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Molecular motion of alcohols adsorbed in ACF hydrophobic nanoslits as studied by solid-state NMR
The molecular motion and local structure of methanol-d 1 (CH 3 OD) and ethanol-d 1 (C 2 H 5 OD) in activated carbon fiber (ACF) with a slit width of 0.7 and 1.1 nm have been investigated by solid-state 1 H and 2 H NMR. 2 H NMR spectra for alcohols confined in ACF gave a so-called Pake doublet below...
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Published in: | Adsorption : journal of the International Adsorption Society 2015-05, Vol.21 (4), p.273-282 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | The molecular motion and local structure of methanol-d
1
(CH
3
OD) and ethanol-d
1
(C
2
H
5
OD) in activated carbon fiber (ACF) with a slit width of 0.7 and 1.1 nm have been investigated by solid-state
1
H and
2
H NMR.
2
H NMR spectra for alcohols confined in ACF gave a so-called Pake doublet below 140 K, which were characterized by the
2
H quadrupole coupling constant (QCC) of 185 kHz and the asymmetric parameter of the electric-field-gradient tensor (
η
) of 0.1. The QCC value of the deuteron was indicative of the hydrogen bond formation with the O···O distance of ca. 0.27 nm, suggesting the solid-like feature of alcohols in ACFs. The quadrupole broadening vanished on heating and the single isotropic resonance line was observed. Alcohol molecules were undergoing a rapid motion like as in the bulk liquid, indicating a transition from solid to liquid in ACFs. Temperature-dependent
1
H NMR spectra were used for evaluating the
E
a
value for reorientation in solid-like alcohols, whereas the
2
H NMR spectra were used for obtaining the translation accompanying reorientation in liquid-like alcohols. Alcohols with a bilayered structure in ACF with a slit width of 1.1 nm gave similar
E
a
values, whereas C
2
H
5
OD in ACF with a slit width of 0.7 nm, in which a monolayered structure is expected, exhibited a cross-over of two activation processes at 172 K. The variation in
E
a
was probably caused by the structural relaxation concerning with the hydrogen bonding formation and/or excitation of the large amplitude local motion. The competition of the directionality of hydrogen bonds and a freedom of molecular orientation plays an important role to characterize the intermolecular structure as well as the physicochemical properties in amphipathic molecules in confinement. |
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ISSN: | 0929-5607 1572-8757 |
DOI: | 10.1007/s10450-015-9669-5 |