Unsteady flows in a fluid in pipes made of viscoelastic active material

Results are given of the numerical solution of the equations which describe in the quasione-dimensional approximation the motion of a fluid in a pipe made of a viscoelastic material with a nonmonotonic S-shaped static characteristic curve. Some possible ways are considered for obtaining simpler models with lumped parameters for such flows. There is a discussion of the application of the theory to the problem of the tone of blood vessels.     

One- and two-dimensional NMR study of structure of 1,2-disubstituted p-tert-butylthiacalix[4]arene containing amide fragment

The structure of 5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-bis[N-(4′-nitrophenyl)aminocarbonylmethoxy] thiacalix[4]arene I was examined by 1D and 2D (NOESY) ¹H and ¹³C NMR methods in a CDCl₃ solution using numerical simulation (semi-empirical quantum-chemical calculations, PM3 method). Compound I was found to exist in the 1,2-alternate conformation, where bulky substituents OCH₂C(O)· NHPhNO₂ are in the endo-position relative to the macrocycle cavity.     

Dependence of cooperative emission spectra of complex organic molecules on exciting photon energy

The dependences of cooperative emission, superfluorescence, and lasing spectra on the wavelength of pumping radiation were studied for highly concentrated ethanol solutions of organic dye molecules. The relative intensity of cooperative emission was found to decrease, while the intensities of superfluorescence and lasing were found to increase with the energy of exciting photons. The increase in the dephasing rate of quantum states is supposed to be primarily responsible for the change in proportion between these intensities. On increasing the dephasing rate, with all other factors being the same, the concentration of the excited molecules with the phased quantum states decreases, while that with the dephased states increases. The ratio between the cooperative emission and dephasing rates determines the proportion between the intensities of cooperative emission, superfluorescence, and lasing. The onset of lasing at high concentrations (above >10¹⁹ cm^?3) in the case of pumping by high-energy photons occurs due to the high dephasing rates of quantum states resulting from a more rapid redistribution of vibrational energy.     

Near field of a low-frequency source of forced vibration on a layered biological tissue

Results of a numerical calculation of the near acoustic field excited by a low-frequency source of forced vibration positioned on the surface of a layered biological tissue are presented. The latter is simulated by a soft tissue layer varying in thickness and overlying a solid halfspace. The spatial distribution of the acoustic field in the biological tissue is obtained.     

Influence of medium on thermodynamic parameters of conformational conversions of certain partially fixed seven-membered 1,3-heterocycles,as indicated by NMR data

It has been shown that the observed differences in enthalpy and entropy between equilibrium states of seven-membered 1,3-heterocycles contained contributions from the temperature dependence of permittivity of the medium. These contributions are comparable in magnitude to the differences in enthalpy and entropy characterizing the conformational equilibrium in such solvents as chloroform and methylene chloride. The changes in enthalpy difference of the conformers chair twist and chair boat in these compounds when the medium is changed are in good agreement with reactive field theory.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1300–1303, June, 1990.     

2M-NMR detection of spectrally nonobservable components of a conformational exchange

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 224–225, January, 1990.     

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.     

Theoretical and experimental study of imine-enamine tautomerism of condensation products of propanal with 4-aminobenzoic acid in ethanol

The tautomerism of the reaction products of propanal with 4-aminobenzoic acid in ethanol was studied by J-modulated spin-echo (JMOD) ¹³C NMR spectroscopy and gradient-enhanced heteronuclear (ge-2D) ¹H–¹³C HSQC spectroscopy. The existence of imine and enamine tautomeric forms of the reduced compounds in solution was established. The tautomeric equilibrium of the condensation product of propanal with 4-aminobenzoic acid in ethanol was found to be shifted toward the imine form. Quantum chemical calculations by the density functional theory (DFT) method demonstrated that the 4-(N-propylidene)aminobenzoic acid molecule forms a stronger hydrogen bond with an ethanol solvent molecule compared to the enamine molecule, resulting in a higher stability of the ethanol adduct of azomethine compared to the adduct of enamine.     

p-tert-Butyl thiacalix[4]arenes functionalized at the lower rim by amide, hydroxyl and ester groups as anion receptors

New p-tert-butyl thiacalix[4]arenes differently substituted at the lower rim with amide, hydroxyl and ester groups were synthesized. Binding properties of the compounds toward some tetrabutylammonium salts n-Bu(4)NX (X = F(-), Cl(-), Br(-), I(-), CH(3)CO(2)(-), H(2)PO(4)(-), NO(3)(-)) were studied by UV spectroscopy. It was found that the stoichiometry of the complexes, generally, is 1 : 1, and the association constants are in the range of 10(3)-10(5) M(-1). The p-tert-butyl thiacalix[4]arenes containing secondary amide groups trisubstituted at the lower rim bind the studied anions most effectively. Selective receptors for fluoride and dihydrogen phosphate salts of tetrabutylammonium were found.