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Nuclear magnetic resonance study of alkane conformational statistics
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10.1063/1.3665139
/content/aip/journal/jcp/135/23/10.1063/1.3665139
http://aip.metastore.ingenta.com/content/aip/journal/jcp/135/23/10.1063/1.3665139

Figures

Image of FIG. 1.
FIG. 1.

Calculated NMR spectra of orientationally ordered ethane, propane and butane in 5CB at 298.5 K.

Image of FIG. 2.
FIG. 2.

Ratio of propane dipolar couplings (top) and potential energy parameters (bottom) to the intramethyl dipolar couplings and the potential parameters of ethane in the liquid crystals 5CB (left) and 1132 (right) as a function of temperature. The maximum percent changes for 5CB are: D 12 of 0.4%, D 14 of 2.0%, D 16 of 0.3%, D 45 of 1.2%, Gβ xx of 0.8%, and Gβ yy of 4.0%, while those for 1132 are: D 12 of 0.9%, D 14 of 2.9%, D 16 of 0.6%, D 45 of 1.9%, Gβ xx of 0.7%, and Gβ yy of 3.9%. The axis of propane is perpendicular to the plane of the three carbons and is the c2 axis.

Image of FIG. 3.
FIG. 3.

Calculated D ij for butane in 1132 as a function of dihedral angle (for the full potential fit ST at T = 293.5 K). The order parameters from the fit (that are used to calculate these couplings) are: , and There is a discontinuity in dipolar couplings at 60°–65° because these angles give a maximum in the potential energy and hence are used as the division between trans and gauche conformers which have quite different order parameters. The conformer population at these angles is very low because of the high potential energy.

Image of FIG. 4.
FIG. 4.

Ratio of butane dipolar couplings to the intramethyl dipolar couplings of ethane in the liquid crystal 5CB (left) and 1132 (right) as a function of temperature. Note the change in scale of the axis.

Image of FIG. 5.
FIG. 5.

E tg as a function of temperature where “CCd chord” is the modified chord model fit to 1132.12

Image of FIG. 6.
FIG. 6.

Temperature dependence of butane trans and gauche conformer probabilities from Table III fit ST where n(total) is the probability of conformer n (which includes and contributions to the Boltzmann factor) and where n(iso) is the probability calculated if we ignore the anisotropic contribution. The gas-phase probabilities calculated from Eq. (9) (setting and ) are also shown. The high-temperature limit is .

Image of FIG. 7.
FIG. 7.

Ratio of butane potential parameters to those of ethane as a function of temperature in 5CB (left) and in 1132 (right). The points are values from fits ST which include a linear temperature dependence of E tg . The lines are the constant values obtained from the fits UT (scaling to anisotropic potential parameters with a linear temperature dependence of E tg ). The axis is the PMI axis with lowest moment of inertia, is the c2 axis and is perpendicular to and .

Tables

Generic image for table
Table I.

Experimental dipolar couplings in Hz of n-butane and tcb in 5CB as a function of temperature.

Generic image for table
Table II.

Parameters obtained from RIS fits to the temperature dependence of butane dipolar couplings

Generic image for table
Table III.

Parameters obtained from full potential fits (integration over all dihedral angles) to the temperature dependence of butane dipolar couplings

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/content/aip/journal/jcp/135/23/10.1063/1.3665139
2011-12-19
2014-04-20
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Nuclear magnetic resonance study of alkane conformational statistics
http://aip.metastore.ingenta.com/content/aip/journal/jcp/135/23/10.1063/1.3665139
10.1063/1.3665139
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