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1.
《Chemical physics》1987,116(1):101-111
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, τθ, and the angular momentum correlation times, τJ, 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· 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 τθ 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·) 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.  相似文献   

2.
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, τC(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.  相似文献   

3.
The viscosity dependent radiationless relaxation of several cyanine dyes has been studied by picosecond laser spectroscopy. It was found that the relaxation rate is proportional to η. The value of α, however, is not constant for a certain dye molecule, but is strongly dependent on the kind of solvent used. In n-alcohols for instance α is typically about 1. In glycerol/methanol or glycerol/water mixtures on the other hand α ≈ 0.5. A comparison is made with literature data on orientational relaxation lifetimes of some dyes in similar solvents. It is shown that the radiationless relaxation of cyanine dyes and the orientational relaxation of for instance xanthene dyes changes in roughly the same way as the solvent is changed. This is taken as proof of the proposal that a torsional motion of the heterocyclic quinolyl rings is the main course of the viscosity dependent relaxation of the cyanine dyes studied.  相似文献   

4.
1-Ethyl-3-methylimidazolium acetate was studied by NMR relaxation. The temperature dependences of the spin-lattice relaxation rates (1/T 1) for 1H and 13C were obtained. The curves with maxima were observed for the majority of the temperature dependences 1/T 1, which provided a reliable temperature dependence of the correlation times (τc). In the low-temperature range, the proton relaxation rates tend to an asymptotic value, which is related, most likely, to spin diffusion manifested in the studied samples. The values of correlation times τc calculated for 1H and 13C of the same functional group almost coincide at high temperatures, which confirms that the used approach is adequate for the determination of characteristic times of rotational reorientation of counterions in the studied ionic liquid.  相似文献   

5.
《Chemical physics》1987,112(1):23-41
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 (τx = τy = τz) 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 τx, τy, τz. 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.  相似文献   

6.
A comparison between viscosities in the systems sodium octanoate — water —n-decanol/n-pentanol is made. It is found that the isoviscosities behave quite differently. In the case ofn-decanol as co-surfactant the isoviscosities rapidly increases as the phase border towards the mesophaseF is reached. In the pentanol system we register no large increase in the viscosity for higher micellar contents. The effect of an addition withn-octane to the two systems is also quite different. The decrease in viscosity, as the content ofn-octane is increased, is much more pronounced for the long-chain alcohol system. These data support earlier findings that then-pentanol system should exhibit a much lower content of micellization.  相似文献   

7.
The Raman band shape analysis of the CO stretching mode of vibration of methyl-isobutyl ketone in solution phase reveals that macroscopic consideration of hydrodynamic force is not sufficient to correlate the vibrational relaxation rate (τv−1) with parameter ƒ(ϱ, η, n), involving dynamic viscosity (η). The band shape analysis was therefore attempted taking the microscopic parameter, microviscosity (ηm) into account through a modified parameter ƒm. The correlation of τv−1 with ƒm is reported.  相似文献   

8.
At certain magnetic fields, when the energy separations between the τx — τy spin levels and between the τy — τz 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 τx — τy spin levels of one triplet coumarin molecule and the τy — τz 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.  相似文献   

9.
Rotational correlation times (τT) of the 5′-AMP molecule deduced from spin-lattice relaxation times (T1) of different protons in the molecule agree fairly well with each other in the temperature range of 3.5–74°C. The same is true with τT values deduced from 13CT1 values. These results indicate that the internal motions are slow as compared to the overall rotation of the 5′-AMP molecule.  相似文献   

10.
The overall rotational correlation times of symmetric tetraalkylammonium ions, R 4N+ (R = ethyl, n-propyl, n-butyl, and n-pentyl), in various solvents were determined by the measurements of the 13C NMR spin-lattice relaxation times and the nuclear Overhauser enhancement factors of each α-carbon, considering the contribution of the internal rotation around the N—C bond. Except in water, the observed solvent dependencies of the rotational correlation times, τr, showed good correlations with those predicted from an electrohydrodynamic (Hubbard–Onsager–Felderhof) model. The correlation times of R 4N+ increased as the size of the alkyl groups became larger. In the case of the n-Bu4N+ and the n-Pen4N+ ion, the τ r values were similar to or even higher than those predicted by the HOF model under the stick hydrodynamic boundary condition, in spite of the fact that the ions were too small to allow the solvent to be regarded as a hydrodynamic or a dielectric continuum. A comparison of the results with the rotations of other pseudotetrahedral ions, e.g., tetraphenylborate and tetraphenylarsenium ions and with the translation of the R 4N+ ions suggests that a considerable part of the rotational friction for R 4N+ is brought about by pushing aside the solvent in the spaces between the alkyl groups of R 4N+. A significant slowing in the rotation in water was observed for the n-Pr4N+, n-Bu4N+, and n-Pen4N+ions; the extent of this effect increased with increasing size of the alkyl group. The increase in friction was related to the hydrophobic hydration of the R 4N+ ions.  相似文献   

11.
The 13C relaxation times (T1 and T2) and isotropic contact shifts (Δω) of a one molar aqueous solution of l-proline at pH = 11 (or pD = 11.4) containing ca 10?4 M copper(II) perchlorate are measured at 62.86 MHz over a temperature range of 26–70°C. The purely dipolar longitudinal relaxation of carbon-13 nuclei contrasting with purely scalar transverse relaxation allowed us to extract carbon-to-metal distances (through T1 measurements) and hyperfine coupling constants and dynamic parameters (from T2 and Δω measurements). The structure of the complex in solution is found closely similar to that in the solid state. Curve-fitting procedures allowed us to derive the hyperfine electron—carbon coupling constants Ac = ?1.95, + 0.42, + 1.90 and ?1.70 MHz for carbons α, β, γ, δ, of the pyrrolidinic ring, the reorientation correlation time of the complex, τR (25°C) = 1.15 × 10?10 sec, the l-proline exchange rate, kM (25°C) = 4.0 × 105 sec?1 (and the corresponding activation parameters ΔH = 9.0 kcal mol?1 and ΔS = ?0.7 e.u.), and the electronic relaxation time, T1e = 1.13 × 10?8 sec (at 25°C). The latter value was found in agreement with the one computed from ESR data and the above τR value, showing the predominant contributions of spin—rotation interaction and, to a lesser extent, of the effect of g-tensor anisotropy to the electronic relaxation rate.  相似文献   

12.
13C T1 relaxation times have been determined for n-butanol in C6D12 in the concentration range 0.001 ? x ? 1 (using 13C labelled alcohol) and for t-butanol in the range 0.01 ? x ? 1. In the former case, this has allowed us to probe molecular mobility down to the region of monomeric alcohols. Comparison with previous results from measurements of 1H chemical shifts, dielectric relaxation, and the picosecond dynamics of electron solvation allows us to build up a detailed picture of molecular clustering and liquid structure in alkane—alcohol mixtures. For n-butanol, the local liquid structure is established by x = 0.2 while t-butanol does not appear to form aggregates larger than trimers until x = 0.5.  相似文献   

13.
The association of many classes of surface active molecules into micellar aggregates is a well-known phenomenon. Micelles are often drawn as static structures of spherical aggregates of oriented molecules. However, micelles are in dynamic equilibrium with surfactant monomers in the bulk solution constantly being exchanged with the surfactant molecules in the micelles. Additionally, the micelles themselves are continuously disintegrating and reforming. The first process is a fast relaxation process typically referred to as τ1. The latter is a slow relaxation process with relaxation time τ2. Thus, τ2 represents the entire process of the formation or disintegration of a micelle. The slow relaxation time is directly correlated with the average lifetime of a micelle, and hence the molecular packing in the micelle, which in turn relates to the stability of a micelle. It was shown earlier by Shah and coworkers that the stability of sodium dodecyl sulfate (SDS) micelles plays an important role in various technological processes involving an increase in interfacial area, such as foaming, wetting, emulsification, solubilization and detergency. The slow relaxation time of SDS micelles, as measured by pressure-jump and temperature-jump techniques was in the range of 10−4–101 s depending on the surfactant concentration. A maximum relaxation time and thus a maximum micellar stability was found at 200 mM SDS, corresponding to the least foaming, largest bubble size, longest wetting time of textile, largest emulsion droplet size and the most rapid solubilization of oil. These results are explained in terms of the flux of surfactant monomers from the bulk to the interface, which determines the dynamic surface tension. The more stable micelles lead to less monomer flux and hence to a higher dynamic surface tension. As the SDS concentration increases, the micelles become more rigid and stable as a result of the decrease in intermicellar distance. The smaller the intermicellar distance, the larger the Coulombic repulsive forces between the micelles leading to enhanced stability of micelles (presumably by increased counterion binding to the micelles). The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped-flow and pressure-jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results show relaxation times τ2 in the range of seconds for Triton X-100 to minutes for polyoxyethylene alkyl ethers. The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation time τ2 was related to dynamic surface tension and foaming experiments. A slow break-up of micelles, (i.e. a long relaxation time τ2) corresponds to a high dynamic surface tension and low foamability, whereas a fast break-up of micelles, leads to a lower dynamic surface tension and higher foamability. In conclusion, micellar stability and thus the micellar break-up time is a key factor in controlling technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification and oil solubilization. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the break-up of micelles. Especially when the free monomer concentration is low, as indicated by a low CMC, the micellar break-up time is a rate limiting step in the supply of monomers, which is the case for many nonionic surfactant solutions. Therefore, relaxation time data of surfactant solutions enables us to predict the performance of a given surfactant solution. Moreover, the results suggest that one can design appropriate micelles with specific stability or τ2 by controlling the surfactant structure, concentration and physico-chemical conditions, as well as by mixing anionic/cationic or ionic/nonionic surfactants for a desired technological application.  相似文献   

14.
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 τorot is discussed in terms of anisotropic relaxation and calculations of τ0.  相似文献   

15.
The relaxation time, τ1, of the first mode in the Rouse—Zimm analysis of intramolecular motion in polymers has been evaluated from an analysis of Rayleigh scattered light using intensity fluctuation spectroscopy. Linear polystyrene in the molecular weight range of 5 × 106 to 20 × 106 has been investigated in theta and non-theta dilute solution conditions. Values compatible with the free-draining Rouse model are obtained and a molecular weight dependence approximately as τ1 α M1.5 is deduced.  相似文献   

16.
The pressure dependence of the reorientational correlation function for chloroform has been measured by analysis of the Raman 3019 cm? 1 A1 CH stretching lineshape at 1, 1000, and 2000 bar and 23°C. These reorientational correlation functions were obtained using the method of spectral Fourier deconvolution introduced by Bratos. The results are compared to the correlation times obtained from the NMR deuteron T1 relaxation times for CDCl3 and those calculated from high pressure viscosity measurements.  相似文献   

17.
Glycerol is a by-product of biodiesel production and is an important readily available platform chemical. Valorization of glycerol into value-added chemicals has gained immense attention. Herein, we carried out the conversion of glycerol to formic acid and glycolic acid using H2O2 as an oxidant and metal (III) triflate-based catalytic systems. Aluminum(III) triflate was found to be the most efficient catalyst for the selective oxidation of glycerol to formic acid. A correlation between the catalytic activity of the metal cations and their hydrolysis constants (Kh) and water exchange rate constants was observed. At 70 °C, a formic acid yield of up to 72% could be attained within 12 h. The catalyst could be recycled at least five times with a high conversion rate, and hence can also be used for the selective oxidation of other biomass platform molecules. Reaction kinetics and 1H NMR studies showed that the oxidation of glycerol (to formic acid) involved glycerol hydrolysis pathways with glyceric acid and glycolic acid as the main intermediate products. Both the [Al(OH)x]n+ Lewis acid species and CF3SO3H Brønsted acid, which were generated by the in-situ hydrolysis of Al(OTf)3, were responsible for glycerol conversion. The easy availability, high efficiency, and good recyclability of Al(OTf)3 render it suitable for the selective oxidation of glycerol to high value-added products.  相似文献   

18.
The photophysics of several triphenylmethane (TPM) dyes has been studied in n-alcohols and glycerol/water solutions. We have been able to measure the rate and characterize the viscosity dependence of several radiationless decay channels. We propose a kinetic scheme to account for our observations. Important points in this model are, the existence of more than one ground-state species (in some dyes), a rapid excited-state equilibrium between states of differing geometry and solvent-induced spectral shifts.  相似文献   

19.
The vaporization of perhydrotriphenylene and its inclusion compounds with higher evennumberedn-alcohols from C8 to C16 has been studied using thermogravimetric analysis. For the inclusion compounds of perhydrotriphenylene with the C8, C10 and C12 n-alcohols, kinetic analysis shows that the vaporization processes of then-alcohol and the perhydrotriphenylene are partially superimposed, while for the inclusion compound perhydrotriphenylene-hexadecanol-1, the above processes are totally superimposed. From the experimental kinetic parameters the values of the preexponential factors corresponding to the vaporization of then-alcohols and perhydrotriphenylene were calculated, as were the compositions of the inclusion compounds of perhydrotriphenylene with the C8-C12 n-alcohols.  相似文献   

20.
Differences in the temperature and pressure dependences of the relaxation times of a slow diffusional process and the α structural relaxation pose an interesting problem. This feature, observed by dynamic light scattering in amorphous poly(phenylmethyl siloxane), is related to another basic feature of lack of thermorheological simplicity discovered by Plazek in polystyrene, poly(vinyl acetate), and amorphous polypropylene. A quantitative explanation based on the predictions of a general coupling theory of relaxations has been found. The coupling theory also predicts the Kohlrausch fractional exponential time correlation function exp[?(tτ*)1?n] at long times, as observed by photon correlation spectroscopy, and crossover to an exponential time dependence exp–(t0) at short times, as frequently assumed in Brillouin scattering. An additional relation between τ* and τ0 predicted by the theory is confirmed also by the experimental data.  相似文献   

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