Search results for "Relaxation"
showing 10 items of 1162 documents
Surface Relaxivity of Cement Hydrates
2014
Numerous aspects of the physical chemistry of colloidal systems are conditioned by the solid–liquid interface, and this is also the case for hydrated cement systems. Estimating the surface area is thus essential for studying the kinetics of cement hydration and admixture adsorption. Proton nuclear magnetic resonance (NMR) relaxation techniques have already proven useful for this objective, but, for hydrating cements at early ages, it is necessary to know the surface relaxivities of all of the individual phases present to correctly interpret the relaxation data. This paper reports the results of a comparison of NMR relaxometry and Brunauer–Emmett–Teller gas adsorption measurements on various…
The Relationship between Gray Matter Quantitative MRI and Disability in Secondary Progressive Multiple Sclerosis
2016
Purpose: In secondary progressive Multiple Sclerosis (SPMS), global neurodegeneration as a driver of disability gains importance in comparison to focal inflammatory processes. However, clinical MRI does not visualize changes of tissue composition outside MS lesions. This quantitative MRI (qMRI) study investigated cortical and deep gray matter (GM) proton density (PD) values and T1 relaxation times to explore their potential to assess neuronal damage and its relationship to clinical disability in SPMS. Materials and Methods: 11 SPMS patients underwent quantitative T1 and PD mapping. Parameter values across the cerebral cortex and deep GM structures were compared with 11 healthy controls, and…
A method for measurement of spin-spin couplings with sub-mHz precision using zero- to ultralow-field nuclear magnetic resonance.
2017
We present a method which allows for the extraction of physical quantities directly from zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) data. A numerical density matrix evolution is used to simulate ZULF NMR spectra of several molecules in order to fit experimental data. The method is utilized to determine the indirect spin-spin couplings ($J$-couplings) in these, which is achieved with precision of $10^{-2}$--$10^{-4}$ Hz. The simulated and measured spectra are compared to earlier research. Agreement and precision improvement for most of the $J$-coupling estimates are achieved. The availability of an efficient, flexible fitting method for ZULF NMR enables a new generation of…
CoIILnIII dinuclear complexes (LnIII = Gd, Tb, Dy, Ho and Er) as platforms for 1,5-dicyanamide-bridged tetranuclear CoII2LnIII2 complexes: A magneto-…
2012
Five acetate-diphenoxo triply-bridged Co-II-Ln(III) complexes (Ln(I) = Gd, Tb, Dy, Ho, Er) of formula [Co(mu-L)(mu-Ac)Ln(NO3)(2)] and two diphenoxo doubly-bridged Co-II-Ln(III) complexes (Ln(III) = Gd, Tb) of formula [Co(H2O)(mu-L)Ln(NO3)(3)]center dot S (S = H2O or MeOH), were prepared in one pot reaction from the compartmental ligand N,N',N ''-trimethyl-N,N ''-bis(2-hydroxy3-methoxy-5-methylbenzyl)diethylene triamine (H2L). The diphenoxo doubly-bridged Co-II-Ln(III) complexes were used as platforms to obtain 1,5-dicyanamide-bridged tetranuclear Co-II-Ln(III) complexes (Ln(III) = Gd, Tb, Dy, Ho, Er). All exhibit ferromagnetic interactions between the Co-II and Ln(III) ions and in the case …
Molecular association and dynamics in n-pentanol and 2-methyl-2-butanol
1985
Static dielectric constant, viscosity, density, ultrasonic and dielectric relaxation measurements are reported for two pentanol isomers (n-pentanol and 2-methyl-2-butanol) in a wide range of temperatures. The results show a different extent of H-bonds self-association in relation to the molecular geometry of the alcohol. The activation energies obtained from dielectric and ultrasonic studies are compared with those obtained from viscous flow. Such a comparison, together with the analysis of all the dielectric and ultrasonic relaxation data, show that in n-pentanol a correlation between viscosity and ultrasonic relaxation exists, while dielectric relaxation is not correlated with the other t…
13C-Decoupled J-Coupling Spectroscopy Using Two-Dimensional Nuclear Magnetic Resonance at Zero-Field
2017
We present a two-dimensional method for obtaining 13C-decoupled, 1H-coupled nuclear magnetic resonance (NMR) spectra in zero magnetic field using coherent spin-decoupling. The result is a spectrum determined only by the proton–proton J-coupling network. Detection of NMR signals in zero magnetic field requires at least two different nuclear spin species, but the proton J-spectrum is independent of isotopomer, thus potentially simplifying spectra and thereby improving the analytical capabilities of zero-field NMR. The protocol does not rely on a difference in Larmor frequency between the coupled nuclei, allowing for the direct determination of J-coupling constants between chemically equivalen…
NMR spectroscopy in environmental chemistry:1H and13C NMR parameters of tricyclic polychlorinated C10 hydrocarbons and their oxy derivatives based on…
1999
Two-dimensional homo- and heteronuclear NMR chemical shift correlation techniques were applied in the characterization of five tricyclic polychlorinated C10 hydrocarbons, chlordene (1), heptachlor (2), trans-nonachlor (3), α-chlordene (4) and γ-chlordene (5), which are spread globally in the environment owing to their use as insecticides. Approximate and partly contradictory 1H and 13C NMR chemical shifts reported in the literature were corrected in this work. The chemical shift assignments of 1–5 were based on DQF COSY, HMQC and HMBC experiments. In addition, an INADEQUATE experiment was needed to ascertain the 13C chemical shifts assignment of 2. The nJ(H,H)s of 1–5 were solved by compute…
Structural relaxation in a molten salt probed by time-dependent dc conductivity measurements
1998
Abstract Time-dependent dc conductivity was measured after cooling steps of about 2 K in the glass transformation range of 2Ca(NO 3 ) 2 -3RbNO 3 . The shape and time scale of the structural relaxation function was thus monitored for times 60 s t 6 s. The time scale could be compared with results from scanning calorimetry measurements and good agreement was found. From the heat capacity data and from the solid state conductivity the expected liquid state conductivity relaxation time was calculated using several models. The good compatibility of these calculations with the experimental results provides evidence that near the calorimetric glass transition the mobile ions perform concerted moti…
Modelling of thermo-chemical properties over the sub-solidus MgO–FeO binary, as a function of iron spin configuration, composition and temperature
2014
Thermo-chemical properties and T–X phase relations diagram of the (Mg,Fe)O solid solution are modelled using mixing Helmholtz energy, ΔF(T,x)mixing, calculated by quantum mechanical and semi-empirical techniques. The sub-solidus MgO–FeO binary has been explored as a function of composition, with iron either in high-spin (HS) or low-spin (LS) configuration. Only the HS model provides physically sound results at room pressure, yielding a correct trend of cell edge versus composition, whereas LS’s issues are at variance with observations. Mixing Helmholtz energy has been parametrized by the following relationship: ΔF(T,x)mixing = x × y × [U0(T) + U1(T) × (x – y) + U2(T) × (x − y)2]−T × S(x,y)c…
Self-trapped exciton luminescence in crystalline α-quartz under two-photon laser excitation
2003
Abstract The luminescence of pure crystalline α-quartz is studied under pulsed ArF laser excitation. The luminescence parameters obtained correspond well with those of self-trapped excitons (STEs) in α-quartz, indicating that the excitation process is two-photon. The efficiency of two-photon excitation is of the same order of magnitude as the one-photon excitation of sodium salicylate. The STE luminescence decay kinetics and their temperature dependence under photoexcitation were recorded with higher accuracy than previously. Changes in the decay kinetics with temperature are explained by the splitting of the STE triplet state in zero magnetic field and are analyzed with the assumption of t…