New type of phase transformation in gas hydrate forming system at high pressures. Some experimental and computational investigations of clathrate hydrates formed in the SF6-H2O system

In this work, we present a new, previously unknown type of structure transformation in the high-pressure gas hydrates, which is related to the existence of two different isostructural phases of the sulfur hexafluoride clathrate hydrates. Each of these phases has its own stability field on the phase diagram. The difference between these hydrates consists of partial filling of small D cages by SF(6) molecules in the high-pressure phase; at 900 MPa, about half of small cages are occupied. Our calculations indicate that the increase of population of small cavities is improbable, therefore, at any pressure value, a part of the cavities remains vacant and the packing density is relatively low. This fact allowed us to suppose the existence of the upper pressure limit of hydrate formation in this system; the experimental results obtained confirm this assumption.     

Megawatt-Level Repetitively Pulsed Erbium 3-µm Laser with Strong Thermal Lens Compensation

JETP Letters - We report on an increase in the average power of a repetitively pulsed (10 Hz) nanosecond 3-µm laser due to the compensation of a strong thermal lens in the Cr:Er:YSGG laser...     

[4+2]-Cycloaddition of 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines with alkenes

A number of 1,4-dihydropyridazines and pyridazines were prepared by the Diels-Alder reaction with an inverse electron demand from cyclic heterodiene systems, 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines, and some enamines as well as from 4-vinylpyridine, butyl vinyl ether, phenylacetylene, and acrylamide. The reaction of 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine with styrene afforded 4,5-dihydropyridazine, which was readily oxidized by atmospheric oxygen to form the corresponding pyridazine. Electron-withdrawing substituents (Br or Cl) in the pyrazole rings accelerate [4+2]-cycloaddition. When heated, 1,4-dihydropyridazines, which were synthesized from tetrazines and enamines, eliminated amine to give pyridazines. The reactivities of tetrazines were evaluated by quantum-chemical methods. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 354–360, February, 2000.     

Theoretical estimation of the possibility of formation of oxadiazocines in the nucleophilic addition of resorcinol to pyrimidines and synthesis of new azoloannelated benzooxadiazocines

A method was proposed for estimating the possibility of cyclization of adducts of resorcinol with pyrimidines to form oxadiazocines based on the conformational criterion and analysis of charges on the C(5) atom of the pyrimidine ring and on the H atom of the hydroxy group of the resulting adducts. A series of new derivatives of 4,5-dihydro-11H-5,11-methanobenzo[g]azolo[1,3,5]oxadiazocines were synthesized. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1573–1577, August, 1999.     

Effect of the Molecular Weight of AB Diblock Copolymers on the Lamellar Orientation in Thin Films: Theory and Experiment

We propose a theoretical explanation of the parallel and perpendicular lamellar orientations in free surface films of symmetric polystyrene‐block‐polybutadiene diblock copolymers on silicon substrates (with a native SiO_x layer). Two approaches are developed: A correction to the strong segregation theory and a qualitative analysis of the intermediate segregation regime. We show that the perpendicular orientation of the lamellae formed by the molecules of high molecular weight is stabilized by A–B interfacial interactions. They are weaker in the case of the perpendicular orientation of the lamellae, whereas the surface tension coefficient of the A–B interface decreases with the increase of the molecular weight.

     

Optical Harmonics Generation under the Interaction of Intense (up to 1014 W/cm2) Mid-Infrared Femtosecond Laser Radiation of a Fe:ZnSe Laser System with a Dense Laminar Gas Jet

JETP Letters - Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (I ~ 1014 W/cm2) femtosecond mid-infrared radiation of a laser...     

High-power mid-IR (4–5 μm) femtosecond laser system with a broadband amplifier based on Fe<Superscript>2+</Superscript>:ZnSe

Concept of a solid-state femtosecond laser system with a multigigawatt power level in the 4–5 μm range has been proposed. The system contains an ultrashort pulse seeder, a two-stage chirped pulse amplifier based on a broadband Fe²⁺:ZnSe active element with optical pumping by a YSGG:Cr:Er laser, and an output stage that provides additional nonlinear optical compression of an amplified pulse to approximately 30 fs in a dielectric CaF₂ medium with anomalous group velocity dispersion in this spectral range.     

Real-Time Monitoring of the Energy Deposition under the Tight Focusing of Femtosecond Laser Radiation into a Bulk Transparent Dielectric Based on Third Harmonic Signal

It has been found that the third harmonic generated under the tight focusing of a near-infrared (1.24-μm) femtosecond laser pulse indicates the energy deposition into the bulk transparent dielectric and can be used as a feedback system in the process of microstructuring. A third harmonic signal is sensitive to a change in both the size of a laser-induced microplasma and the free-electron density, which allows the detection of the microplasma with submicron longitudinal sizes. It has been shown that the method based on the third harmonic is universal and can be applied in both single-pulse and two-color microstructuring.     

Intermolecular interactions in heteromolecular crystals of tetrazine derivatives with azoles

Heteromolecular crystal structures formed by symmetrically and unsymmetrically 3,6-disubstituted tetrazine derivatives with NH donor azoles are investigated. The main crystal motifs and the intermolecular interactions responsible for their formation are identified.