Cyclic alkylene carbonates. Experiment and first principle calculations for prediction of thermochemical properties

The standard molar enthalpies of formation of ethylene carbonate, propylene carbonate, and butylene carbonate were measured using combustion calorimetry. Ab initio calculations of molar enthalpies of formation of alkylene carbonates were performed using the G3MP2 method. The calculated values are in excellent agreement with available experimental data. Ring strain corrections were quantified for the refinement of the group-contribution method for prediction of enthalpies of formation and vaporization of alkylene carbonates.     

Theoretical and experimental studies of collision-induced electronic energy transfer from v=0-3 of the E(0g +) ion-pair state of Br2: collisions with He and Ar

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.     

Vapour pressure and enthalpy of vaporization of aliphatic dialkyl carbonates

Molar enthalpies of vaporization of aliphatic alkyl carbonates: dimethyl carbonate [616-38-6], diethyl carbonate [105-58-8], di-n-propyl carbonate [623-96-1], di-n-butyl carbonate [542-52-9], and dibenzyl carbonate [3459-92-5] were obtained from the temperature dependence of the vapour pressure measured by the transpiration method. A large number of the primary experimental results on temperature dependences of vapour pressures have been collected from the literature and have been treated uniformly in order to derive vaporization enthalpies of dialkyl carbonates at the reference temperature 298.15 K. An internal consistency check was performed on enthalpy of vaporization values for dialkyl carbonates studied in this work.     

Studies toward the synthesis of 4-(2-R-ethyl)amino-2,2,5,5-tetramethyl-3-imidazoline 1-oxyls. nucleophilic substitution of bromide in the N-Alkyl chain of the 1,2,4-oxadiazol-2-one precursor

A synthetic approach to access the new nitroxides of the amidine type exhibiting pH-dependent EPR spectra through substitution of a halide in the exo-N-halogenoalkyl chain of 1-(2-bromoethyl)-6-oxyl-5,5,7,7-tetramethyltetrahydroimidazo[1,5-b][1,2,4]oxadiazol-2-one is reported. In this approach, an oxycarbonyl moiety of the oxadiazolone heterocycle plays the role of a "protecting group" for the amidine functionality. A nucleophilic cleavage of the oxadiazolone heterocycle under mild nonbasic conditions, applicable to substrates bearing substituents vulnerable to attack by strong basic nucleophiles, is elaborated. The approach allows for the new amidine nitroxides bearing various functional groups (e.g., such as CN, N3, NH2, COOEt) to be synthesized. A series of nitroxides obtained through the Staudinger/intermolecular aza-Wittig reaction of the azido derivative is also described. The nitroxides synthesized here were found to have pH-dependent two-component EPR spectra indicative of a slow, on the EPR time scale, R* left arrow over right arrow R*H+ chemical exchange, and pKa values ranging from 2.8 to 12.5 units. The guanidine derivatives synthesized in this work show the highest pKa values (pKa = 10.2 and 12.5, respectively) ever reported for the nitroxide pH-probes of a "basic type".     

Study of structural characteristics of ferromagnetic silicon implanted with manganese

The structure of manganese-implanted (dose 2 × 10¹⁶ cm^?2) n-type and p-type silicon, ferromagnetic at room temperature, has been studied. During implantation, an amorphized layer is formed in the silicon wafer. Subsequent vacuum annealing improves the structural quality of the implanted material and leads to the formation of a vacancy solid solution of manganese in silicon. A difference in the degree of structural quality of silicon implanted by different impurities has been experimentally established.     

Neutron diffraction and X-ray diffraction investigations of langasite crystals

Crystals of langasite La₃Ga₄(GaSi)O₁₄ grown by the Czochralski method are studied using neutron diffraction for the first time. It is established that the compositions of the upper and lower parts of an orange crystal grown from the La₃Ga_5.14Si_0.86O₁₄ seed in an (Ar + O₂) atmosphere (the 〈 0001 〉 growth direction) can be written as(La_2.85(2)□_0.15)(Ga_0.95(2)□_0.05) Ga₃(Ga_1.15 Si_0.85(5))(O_13.72□_0.28(7)) and(La_2.89(1)□_0.11)·(Ga_0.98(1)□_0.02) Ga₃(Ga_1.06Si_0.94(4))(O_13.81□_0.19(9)), respectively. The La content in the upper and lower parts of this crystal is lower and higher than the Ga content, respectively, and the Ga content exceeds the Si content in the (GaSi) position. By contrast, in a colorless crystal of the composition(La _2.97(4)□_0.03) Ga(1)(Ga_2.94(9)□_0.06)(Ga_0.7(1) Si_1.3)(O_13.9□_0.1(1)), which is grown from the La₃ Ga₅SiO₁₄ seed in an argon atmosphere (the 〈 01\(\bar 1\)1 ?growth direction), the Ga content in the (GaSi) position is lower than the Si content. A relation between the Ga: Si ratio and the (Ga,Si)-3O interatomic distances is found.     

Optical properties of trigonal single crystals (Yb,Tm)Al3(BO3)4 grown from fluxes based on the bismuth and lithium molybdates

The conditions for synthesis of Yb _x Tm_{1 − x} Al₃(BO₃)₄ (x = 0, 0.1, 0.2, 1.0) single crystals from fluxes based on bismuth trimolybdate Bi₂Mo₃O₁₂ and lithium molybdate Li₂MoO₄ are investigated. It is proposed to grow them by the group method on seeds. The polarized optical absorption spectra are measured for two mutually orthogonal linear polarizations at temperatures of 100 and 300 K.     

Single-crystal growth of trigonal DyFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and study of magnetic properties

Fluxes with a wide stability range of trigonal DyFe₃(BO₃)₄ were selected by the successive-approximation method using direct phase probing in the Bi₂Mo₃O₁₂-B₂O₃-Dy₂O₃-Fe₂O₃ system. Crystallization parameters of the fluxes were determined. The conditions for single-crystal growth were recommended using both spontaneous crystal nucleation and seeded growth according to the Kyropoulos method. An analysis of the measured temperature and field dependences of magnetization of crystals in fields parallel or perpendicular to the threefold axis led to the conclusion that DyFe₃(BO₃)₄ undergoes a phase transition to an ordered easy-axis state and is characterized by a field-induced spin-reorientation transition with the predominant alignment of Dy³⁺ spins along the applied field.     

Tuning the Electrochemical Performance of Titanium Carbide MXene by Controllable In Situ Anodic Oxidation

MXenes are a class of two‐dimensional (2D) transition metal carbides, nitrides and carbonitrides that have shown promise for high‐rate pseudocapacitive energy storage. However, the effects that irreversible oxidation have on the surface chemistry and electrochemical properties of MXenes are still not understood. Here we report on a controlled anodic oxidation method which improves the rate performance of titanium carbide MXene (Ti₃C₂T_x, T_x refers to ‐F, =O, ‐Cl and ‐OH) electrodes in acidic electrolytes. The capacitance retention at 2000 mV s^?1 (with respect to the lowest scan rate of 5 mV s^?1) increases gradually from 38 % to 66 % by tuning the degree of anodic oxidation. At the same time, a loss in the redox behavior of Ti₃C₂T_x is evident at high anodic potentials after oxidation. Several analysis methods are employed to reveal changes in the structure and surface chemistry while simultaneously introducing defects, without compromising electrochemically active sites, are key factors for improving the rate performance of Ti₃C₂T_x. This study demonstrates improvement of the electrochemical performance of MXene electrodes by performing a controlled anodic oxidation.