Fitting of deuterium quadrupole echo spectra with multiple motional models

Eastman, Margaret A.; Nanny, Mark A.

doi:10.1016/j.jmr.2006.11.003

Cited by 4 publications

(6 citation statements)

References 21 publications

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“…Because pure benzene is in liquid and vapor form at 25 °C, the C 6 or SAW pattern signifies that benzene- d 6 experiences restricted motion due to its association with the humic and fulvic acids. In an initial fitting of some of the spectra, both SAW and C 6 models were tried, and it appeared that SAW fit these spectra as a group slightly better as gauged by values of R , a measure of the difference between fitted and experimental spectra; , therefore, SAW was chosen to match the broad pattern in all further fits. Differences between spectra with echo delay (τ) values of 25 and 50 μs did not assist in distinguishing between the two models; rather, spectra for both delays were generally better fit with the SAW model.…”

Section: Resultsmentioning

confidence: 99%

“…22,30,40 Errors in the percentages of motional models given below are based on the noise standard deviation of each experimental spectrum and are one-half of the error provided by DFP, which is based on twice the noise standard deviation. 37 Inversion recovery spectra were fit after inverting negative spectra so that the amplitude of each spectrum was positive. Spectra close to a null with both positive and negative intensity were rejected from the analysis.…”

Section: ' Experimental Methodsmentioning

confidence: 99%

“…Three libraries of spectra were used in the fitting. The sorbed isotropic motion (ISO s ) and vapor isotropic motion (ISO v ) libraries used the 32-site icosahedral isotropic motional model, , with AEU kinetic mode (all sites exchange, unequal rates, with the rate depending upon the jump angle between sites). For the isotropic libraries, a quadrupolar asymmetry parameter of 0.00 rather than 0.04 was used.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…The 30-site 2-frame small angle wobble (SAW) library, with the 3-site C 6 rotation model and 10 wobble sites, also employed the AEU kinetic mode and had wobble rates from 5 × 10 3 to 5 × 10 5 rad/s, wobble angles from 2 to 16 degrees, and C 6 rotation rates from 1 × 10 6 to 1 × 10 8 rad/s. Euler angles representing the SAW model were chosen such that the quadrupole principal axis system in each site has the z -axis along the C−D bond, x -axis in the plane of the benzene ring, and y -axis perpendicular to the plane of the ring. ,, Errors in the percentages of motional models given below are based on the noise standard deviation of each experimental spectrum and are one-half of the error provided by DFP, which is based on twice the noise standard deviation …”

Section: Experimental Methodsmentioning

confidence: 99%

“…To derive motional parameters and percentages of different motional models, experimental spectra were matched to spectra in spectral libraries calculated with the Deuterium Motional Simulation (DMS) program by fitting with the Deuterium Fitting Program (DFP). 37 Quadrupole parameters for benzened 6 , χ( 2 H) = 190 kHz and η Q = 0.04, were the same used by Laaksonen et al 38 A range of values, both smaller and larger than this, for the quadrupole coupling constant of C 6 D 6 determined by experiment and calculation have appeared in the literature. 39 For the broader patterns seen here, the effect of χ would be to alter the splitting between the horns, which can also be affected by rates and angles specified for the motional model.…”

Section: ' Experimental Methodsmentioning

confidence: 99%

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²H NMR Study of Dynamics of Benzene-d₆ Interacting with Humic and Fulvic Acids

2011

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Samples of three humic acids and one fulvic acid with 1% loading of benzene-d(6) in sealed glass tubes have been studied with solid-state deuterium quadrupole-echo nuclear magnetic resonance spectroscopy. Calculated spectra combining three motional models, two isotropic models and a third more restricted small-angle wobble (SAW) motional model, are fit to the experimental spectra. One isotropic motion (ISO(v)) is assigned to vaporous benzene-d(6) due to the small line width, short T(1), and the loss of this component by about -25 °C when the temperature is lowered. The remaining two motional components, ISO(s) and SAW, are sorbed by the humic or fulvic acid. Benzene-d(6) slowly interacts with the humic substances, progressively filling SAW sites as ISO(s) motion diminishes. Both the sorption and increase in percentage of SAW motion are for the most part complete within 200 days but continue to a lesser extent over a period of a few years. For the SAW motion there are at least two and most likely a series of T(1) values, indicating more than one adsorption environment. Enthalpies of sorption, obtained from application of the van't Hoff equation to the percentages of the different motional models derived from a series of variable temperature spectra, are comparable in magnitude to the enthalpy of vaporization of benzene. In Leonardite humic acid, ΔH and ΔS for the ISO(s) to SAW transition change from positive to negative values with age, implying a transition in the driving force from an entropic effect associated with expansion and deformation in the molecular structure of the humic substance to accommodate benzene-d(6) to an enthalpic effect of strong benzene-d(6)-humic substance interactions. In contrast, at advanced ages, Suwannee River humic and fulvic acids have small positive or near zero ΔH and positive ΔS for the ISO(s) to SAW transition.

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Section: Resultsmentioning

confidence: 99%