Molecular Structure,Thermodynamic and Spectral Characteristics of Metal-Free and Nickel Complex of Tetrakis(1,2,5-thiadiazolo)porphyrazine

The Knudsen effusion method with mass spectrometric control of the vapor composition was used to study the possibility of a congruent transition to the gas phase and to estimate the enthalpy of sublimation of metal-free tetrakis(1,2,5-thiadiazolo)porphyrazine and its nickel complex (H₂TTDPz and NiTTDPz, respectively). The geometrical and electronic structure of H₂TTDPz and NiTTDPz in ground and low-lying excited electronic states were determined by DFT calculations. The electronic structure of NiTTDPz was studied by the complete active space (CASSCF) method, following accounting dynamic correlation by multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D_2h and D_4h symmetry was obtained for H₂TTDPz and NiTTDPz, respectively. According to data obtained by the MCQDPT2 method, the nickel complex possesses the ground state ¹A_1g, and the wave function of the ground state has the form of a single determinant. Electronic absorption and vibrational (IR and resonance Raman) spectra of H₂TTDPz and NiTTDPz were studied experimentally and simulated theoretically.     

Modeling of Turbulent Natural Convection in Enclosed Tall Cavities

It was shown in our previous work (J. Appl. Mech. Tech. Phys 57 (7), 1159–1171 (2016)) that the eddy-resolving parameter-free CABARET scheme as applied to two-and three-dimensional de Vahl Davis benchmark tests (thermal convection in a square cavity) yields numerical results on coarse (20 × 20 and 20 × 20 × 20) grids that agree surprisingly well with experimental data and highly accurate computations for Rayleigh numbers of up to 10¹⁴. In the present paper, the sensitivity of this phenomenon to the cavity shape (varying from cubical to highly elongated) is analyzed. Box-shaped computational domains with aspect ratios of 1: 4, 1: 10, and 1: 28.6 are considered. The results produced by the CABARET scheme are compared with experimental data (aspect ratio of 1: 28.6), DNS results (aspect ratio of 1: 4), and an empirical formula (aspect ratio of 1: 10). In all the cases, the CABARET-based integral parameters of the cavity flow agree well with the other authors’ results. Notably coarse grids with mesh refinement toward the walls are used in the CABARET calculations. It is shown that acceptable numerical accuracy on extremely coarse grids is achieved for an aspect ratio of up to 1: 10. For higher aspect ratios, the number of grid cells required for achieving prescribed accuracy grows significantly.     

A study of the reaction mechanism of 3-nitro-4-R-furoxans formation by nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro-1-R-ethanes

The mechanism proposed earlier for explanation of the furoxan ring formation in the nitrosation of dipotassium salts of 1-hydroxyimino-2,2-dinitro-1-R-ethanes with NaNO₂/AcOH was confirmed experimentally by determining the ionization constants of the dinitromethyl and oxime fragments in the starting dipotassium salt and by examining ¹H, ¹³C, ¹⁴N, and ¹⁵N NMR and mass spectra of isomeric 3(4)-nitro-4(3)-R-furoxans with the ¹⁵N(5) and ¹⁵N(2) ring atoms, respectively, and 3,4-diarylfuroxan with both ¹⁵N-labeled ring atoms. A comparative analysis of the IR and Raman spectra of the ¹⁵N-labeled furoxan derivatives obtained and their unlabeled analogs allowed refinement of the literature data on interpretation of the vibrational spectra of furoxan derivatives.     

Synthesis of isomeric 1,3- and 1,4-bis[3(4)-nitrofuroxan-4(3)-yl]nitrobenzenes by nitration of the corresponding isomeric 1,3- and 1,4-bis[3(4)-nitrofuroxan-4(3)-yl]benzenes

Isomeric 1,3- and 1,4-bis[3(4)-nitrofuroxan-4(3)-yl]nitro(dinitro)benzenes were synthe-sized in high yields by nitration of the corresponding 1,3- and 1,4-bis[3(4)-nitrofuroxan-4(3)-yl]benzenes with a mixture of 100% nitric acid and concentrated sulfuric acid. The influence of 3- and 4-nitrofuroxanyl fragments on the regioselectivity of the nitration was revealed. The structure of 1,3-bis(4-nitrofuroxan-3-yl)-4-nitrobenzene was confirmed by X-ray diffraction analysis. 1,3- and 1,4-Bis(3-nitrofuroxan-4-yl)nitrobenzenes underwent thermal isomerization to more thermodynamically favorable 1,3- and 1,4-bis(4-nitrofuroxan-3-yl)nitrobenzenes.     

Synthesis and nitration of 3-R-4-(2,2,2-trinitroethyl)aminofuroxans

Simple methods for the synthesis of 3-R-4-(2,2,2-trinitroethyl)aminofuroxans were developed based on the Mannich reaction of N,N′-bis(3-R-furoxan-4-yl)methylenediamines with trinitromethane or 3-R-4-aminofuroxans with trinitroethanol or with CH₂O and trinitromethane. The Mannich bases obtained were studied in the nitration reaction, which showed their ability to form nitramines depending on substituents on the furoxan ring.     

Study of electronic structure of certain phenazasilines by quantum chemistry and UV spectroscopy methods

A spectroscopic and quantum-chemical investigation of heterocyclic derivatives of 9,10-dihydroanthracene, viz., phenazasilines EtN(C₆H₄)₂SiMe₂ (I), MeN(C₆H₄)₂SiMe₂ (II), and EtN(C₆H₄)₂SiHME (III), was carried out. The experimental absorption spectra were theoretically interpreted by calculating the electronic structure of the molecules by the CNDO/S3 method, whose system of semiempirical parameters was supplemented by the silicon parameters. The nonplanar structure of the phenazasiline heterocyclic ring and the presence of two butterfly type conformations in the molecule of III was taken into account. The calculated bond lengths and oscillator forces of the electronic transitions agree well with the UV spectroscopy data and the photoelectronic spectrum of the molecule of II. The polarization spectra of phenazasilines I–III were predicted.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 229–232, March–April, 1985.     

The First Synthesis of Furoxan and 1,3,4‐Oxadiazole Ring Ensembles

Previously unknown furoxan and 1,3,4‐oxadiazole ring ensembles incorporating two, three, and five furoxan and 1,3,4‐oxadiazole rings in different combinations were for the first time synthesized from accessible azides and hydrazides of furoxancarboxylic acids. An interdependence of furoxan and 1,3,4‐oxadiazole rings on their geometric parameters was revealed by the X‐ray diffraction method.     

Reactions of furoxanyl and furazanyl diazonium salts with NaNO2 in weakly acidic medium, a new approach to the preparation of nitrofuroxans and nitrofurazans

A new approach to the preparation of nitrofuroxans and nitrofurazans is suggested based on the diazotization of aminofuroxans and aminofurazans in aqueous organic medium at pH = 4?C5 in the presence of excess NaNO₂.     

Synthesis and nitration of <Emphasis Type="Italic">N,N</Emphasis>´-bis(3-R-furoxan-4-yl)methylenediamines

A Mannich reaction of 4-amino-3-R-furoxans with paraformaldehyde in 10% aqueous H₂SO₄ led to the high yields of N,N′-bis(3-R-furoxan-4-yl)methylenediamines, whose structure (using R = Me as an example) was confirmed by X-ray diffraction study. The N,N′-bis-(3-R-furoxan-4-yl)methylenediamines obtained were nitrated to N,N′-dinitro-N,N′-bis-(3-R-furoxan-4-yl)methylenediamines upon the action of 100% HNO₃ in acetic or trifluoroacetic anhydride.     

Parameter-free numerical method for modeling thermal convection in square cavities in a wide range of Rayleigh numbers

Some numerical results for the two- and three-dimensional de Vahl Davis benchmark are presented. This benchmark describes thermal convection in a square (cubic) cavity with vertical heated walls in a wide range of Rayleigh numbers (10⁴ to 10¹⁴), which covers both laminar and highly turbulent f lows. Turbulent f lows are usually described using a turbulence model with parameters that depend on the Rayleigh number and require adjustment. An alternative is Direct Numerical Simulation (DNS) methods, but they demand extremely large computational grids. Recently, there has been an increasing interest in DNS methods with an incomplete resolution, which, in some cases, are able to provide acceptable results without resolving Kolmogorov scales. On the basis of this approach, the so-called parameter-free computational techniques have been developed. These methods cover a wide range of Rayleigh numbers and allow computing various integral properties of heat transport on relatively coarse computational grids. In this paper, a new numerical method based on the CABARET scheme is proposed for solving the Navier–Stokes equations in the Boussinesq approximation. This technique does not involve a turbulence model or any tuning parameters and has a second-order approximation scheme in time and space on uniform and nonuniform grids with a minimal computational stencil. Testing the technique on the de Vahl Davis benchmark and a sequence of refined grids shows that the method yields integral heat f luxes with a high degree of accuracy for both laminar and highly turbulent f lows. For Rayleigh numbers up to 10¹⁴, a several percent accuracy is achieved on an extremely coarse grid consisting of 20 × 20 cells refined toward the boundary. No definite or comprehensive explanation of this computational phenomenon has been given. Cautious optimism is expressed regarding the perspectives of using the new method for thermal convection computations at low Prandtl numbers typical of liquid metals.