0} } \right), \) entropy \( \left( {\Delta_{{{\text {T}}^{\prime}}}^{\text{T}} S_{\text{m}}^{0} } \right) \) and Gibbs energy \( \left( { - \Delta_{{{\text {T}}^{\prime}}}^{\text{T}} G_{\text{m}}^{0} } \right) \) . The temperature dependences of their molar heat capacities are determined using the least squares method. The thermodynamic properties are calculated: entropy, enthalpy and Gibbs function.     

The low-temperature heat capacity of CaUO4 and SrUO4

The heat capacity of calcium monouranate CaUO₄ and strontium monouranate SrUO₄ have been measured over the temperature range (2 to 300) K. The results are significantly different from earlier measurements, confirming the conclusions from our previous study on BaUO₄. The standard entropy at T = 298.15 K of orthorhombic α-SrUO₄ is consistent with the values for the isostructural BaUO₄ as well as the alkali uranates Na₂UO₄ and Cs₂UO₄. The standard entropy of the rhombohedral CaUO₄ is appreciably different, which is attributed to the different structures of the uranium sublattices.     

Low-temperature heat capacity and thermodynamic properties of endo-Tricyclo[5.2.1.0]decane

Endo-Tricyclo[5.2.1.0^2,6]decane (CAS 6004-38-2) is an important intermediate compound for synthesizing diamantane. The lack of data on the thermodynamic properties of the compound limits its development and application. In this study, endo-Tricyclo[5.2.1.0^2,6]decane was synthesized and the low temperature heat capacities were measured with a high-precision adiabatic calorimeter in the temperature range from (80 to 360) K. Two phase transitions were observed: the solid-solid phase transition in the temperature range from (198.79 to 210.27) K, with peak temperature 204.33 K; the solid-liquid phase transition in the temperature range from 333.76 K to 350.97 K, with peak temperature 345.28 K. The molar enthalpy increments, ΔH_m, and entropy increments, ΔS_m, of these phase transitions are ΔH_m=2.57 kJ · mol⁻¹ and ΔS_m=12.57 J · K⁻¹ · mol⁻¹ for the solid-solid phase transition at 204.33 K, and, Δ_fusH_m=3.07 kJ · mol⁻¹ and Δ_fusS_m=8.89 J · K⁻¹ · mol⁻¹ for the solid-liquid phase transition at 345.28 K. The thermal stability of the compound was investigated by thermogravimetric analysis. TG result shows that endo-Tricyclo[5.2.1.0^2,6]decane starts to sublime at 300 K and completely changes into vapor when the temperature reaches 423 K, reaching the maximal rate of weight loss at 408 K.     

The low-temperature heat capacity of alkali and alkaline-earth metal uranoborates

The results of heat capacity measurements for several crystalline uranoborates over the temperature range 0–340 K were discussed and analyzed. Low-temperature heat capacities (T < 50 K) were considered using the Debye theory of the heat capacity of solids and its multifractal generalization. The fractal dimensions of compounds were calculated and the heterodynamic characteristics of their structures determined.     

Determination of low level methyl tert-butyl ether, ethyl tert-butyl ether and methyl tert-amyl ether in human urine by HS-SPME gas chromatography/mass spectrometry

Methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) are oxygenated compounds added to gasoline to enhance octane rating and to improve combustion. They may be found as pollutants of living and working environments. In this work a robotized method for the quantification of low level MTBE, ETBE and TAME in human urine was developed and validated. The analytes were sampled in the headspace of urine by SPME in the presence of MTBE-d12 as internal standard. Different fibers were compared for their linearity and extraction efficiency: carboxen/polydimethylsiloxane, polydimethylsiloxane/divinylbenzene, and polydimethylsiloxane. The first, although highly efficient, was discarded due to deviation of linearity for competitive displacement, and the polydimethylsiloxane/divinylbenzene fiber was chosen instead. The analysis was performed by GC/MS operating in the electron impact mode. The method is very specific, with range of linearity 30-4600 ng L⁻¹, within- and between-run precision, as coefficient of variation, <22 and <16%, accuracy within 20% the theoretical level, and limit of detection of 6 ng L⁻¹ for all the analytes. The influence of the matrix on the quantification of these ethers was evaluated analysing the specimens of seven traffic policemen exposed to autovehicular emissions: using the calibration curve and the method of standard additions comparable levels of MTBE (68-528 ng L⁻¹), ETBE (<6 ng L⁻¹), and TAME (<6 ng L⁻¹) were obtained.     

Nonempirical studies of the molecular properties and thermodynamic functions of urea in the ideal gas state

Quantum-chemical calculations were performed for the equilibrium structure of isolated urea molecules using the 6-311++G** basis set and second-order M?ller-Plesset perturbation theory, density functional theory, and the coupled cluster method with the local inclusion of electron correlation. The results were used to perform statistical calculations of the standard thermodynamic functions of urea in the ideal gas state taking into account anharmonicity of normal vibrations. The contributions of dimerization and isomerization of urea molecules in the vapor phase were determined. The recommended values were compared with the available experimental data and the results of preceding calculations. Original Russian Text ? A.V. Kuznetsov, A.V. Stolyarov, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 2, pp. 339–345.     

The thermodynamic properties and stability of tert-butyl peroxyesters of o-and m-carborane-1-carboxylic acids

The paper presents the results of experimental determination of the enthalpies of combustion and formation and the temperature dependence of the heat capacity of the o-and m-derivatives of tert-butyl peroxyesters of carborane-1-carboxylic acid. The entropies and Gibbs energies of formation of these compounds were calculated. The enthalpies, entropies, and Gibbs energies of the o-m transitions of the isomers in the condensed and gaseous states were determined.     

The low-temperature heat capacity and standard entropy of synthetic huttonite ThSiO4

The low-temperature heat capacity of synthetic huttonite ThSiO₄ has been measured from T = (2 to 300) K. The sample was synthesised successfully from SiO₂ and ThO₂ by solid-state reaction at T = 1873 K at atmospheric pressure. From the calorimetric results, the value for the standard entropy $S_{\text{m}}^{°}$ (ThSiO₄, huttonite, 298.15 K) = (104.3 ± 2.0) J · K^?1 · mol^?1 has been obtained. This value indicates that the entropy of reaction from SiO₂ and ThO₂ is negative, giving a positive entropy term (?T · Δ_rS) of the Gibbs free energy of reaction. The implications of this finding are discussed extensively.     

Dual capillary gas chromatographic analysis of alcohols and methyl tert-butyl ether in gasolines

A gas chromatographic method has been developed for the identification and direct determination of alcohols and methyl tert-butyl ether (MTBE) in gasolines. The technique involves simultaneous injection of the gasoline without any sample preparation onto two fused silica capillary columns of differing polarities. The method permits simultaneous determinations of methanol, ethanol, 2-propanol, tert-butanol, 1-propanol, sec-butanol, 1-butanol, and MTBE. By using an automatic sampler in combination with electronic pressure programming and BASIC programming, the determinations were performed automatically and reproducibly with a relatively short analysis time.     

Low-temperature polymerization of isobutyl vinyl ether

Isobutyl vinyl ether has been polymerized under conditions well known to yield isotactic polymer, viz., with boron trifluoride etherate at 78°C. in a nonpolar hydrocarbon diluent. A particular mixed solvent ratio and previous dissolution of catalyst enabled the polymerization to proceed homogeneously at the beginning. By following the temperature rise in an initially thermostatted system, we showed that the progress of the reaction eventually proceeded via a homogeneous phase to a gellike phase. Isotactic polymer is shown to be produced in both steps by a mechanism of slow chain propagation.     

Method for determination of methyl tert-butyl ether in gasoline by gas chromatography

A simple method for the determination of methyl tert-butyl ether (MTBE) in gasoline has been developed. The separation of MTBE from other analytes was controlled by the use of gas chromatography–mass spectrometry in the full scan mode using the characteristic primary, secondary and tertiary ions m/z 73, 57 and 43. The sample mass spectrum did not show any superimposition of other analytes. The separation from the common gasoline component 2-methylpentane was sufficient for reliable quantitation. An application of the developed conditions using gas chromatography with flame ionization detection was performed by the analysis of regular, euro super, super premium unleaded and ‘Optimax’ gasoline from petrol stations in the area of Frankfurt/Main, Germany. Regular unleaded gasoline shows an average MTBE content of 0.4% (w/w), whereas the MTBE content in euro super gasoline varies between 0.4 and 4.2% (w/w). The blending of MTBE to super premium has increased from 8.2% (w/w) in 1998 to 9.8% (w/w) on average in 1999. The recently introduced gasoline ‘Optimax’ shows an average MTBE content of 11.9% (w/w). The presented method might also be used for the analysis of other ethers, such as ethyl tert-butyl ether, which requires the use of another internal standard.