NO2 lifetimes by hanle effect measurements

We explain controversial results of previous Hanle effect (zero magnetic field level crossing) experiments on NO₂. It is shown that the magnetic resonance signal is in general associated with two characteristic times, the radiative lifetime τ_R ≳ 40 μs and an intramolecular decay time τ₀ ≈ 3 μs. The contributions of both times to the magnetic resonance signal have to beclearly separated to give reliable results. The two times τ_R and τ₀ are observed for all investigated absorption lines near 593 and 514 nm.     

The Fokker-Planck-Langevin model for rotational brownian motion. IV. Asymmetric top molecules

The general series expansion for the joint angular velocity-orientation conditional probability density for a fluid composed of asymmetric top molecules is derived, and expressions for the reorientational correlation functions, correlation times, spectral densities, and the correlation times relevant to magnetic relaxation via spin-rotation interactions are presented. The reorientational correlation times and spin-rotation functions computed using this Fokker-Planck-Langevin (FPL) model with isotropic friction tensor (τ_x = τ_y = τ_z) are compared with the corresponding times and functions computed with the J-diffusion limit of the extended diffusion (EDJ) model. The models are found to predict significantly different behaviour only in the regime where free rotation and precessional effects become important. The reorientation self- and cross-correlation times are shown to follow Hubbard relations in the limit of short τ_x, τ_y, τ_z. Numerical calculations of the FPL reorientational correlation functions, correlation times and spin-rotation functions show that these properties are sensitive to the anisotropies in the friction tensor in the rotational Langevin equation.     

Estimation of the kinetic characteristics of sorption of an organic ion on a heterogeneous crosslinked polymer sorbent within the framework of a bidisperse model

For the sorption of rubomycin, an antitumor athracycline-type antibiotic, on BDM-12 carboxyl-containing heterogeneous crosslinked polymer sorbent, it was shown that the measured time dependences of the extent of process are determined by two characteristic times: τ₁ (in the range of short times) and τ₂ (at long times). A phenomenological theory of the kinetics of sorption on the heterogeneous sorbent was developed on the basis of a biporous sorbent model. The dependences of the characteristic times τ₁ and τ₂ on the sorbent grain radius were obtained. It was concluded that the theory makes predictions in good agreement with experimental data and allows calculating the most important kinetic parameters of sorption of organic ions on polymer sorbents: the time of diffusion of the sorbate into microgranules, the diffusion coefficient of the sorbate in transport pores, the effective coefficient of the sorbate diffusion into the heterogeneous sorbent, etc.     

INTRINSIC POLARIZATION AND ROTATIONAL DEPOLARIZATION OF FLUORESCENCE OF DNA BASES IN AQUEOUS SOLUTION AT ROOM TEMPERATURE*

Abstract— A counting procedure is described for determining the polarization characteristics of very weak emission. The degree of polarization of weak emission of DNA bases in aqueous solution at room temperature is found to he high and positive, and shows an inverse correlation with their emission quantum yields which can be understood in terms of a competition between emission lifetimes and rotational relaxation times. For 5-methylcytosine the emission quantum yield can be changed by varying the acidity and the corresponding degrees of polarization show the behavior expected from Perrin's relation. The ratio τ_o/τ_rot is discussed in terms of anisotropic relaxation and calculations of τ₀.     

Nuclear relaxation and molecular motion of 1,3,5-trifluorobenzene-d3 in liquid solutions

The ²D and ¹⁹F spin-lattice relaxation times of 1,3,5-trifluorobenzene-d₃ in 0.15 mol fraction solutions in various solvents have been measured over the temperature range 270–400 K. The relationship between the reorientational correlation times, τ_θ, and the angular momentum correlation times, τ_J, obtained from the nuclear relaxation data has been compared with the theoretical relationships predicted by the J-diffusion limit of the extended diffusion (EDJ) model and the Fokker-Planck-Langevin (FPL) model for symmetric top molecules. It was found that the rotational motion of 1,3,5-trifluorobenzene-d₃ in most solutions was better described by the FPL model with frictional anisotropy τ₆/τ_· in the range 1–2. (τ₆ and τ_· are the correlation times for the angular velocity components along the axes parallel and perpendicular to the molecular symmetry axis.) The viscosity and temperature dependence of τ_θ has been analyzed in terms of a modified Debye equation and values for the anisotropic interaction parameter, κ, in each solvent are reported. The variation in κ with solvent is attributed mainly to the variation in the dipole moment of the solvent molecules. The frictional anisotropy (τ₆/τ_·) is found to decrease as κ increases. This is interpreted in terms of the effects of molecular shape and electrostatic interactions between CF bond dipole moment and solvent dipole moments.     

FRAGMENTATION OF PRIMARY FLOCS IN EMULSIONS AND THE SUBSEQUENT REDUCTION OF COALESCENCE

In existing theories emulsion desiabilization is considered as the combined processes of irreversible flocculation and coalescence of dispersed droplets. This approach can be justified when the potential pit characterizing the energy of droplet interaction is sufficiently deep, i.e. excluding small droplet dimensions, strong electrosiatic repulsion and low electrolyte concentrations. For smaller droplet dimensions and stronger electrostatic repulsions the emulsion instability must be considered as a combined process of reversible flocculation and coalescence. In this paper a mathematical model that couples the kinetics of flocculation, coalescence and floe fragmentation is developed in order to quantify the kinetic instability of emulsions with charged submicron droplets. The characteristic limes for flocculation (Smoluchowski's time τ_c) for coalescence (coalescence time τ_c) and for disaggregation (doublet lifetimeτ_d) are considered model parameters. The mathematical model applies to the case when and τ_d<< τ_c, which corresponds to a situation with a small multiplet concentration compared to the concentration of doublets and a singlet-doublet quasi-equilibrium. It is established that at singlet-doublet quasi-equilibrium the rate of the decline in the total droplet concentration is described by second order kinetics in distinction to the exponential time dependence valid for coalescence at irreversible flocculation. The double disintegration reduces the entire coalescence rate, expressed as τ_sm/ τ_d. This reduction is very large at small values of Td. The mathematical model presented can hased on the spontaneous disintegration of doublets predict changes in emulsion stability for model systems and also for technologically important emulsions.     

Relation between momentum and angular momentum correlation times. analysis of the uncorrelated successive binary-collision approximation

A relation between the correlation times τ_P and τ_J for a system of hard spheres of arbitrary roughness has been found in the uncorr     

含氟离聚体的多相结构及大分子链运动的NMR研究

 本文用¹³C自旋-自旋弛豫时间T₂表征了以丙烯酸-1,1,5-三氢全氟戊酯-丙烯酸共聚物为基础的离聚体体系的多相结构和大分子链段运动特性,结果表明:离子微区和聚合物基体之间存在界面层,聚合物主链的运动活性与离聚体的共聚物组成、金属离子特性、离子化程度、离子微区的稳定性和离子微区内的精细结构均有密切关系.     

Depolarized Rayleigh scattering studies of molecular motion of 1,3,4-[D2] thiadiazole

Depolarized Rayleigh scattering spectra have been measured using a Fabry-Pérot interferometer for 1,3,4-[D₂] thiadiazole at temperatures between 20 and 92.5°C, covering a large portion of the liquid and supercooled liquid regions. From the spectral halfwidth measurements, the Rayleigh relaxation times, τ_Ray, have been obtained. A semilog plot of τ_Ray versus T^?1 shows that the data fit well to an Arrhenius equation, with activation energy equal to 2.4 kcal/mole. The relation of τ_Ray to pair orientation correlations is discussed.     

Comparison of the mechanical properties of styrene‐based ionomers containing either crown ether or pentaethylene glycol with those of underneutralized ionomers

Dynamic mechanical properties of styrene‐based ionomers containing varying amounts of either 15‐crown‐5 ether (CE) or pentaethylene glycol (PG) are compared with those of ionomers of varying degree of neutralization (ND). The cluster T_g (T_g,c) and ionic modulus of the ionomers decrease with increasing amount of CE or PG or decreasing ND. Thus, we propose that the CE binds Na⁺ strongly to form a large‐sized complex. Thus, the electrostatic interactions between charges decrease, leading to lower T_g,c. For the PG‐containing ionomers, the PG acts as polar plasticizer, further lowing the T_g,c. In the case of the underneutralized ionomers, the T_g,c is reduced by the existence of both relatively weak hydrogen bonds between carboxylic acid groups and relatively strong ionic bonds between ion pairs in the multiplets. The small‐angle X‐ray scattering results are also supportive of the above interpretations. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1358–1367     

Reorientation of hydrogen-bonded solvents around a large dipole

Using picosecond spectroscopy, we find rise times τ_R ? 20 ps for p-dimethylaminobenzonitrile S₂ → S₀ fluorescence in the weakly hydrogen-bound alcohols 2,2,2-trifluoroethanol and 2-fluoroethanol. Since we previously obtained τ_R ≈ 40 ps in methanol and ethanol, this provides evidence that hydrogen-bonding-retarded solvent reorientation largely governs the fluorescence in these solvents.     

Magnetic field induced cross relaxation between two different spin transitions of triplet coumarin

At certain magnetic fields, when the energy separations between the τ_x — τ_y spin levels and between the τ_y — τ_z levels of triplet coumarin become equal, intensity changes are observed in the phosphorescence of coumarin doped in a single crystal of durence cooled to 1.6 K that can be attributed to cross relaxation (CR) between the τ_x — τ_y spin levels of one triplet coumarin molecule and the τ_y — τ_z spin levels of another triplet coumarin. Rate equations that describe the time behavior of the populations of the spin levels which are involved in the CR process are described and numerically solved. CR behavior is found to be strongly dependent on the total coumarin triplet concentration.     

Eldor spin echoes and slow motions

It is shown how an ELDOR technique based upon spin echoes and rapid stepping of the magnetic field may be employed to measure rotational correlation times, τ_R for very slow motions. Experiments on PD-Tempone in 85% glycerol/ D₂O at low temperatures led to τ_R values of 10^?4 to 10^?5 s obtained with a simple analysis of the data.     

Dielectric studies of the 4-n-alkyl-4'-thiocyanatobiphenyl (nBT) homologous series (n=2-10) in the isotropic and E phases

Results of the dielectric studies of nine members of the nBT (4-n-alkyl-4'-thiocyanatobiphenyl, n= 2-10) homologous series in the crystal E (E) and isotropic (I) phases are presented. The dependence of the static permittivity ?_s in the isotropic phase, the longitudinal relaxation times τ_is and τ_∥, and the activation enthalpies ΔH _is and ΔH _∥ on the number of carbon atoms in the alkyl chain n are analysed. A considerable increase in the retardation factor g= τ_∥/τ_is with decreasing n is observed. The results are compared with those obtained for similar two-ring homologous series. The parameters characterizing the molecular rotations around the short axis in the E phase (τ_∥ and ΔH _∥) indicate a hardening of this solid-like phase with shortening of the alkyl chain.     

Temperature dependence of rotational correlation times in tribromobenzene

The ¹³C relaxation times and nuclear Overhauser enhancements of the protonated carbons in 1,3,5-tribromobenzene were measured as a function of temperature in the solvent benzene-d₆. Rotational correlation times, τ_C(CH), calculated by the Microviscosity/Free Rotor and Hu-Zwanzig “slip” models are substantially below the measured values. In contrast, correlation times predicted by the Hynes—Kapral—Weinberg model are in near quantitative agreement with experiment at all temperatures studied.     

Temperature dependence of the NQR frequencies and relaxation times in Menshutkin's complexes on the basis of arsenic trichloride and tribromide

The temperature dependence of NQR frequencies and spin-lattice relaxation times in Menshutkin's complexes, prepared on the basis of AsCl₃ and AsBr₃ have been investigated. The rotational oscillation frequencies ν_t and the average life times τ_a of rotational oscillation quanta have been calculated by both the Bayer and Woessner-Gutowsky theories.     

Rheological properties of ionic and nonionic polyacrylamide solutions

Non-Newtonian shear viscosities were measured over six decades of strain rate k for 13 solutions of both the ionic and nonionic forms of polyacrylamide. By using the Weissenberg rheogoniometer with both the cone-and-plate and the parallel-plate attachments, the normal stress functions σ₁ (k²) and σ₂(k²) were obtained for four of the solutions. From the measurements of the shear viscosity and the normal stresses at low rates of strain, characteristic times τ and τ_N, respectively, were determined for each solution. The quantity τ was then used to nondimensionalize the strain rate τk, and when plotted versus the reduced shear viscosity, found successfully to correlate the experimental data for all the polyelectrolyte solutions over the entire range of τk and the data for the concentrated solutions of the nonionic polymer over a smaller range of τk. However, in order to correlate the normal stress data for the polyelectrolyte solutions, a second reduced strain rate (τ_Nk) was used. Thus, two different times were required to correlate all the observed data. The shear viscosity data for the dilute solutions of the nonionic polymer were well represented by the two-parameter, non-Newtonian intrinsic viscosity function that has been computed by Fixman.     

Investigation of Structure and Mobility of Silica Bonded Stationary Acridine Phases by Means of Fluorescence and NMR Spectroscopy

Ligand mobility of silica-based HPLC stationary phases modified by various surface coverages of acridine-9-carboxy(N-aminoethylaminopropyl)amide ligands was investigated by fluorescence spectroscopy, time-resolved fluorescence anisotropy measurements, as well as solid-state ¹³C-CP/MAS- and ¹H-MAS-NMR spectroscopy. Rotational correlation times, τ_R, of the bound acridine fluorophore obtained from fluorescence anisotropy measurements are significantly longer in the bound phase, than in solution. Also, in time-resolved experiments anisotropies do not decay to zero. These results are interpreted in terms of wobble-in-cone ligand motion. The mobility of the fluorophore in the presence of liquid phase correlates strongly with the solubility of the model compound acridine-9-carboxy-n-butylamide in the same solvent. In the good solvent acetonitrile τ_R = 3.2 ns is found, whereas in methanol, τ_R > 80 ns is obtained. NMR measurements of the dry phase yield large linewidths, cross polarization constants, T_CH, and spin-lattice relaxation times, T_1ρH, shifting around the minimum in the correlation time curve. Both fluorescence and NMR data indicate medium to low ligand mobility. No difference in the mobilities of alkyl spacer and aromatic group is observed, probably due to the rigidity of the amide group.     

Intramolecular mobility of side chains of poly(methacrylic acid) in regularly grafted copolyimides in solution

A “dark” copolyimide with regularly grafted side chains of polymethacrylic acid and a luminescence-labeled copolyimide containing an anthracene label covalently attached to side chains with an average polycondensation degree of the polyimide backbone of ～16 and an average polymerization degree of side poly(methacrylic acid) chains of ～100 are synthesized. Relaxation times τ_IMM characterizing the mobility of parts of side chains in solvents of different thermodynamic qualities for the backbone and side chains are determined through the polarized luminescence method. It is shown that, in a “common” solvent for the backbone and side chains, the values of τ_IMM are close to those characterizing the mobility of linear polymethacrylate chains. In selective solvents, changes in τ_IMM are related to not only changes in intramolecular interactions but also changes in the heterogeneity of the dynamic characteristics of parts of grafted chains arranged at different distances from the grafting point.     

Calculating Permittivity and Dielectric Loss Frequency Spectra for Aqueous Electrolyte Solutions

Analytic expressions for dielectric permittivity factor ε₁(ω) and dielectric dissipation factor ε₂(ω) of electrolyte solutions are obtained, based on the ratio between complex factors of dielectric permittivity and specific conductivity. The range of frequency dispersion of dynamic factors ε₁(ω) and ε₂(ω) for aqueous solutions of LiCl, NaCl, KCl, and CsCl is considered. Numerical calculations are performed for friction coefficients β _a and β _b ; relaxation times τ _a , τ _b , and τ _ab ; and factors ε₁(ω) and ε₂(ω) in a wide range of variation for ρ; concentration c; temperature T; and frequencies ω. The resulting theoretically calculated ε₁(ω) and ε₂(ω) values and the Cole–Cole diagram are in quantitative agreement with experimental data.