13C NMR Lineshapes Research Articles

Computational prediction of temperature dependence of 13C NMR lineshapes of planar molecules in structure I clathrate hydrates

We use the molecular dynamics-based method to predict the solid-state 13C NMR lineshape of a planar molecule in a nonspherical cage. Unlike linear molecules, three components of the chemical shielding tensor of a planar molecule are different which gives rise to a different lineshape and temperature dependence in the 13C NMR spectrum. We study the planar ethylene molecule in the oblate large cages of the structure I (sI) clathrate hydrate and extract the lineshapes from the angular distribution and dynamics at four temperatures (70, 150, 200, and 250 K) in the stability range of the hydrate. At low temperature, the C2H4 molecules have limited range of motion in the cages, but as the temperature is raised, they first gain greater rotational freedom about the molecular C=C axis, and then in other directions. These data allow for the calculation of the 13C NMR lineshapes of the C2H4 guests at each temperature.

Molecular Dynamics and Phase Transition in One-Dimensional Crystal of C60 Encapsulated Inside Single Wall Carbon Nanotubes

One-dimensional crystals of 25% 13C-enriched C60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the 13C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C60α, are blocked at room temperature and 68%, labeled C60β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin−lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C60β dynamics. Above the transition, the C60β exhibits continuous rotational diffusion; below the transition, C60β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C60, suggesting a quiet smooth orientational dependence of the interaction between C60β molecules and the inner surface of the nanotubes.

Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations

Nonspherical cages in inclusion compounds can result in non-uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. Herein, we develop a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO(2) molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO(2) guests in terms of a polar angle theta and azimuth angle phi and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO(2) sI clathrate. The experimental (13)C NMR lineshapes of CO(2) guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the (13)C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.

Arylallylic Lithium Compounds, Internally versus Externally Coordinated: Comparison of Their Structures and Dynamic Behavior via X-ray Crystallography and NMR

A comparison of externally coordinated arylallyllithiums with internally coordinated arylallyllithiums using a combination of X-ray and NMR studies shows that 2-bis(2-methoxyethyl)aminomethyl-1-phenylallyllithium 7, and 2-bis(2-methoxyethyl)aminomethyl-1,3-diphenylallyllithi um 8, are indeed fully internally coordinated with all phenyls endo, and lithium is close to one terminal allyl carbon. By contrast, among the externally coordinated analogs 1-phenylallyl-lithium.(HMPT)4 5, and 1-phenylallyllithium.(THF) 6, both phenyls are exo and the proximity of lithium to anion was only detected in compound 6. Lithium-7 NMR clearly identifies the Li(HMPT)4+ complex in 5, whereas a 7Li{1H} HOESY experiment reveals 7Li to be coordinated to THF in 6 and close to the PhC terminal allyl carbon. Carbon-13 NMR shows that all of the above compounds are fully delocalized despite differences among the sites of lithium and their separations from the anions. Changes in the 13C NMR line shapes show that the diphenyl compound 8 undergoes a very fast 1,3-lithium sigmatropic shift, and all of the phenyls in the above compounds undergo fast rotation around their phenyl Cipso-Callyl bonds. Barriers to rotation, DeltaH from NMR line shapes for 5, 6, 7, and 8 respectively, are 19.8, 14.6, 10.2, and 8.9 kcal x mol-1. The decrease in barriers is clearly correlated with a decrease in separation of lithium from an allyl terminal carbon and implies that especially among the latter three, 6, 7, and 8, lithium is involved in the mechanism for phenyl rotation. This question is discussed.

Thermotropic Phase Behavior of a Liquid‐Crystalline Poly(ether ester) Derived from Hydroxydibenzoic Acid, 2‐Methyl‐1,3‐propanediol and R‐1,3‐Butanediol

AbstractThe thermotropic phase behavior of a liquid‐crystalline poly(ether ester) derived from 4′‐hydroxybiphenyl‐4‐carboxylic acid and two different spacer diols (2‐methyl‐1,3‐propanediol and R‐1,3‐butanediol) has been analyzed by differential scanning calorimetry, real time synchrotron X‐ray diffraction and solid‐state 13C NMR. It has been found that the polymer develops a smectic mesophase that presents order inside the layers. This mesophase has a slow rate of formation, in such a way that it is possible to obtain either an amorphous glass or a liquid‐crystalline glass. These two phases exhibit different and independent glass transition temperatures: 95 °C for the amorphous one and 85 °C for the mesophase. The diffractogram of this mesophase shows two clear diffractions in the wide‐angle region and a long spacing (of low intensity) in the small‐angle region. All those features are characteristic of a mesophase of intermediate order, presumably of the type SmI, in contrast to the low‐ordered SmCalt mesophase reported for the racemic polymer. Thus, the chirality of the spacer, which provides structural regularity, makes easier the packing of the chains, so that a more ordered mesophase is obtained. In addition, both the solid‐state 13C NMR line shapes and the value of $T_{1\rho }^H $ are practically the same for the quenched and the annealed samples, indicating that the “ordered” phase obtained by annealing is not a three‐dimensional crystal, thus confirming its assignment as mesophase, with a considerable degree of conformational disorder.magnified image

Line shapes in CP/MAS 13C NMR spectra of cellulose I

The CP/MAS 13C NMR line shape of cellulose I has been qualitatively analyzed by direct simulations using the Ornstein–Uhlenbeck stochastic process and the Kubo model. Both approaches describe a anhydroglucose C4 carbon as a oscillator with fluctuating Larmor frequency. The NMR resonance frequency is written ω = ω ¯ + ω ( t ) , where the fluctuating part with zero mean was modelled as a stationary Markov diffusion process. The simulation results both motivates the use of multiple line shapes when fitting CP/MAS 13C NMR spectra recorded on cellulose I and gives some insights into why signals from crystalline cellulose I give rise to Lorentzian line shapes.

Belt-like C60$minus; electron spin density distribution in the organic ferromagnet TDAE-C60

The measured 13C NMR spectra in the organic ferromagnet TDAE-C60 have been simulated in the temperature range between room temperature and 4 K. It is shown that the sudden increase in the inhomogeneous 13C NMR line width and the corresponding changes of the 13C NMR line shape deep in the ferromagnetic phase can be explained by the freezing out of the pseudo-rotations of the axes of the Jahn–Teller distortions of the C60− ions. A strong correlation between spin ordering and orientational ordering has been found.

Investigation of Eu 6C 60 magnetic properties

A representative rare earth doped fulleride, Eu 6C 60, has been studied using several spectroscopic and magnetometry techniques. We find that europium ions are present both in their magnetic bivalent state and nonmagnetic trivalent state. The Eu 2+/Eu 3+ ratio seems to critically depend on sample preparation conditions. Muon relaxation rates show a large magnetic field distribution near the ordered phase, whereas 13C NMR lineshapes at room temperature are consistent with dipolar field broadening (∼100 G) from neighbouring Eu 2+ magnetic moments.

Jahn–Teller effect in the organic ferromagnet TDAE–C 60

We report on the measurements of the 13C NMR lineshape in the organic ferromagnet TDAE–C 60 between room temperature and 4 K as well as on a simulation of the lineshape in this temperature interval. It is shown that the sudden increase in the inhomogeneous 13C NMR linewidth in the middle of the ferromagnetic phase can be explained by the freeze out of the pseudorotations of the axes of the Jahn–Teller distortions of the C 60 − ions. The correlation times for the Jahn–Teller dynamics of the C 60 − ions have been extracted from a comparison of experimental and simulated 13C NMR lineshapes. A strong correlation between spin ordering and orbital ordering has been found.

Spin−Lattice Relaxation and 13C NMR Line Shape at Multiaxial Reorientation of Molecules in Fullerite C60

Molecular dynamics in the fcc-sc phases of fullerite C60 (13C NMR line shape and spin-lattice relaxation time T1) is described in terms of discrete orientational states of molecules. Molecular reorientations in the low temperature orientationally ordered sc phase are assumed to take place about molecular symmetry axes. Transition into the high-temperature disordered fcc phase is modeled by additional molecular reorientations about C4 symmetry axes of the cubic unit cell. At low temperatures, the NMR line shape functions are demonstrated to depend significantly on the model of molecular reorientations in contrast with T1. In the vicinity of the phase transition at 260 K, T1 changes dramatically for all models, unlike the NMR line shape. The model of relatively fast molecular jumps about a single C3 symmetry axis along with slow reorientations about the other axes is found to adequately reproduce the T1 experiment and qualitatively reflects the temperature changes in NMR line shape.

Chemical derivatization of Athabasca oil sand asphaltene for analysis of hydroxyl and carboxyl groups via nuclear magnetic resonance spectroscopy

Athabasca oil sand asphaltene was methylated with different base catalyst/solvent combinations in order to find an optimum procedure for analysis of the number and types of hydroxyl and carboxyl functional groups. High resolution carbon-13, fluorine-19, and silicon-29 NMR spectra were used to monitor the degree of methylation, trifluoroacetylation, trimethylsilylation, and aromaticity of asphaltene. Tetra- n-butylammonium hydroxide as phase transfer base catalyst and tetrahydrofuran or dichloromethane as solvent result in enhanced O-methylation of asphaltene. At least two types of acidic oxygen containing functionality have been detected, viz. hydroxyl and carboxyl (aliphatic and aryl). On average there are few, ≤4–8, hydroxyl containing groups (including COOH) per asphaltene molecule. 13C NMR lineshapes suggest a broad asymmetric distribution of acidic sites. The NMR and elemental analyses allow for oxygen containing functionalities to be included in an average molecular structure. A sludge phase collected from aqueous and hydrochloric acid extractions of asphaltene has also been analyzed. A correlation is observed between the degree of O-methylation and the dielectric permittivity of the solvent and the acidity of the substrate reaction site.

Studies on Different Types of Hydrogen Bonds in Poly(vinyl alcohol) Films by 1H CRAMPS and Solid-State Two-Dimensional 1H-13C Heteronuclear Correlation Analyses

Different hydrogen-bonded states of OH groups in poly(vinyl alcohol) (PVA) films with different tacticities have been characterized by 1H combined rotation and multiple pulse spectroscopy (CRAMPS), solid-state 13C NMR and two-dimensional (2D) 1H-13C heteronuclear correlation (HETCOR) analyses. The newly developed line shape analysis for the 1H CRAMPS spectra reveals the existence of three types of OH groups in different hydrogen-bonded states below Tg; intramolecular and intermolecular hydrogen bonded OH (OHintra, OHinter) groups and OH (OHf) groups free from hydrogen bonding. Above Tg, another resonance line which can be assigned to highly mobile OH (OHr) groups in the noncrystalline region appears as a result of the reduction in mole fractions of the OHintra and OHinter groups, in good accord with the confirmation of the rubbery component by the solid-state 13C NMR line shape analysis. In contrast, the mole fraction of the OHf groups stays constant at −60-160°C, indicating that these OH groups are preferentially distributed in the crystalline region. In order to examine the correlations of the results obtained by 1H CRAMPS and solid-state 13C NMR analyses, 2D 1H-13C HETCOR spectra have been also measured at room temperature for atactic and main-chain deuterated atactic PVA films. As a result, evident correlations between 1H OH and 13C CH constituent lines are observed at a cross polarization (CP) contact time of 120 μs, indicating the validity of the assignments for the 1H OH and 13C CH resonance lines made previously as well as in this work.

Charge disproportionation in (BEDT-TTF) 2RbZn(SCN) 4

The magnetic behavior in θ-(BEDT-TTF) 2RbZn(SCN) 4 with a 2D closed Fermi surface has been studied by the precise measurements of the angular dependence of 13C-NMR lineshape. The results indicate the existence of charge disproportionation below the transition temperature (190 K).

Lateral diffusion of 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate at the interfaces of C18 and chromatographic solvents

The influence of mobile phase on the lateral diffusion of an amphiphilic dye was studied for four chromatographic interfaces using fluorescence correlation spectroscopy. The fluorescent dye was DiI (1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate); the stationary phase was a covalently bonded monolayer of dimethyloctadecylsiloxane on fused silica; and the four mobile phases were acetonitrile, methanol, tetrahydrofuran, and water. Time-resolved measurements at varying focal positions of the microscope confirmed negligible fluorescence from solution. Single-molecule spectroscopy showed that exchange between mobile and immobile adsorbates was negligible. The lateral diffusion of DiI was the fastest for tetrahydrofuran, D = 5.3 x 10(-5) cm2/s, and slowest for water, D = 2.5 x 10(-6) cm2/s. Methanol and acetonitrile gave intermediate rates of diffusion, D = 3.4 x 10(-6) and 4.8 x 10(-6), respectively. There was no correlation of the lateral diffusion coefficient with solvent viscosity, a weak correlation with interfacial tension, and a strong correlation with 13C NMR line shapes for the terminal methyl group of the hydrocarbon chain. The increased wetting of the C18 interphase in the order water < methanol < acetonitrile < tetrahydrofuran agreed with data for the miscibility of these solvents with n-hexadecane. It is concluded that the wetting of the hydrocarbon interphase by the mobile phase enhances the lateral diffusion of the amphiphile.

13C NMR Lineshapes of Acetone Adsorbed on Silica

13C NMR data, obtained as a function of temperature with magic-angle spinning (MAS) and either cross polarization or direct polarization, are reported on acetone and a sample of acetone (an approximately equal mixture with 13C labels at C-1 or C-2) adsorbed on dry silica gel. Various contributions to the observed linewidths and TC2 values are considered in terms of a previously established model of the acetone/SiO2 system; in that model, acetone species are in equilibrium between a physisorbed-acetone (non-hydrogen-bonded) state and a state consisting of acetone units that are hydrogen bonded to silanol moieties on the silica surface. Spin dynamics simulations are useful in interpreting the effects of variations of experimental parameters. It is concluded that the main linewidth contributions, which increase at lower temperatures, are: (a) a dispersion of chemical shifts in the hydrogen-bonded state, associated with the inhomogeneous character of the silica surface; (b) the interference between MAS averaging of the chemical shift anisotropy (especially for the carbonyl carbon) and molecular motion and/or chemical exchange; and (c) chemical exchange broadening. Prominence of the last of these contributions is most consistent with data obtained as a function of magnetic field strength, MAS speed, and temperature.

Reorientation Dynamics within Ion-Paired Allylic Lithium Compounds: Isolation of Inversion Processes

Among the several ion−ion reorientational processes that take place within several newly studied allylic lithium compounds, the dynamics of transfer of TMEDA-coordinated lithium between the two allyl faces has been determined independently by use of 13C NMR line shape studies of diastereotopic geminal methylsilyl groups strategically incorporated close to the chiral lithium in the organometallic species. With increasing temperature, the 13C shift between these methyls is progressively averaged due to faster rates of lithium transfer (inversion). Typical ΔH⧧ values of 5−7 kcal·mol-1 with ΔS⧧ = ca. −20 eu are encountered. A faster process, the reorientation of coordinated TMEDA on one side of the allyl plane, has been monitored from the NCH3 13C resonance of TMEDA-coordinated Li+, with ΔH⧧ = 7 kcal·mol-1 and ΔS⧧ = ca. −12 eu.

High resolution 13C NMR studies of one- and two-dimensional polymerized C 60 under high pressure

We report on 13C NMR measurements of orthorhombic one-dimensional and rhombohedral two-dimensional polymers of C 60 obtained under high pressure. The obtained 13C MAS NMR spectrum of the orthorhombic polymer shows two resonances at 146 ppm and 73.5 ppm, and the one of the rhombohedral polymer presents six isotropic lines at 149.1, 147.9, 145.2, 139.6, 134.8 and 73.5 ppm. The static distortion of the C 60 molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The 13C NMR lineshape simulation of the obtained spectra are compatible with the suggested polymeric structures where the C 60 molecules are connected by [2 + 2] cycloadditions.

Mechanisms of molecular motion of Mo(CO) 6 in zeolites NaY and HY from 13C-NMR lineshapes

The potential of 13C-NMR spectroscopy of stationary samples for the study of mechanisms of anisotropic molecular motion of Mo(CO) 6 adsorbed in zeolites has been examined. Free diffusional rotation or jump processes about the various symmetry axes of the octahedral complex cause motional averaging of 13C chemical shift anisotropy, allowing for the characterization of the different motional mechanisms. At 110 K a large portion of the Mo(CO) 6 molecules is firmly fixed in zeolites NaY and HY as indicated by a broad (ca. 410 ppm) chemical shift powder pattern. A narrow 13C-NMR component is observed in both zeolites for molecules undergoing isotropic motion or rotation about a three-fold axis. An additional line, which is inverted in shape and reduced in width by a factor of −1/2 compared to the rigid case, is only observed for NaY. This line is assigned to molecules undergoing free rotation about the four-fold axis, but it may also be due to Mo(CO) 6 jumping about a two-fold axis. The changes in the distribution of molecules in different motional states was explored between 110 K and ambient temperature. Molecules undergoing rotations or jumps about the four- or two-fold axes only exist in NaY at lower temperatures when stationary Mo(CO) 6 molecules are also present. At 180 K and higher temperatures, rotation about the three-fold axis or isotropic motion are the sole types of molecular motion.

Induction of Tilted Cholesteric Structures in Microconfined Liquid Crystals

Abstract 13C-NMR measurements of the chiral mesogen cholesteryl-undecylcarbonate (CHUC) adsorbed in Anopore membranes with cylindrical pores of 0.2μm diameter are presented. The pitch of CHUC in the cholesteric phase is of order of the pore diameter, and in view of the planar axial anchoring a strong influence of the walls on the director field can be expected. Simulation of the angular dependence of 13C-NMR line shape allows to determine a mean tilt angle of 60μ of the director with respect to the helical axes which are oriented along the channels. Furthermore, at T=341K a correlation time of 130μs for reorientational motions could be obtained which is connected with molecular diffusion along the pores. Optical and NMR-measurements are compatible with a tilted helical director field for the cholesteric LC material within the pores.

13C NMR study of orientational ordering in solid C 70

The 13C NMR lineshapes of powdered C 70 have been measured between 450 K and 50 K. A comparison of experimental and simulated lineshapes allowed for a determination of the temperature dependence of the order parameter of the orientational phase transition at T c=270 K and of the tilt angle between the direction of the long axis of the C 70 molecules and the < 111 >; direction in the rhombohedral phase. The results essentially confirm the predictions of molecular dynamics calculations of Sprik, Cheny and Klein.