Comments on the interpretation of dynamic deuterium NMR spectra from solid inclusion compounds (original) (raw)
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Physical Review B, 2000
A single-crystal deuterium NMR study of the phase transition of the incommensurate organic inclusion compound of n-nonadecane in urea is presented. In agreement with diffraction measurements, we found that the urea host sublattice is not significantly modulated in the high-symmetry phase. Whereas at room temperature the NMR profiles reflect the fast reorientational disorder of the guest molecules, we report a temperaturedependent inhomogeneous broadening of the resonance lines on approaching T c from above. This effect is assigned to a pretransitional clustering of the guest molecules and a heuristic model is developed to describe it. At very low temperature below T c , NMR measurements give direct information on the mode of ordering of the alkane sublattice inside the urea frame. Remaining static disorder could be attributed to the incommensurate feature of the composite crystal.
Physical Chemistry Chemical Physics, 2009
Partially deuterated 1,4-distyrylbenzene (2PV) is included into the pseudohexagonal nanochannels of perhydrotriphenylene (PHTP). The overall and intramolecular mobility of 2PV is investigated over a wide temperature range by 13 C, 2 H NMR as well as fluorescence spectroscopy. Simulations of the 2 H NMR spectral shapes reveal an overall wobble motion of 2PV in the channels with an amplitude of about 41 at T = 220 K and 101 at T = 410 K. Above T = 320 K the wobble motion is superimposed by localized 1801 flips of the terminal phenyl rings with a frequency of 10 6 Hz at T = 340 K. The activation energies of both types of motions are around 40 kJ mol À1 which imply a strong sterical hindrance by the surrounding PHTP channels. The experimental vibrational structure of the fluorescence excitation spectra of 2PV is analyzed in terms of small amplitude ring torsional motions, which provide information about the spatial constraints on 2PV by the surrounding PHTP host matrix. Combining the results from NMR and fluorescence spectroscopy as well as of time-dependent density functional calculations yields the complete potential surfaces of the phenyl ring torsions. These results, which suggest that intramolecular mobility of 2PV is only reduced but not completely suppressed by the matrix, are corroborated by MD simulations. Unrealistically high potential barriers for phenyl ring flips are obtained from MD simulations using rigid PHTP matrices which demonstrate the importance of large amplitude motions of the PHTP host lattice for the mobility of the guest molecules.
Solid-state deuterium NMR investigation of internal motion in 2'-deoxythymidine
Journal of the American Chemical Society, 1988
molecules it is expected that the extreme narrowing limit holds. Our experiments with the two isotopes of chlorine provide a test for whether exchange is slow. If the line width were controlled by slow exchange, transverse relaxation should be essentially independent of the isotopic identity. The ratio of 35Cl and 37Cl line widths should then be unity.I2 On the other hand, if exchange is fast, the ratio of line widths should go as the square of the ratio of quadrupole moments (1 .262 = 1 .6).99'23'3 Indeed, except for the lowest concentration, the ratio of line widths is in the range 1.6-1.9. Thus, throughout the range of concentrations, exchange between free and associated perchlorate is fast. A possible alternative explanation, that the line broadening derives from exchange between environments with a very large chemical shift difference, is excluded by the observation of only a 38 ppm chemical shift difference between the species. We may conclude that each perchlorate ion must visit a silyl cation many times on the time scale of transverse relaxation, even at 0.584 M.14
Molecular self-diffusion in a columnar liquid crystalline phase determined by deuterium NMR
Physical review. E, Statistical, nonlinear, and soft matter physics, 2002
We report translational diffusion coefficients in a columnar phase of a discotic liquid crystal formed by a triphenylene-based compound. The experiments were performed using 2H stimulated-echo-type pulsed-field-gradient spin-echo NMR applied to a chain-deuterated sample. The diffusion coefficients were found in the range of 1x10(-14)-4x10(-14) m2/s, three orders of magnitude lower than in the isotopic phase of the same compound. This, together with the high activation energy obtained in columnar phase, indicates that the diffusion is dominated by solidlike jump processes.
Chapter 1. Fundamental Intramolecular and Intermolecular Information from NMR in the Gas Phase
Gas Phase NMR
In the gas phase we have a well-defined homogeneous physical system, and the theory for dilute gas behavior is in an advanced stage. In dilute gases, we can expand the molecular electronic property (e.g., nuclear magnetic shielding, J coupling, nuclear quadrupole coupling) in a virial expansion, in which the property virial coefficients can be expressed theoretically in closed form and can be obtained unequivocally experimentally in the binary interaction limit. These experimentally measured quantities depend on two quantum-mechanical mathematical surfaces: the shielding, or J, or electric field gradient (efg) at the nucleus as a function of intermolecular nuclear coordinates and the weak intermolecular interaction potential energy surfaces that are also a function of the same intermolecular nuclear coordinates. Furthermore, we can extrapolate the measured NMR data (shielding, J, efg) to the zero-density limit to obtain these electronic properties for the
Chemical Physics, 1981
13C nuclear spin-lattice relaxation times of I-methylnaphthalene and 'H nuclear spin-lattice relaxation times of the perdeuterated species, both in deuterochloroform solutions, were measured at several different temperatures. The efiects of isotopic substitution on the efiective correlation times are discussed. The Woessner approach to extracting the internai jump rates of the CHZ and CD: groups from these relaxation times was used. Activation energies for the internal motions were calculated by fitting the temperature dependent jump rates to an Arrhenius type expression. The differences between the activation energies of the two isotopic species are discussed_
Deuteron NMR study of molecular dynamics in a compound exhibiting a reentrant nematic phase
Physical Review E, 2000
A deuteron NMR study of molecular dynamics in the partial bilayer smectic Ad and reentrant nematic phases of a pure chain-deuterated compound is presented. The deuteron spin-lattice relaxation times T1Z and T1Q were measured as a function of temperature for two different frequencies (15 and 46 MHz). The experimental results were interpreted in terms of the internal conformational motions of a chain decoupled from the molecular small-step rotational diffusion and the order director fluctuations. The latter motion was found to be essential to the fit of experimental results in the reentrant nematic phase. The fitting parameters obtained by using a global target fitting method are acceptable when compared with those obtained from other deuteron and proton NMR studies of the same mesophases.
The Journal of Physical Chemistry B, 2000
We report a simple and robust NMR approach capable of correlating deuterium quadrupolar splittings with carbon-13 chemical shifts in oriented materials. The technique, which is based on heteronuclear multiplequantum spectroscopy, results in two-dimensional maps that are very useful for the assignment of deuterium NMR spectra. The experiment has been employed to measure and assign all quadrupolar couplings produced by a perdeuterated liquid crystal.