2H NMR Line Shape Research Articles

Static and Dynamic Stereochemistry of N-(1,4-Dimethyl-9-triptycyl)hydroxylamine Derivatives

Abstract Static and dynamic stereochemistry of N-alkyl, O-alkyl, and N,O-dialkyl derivatives of N-(1,4-dimethyl-9-triptycyl)hydroxylamine were studied. X-ray crystallographic analysis revealed that an N-alkyl derivative adopts R*-(−sc)* conformation while N,O-dialkyl derivatives adopt R*-ap conformation. In solution, stereomutation was almost frozen on the NMR timescale at ca. −60 °C. The same conformer as found in the crystal was the major conformer in solution in each compound, and a small amount of a second conformer was found in equilibrium with the major one for the N-alkyl and N,O-dialkyl derivatives. The stereomutation was interpreted in terms of diastereomerization and enantiomerization, and the 1H NMR lineshape analysis afforded the activation parameters for these processes.

Solid state 13C and 1H MAS NMR investigations of C60(ferrocene‒d10)2 complex

The article presents NMR investigations of a molecular crystalline material C60(ferrocene‒d10)2, which consists of two highly mobile molecules. 13C and 1H NMR line shapes and T1 relaxation times are discussed and compared with results obtained for the undeuteriated compound. The mobilities of C60 and ferrocene molecules were found to increase in the complex in comparison with the two components as crystalline materials.

A Solid-State Multinuclear Magnetic Resonance Investigation of Hexamethylborazine

Analyses of 11B, 13C, and 2H NMR spectra of solid hexamethylborazine, I, provide conclusive evidence for rapid in-plane jumps of the borazine ring at room temperature. Boron-11 NMR spectra of magic-angle spinning (MAS) samples, acquired at low (4.7 T), moderate (9.4 T), and high (18.8 T) external applied magnetic field strengths, have been simulated to yield the 11B nuclear quadrupolar coupling constant (CQ), asymmetry parameter, and isotropic chemical shift; their values at 298 K are 2.98 ± 0.03 MHz, 0.01 ± 0.01, and 36.0 ± 0.4 ppm, respectively. Simulations of 13C CP/MAS NMR spectra provide the carbon−boron isotropic indirect spin−spin coupling constant, Jiso, the sign of CQ(11B), the relative orientations of the boron electric field gradient (EFG) and the 13C−11B dipolar coupling tensors, and the motionally averaged 13C−11B dipolar coupling constant. Variable-temperature 2H NMR spectra of a partially deuterated sample of I indicate that the in-plane jumps of the borazine ring are slow with respect to CQ(2H)-1 (i.e., τjump ≥ 10-4 s) at temperatures less than 130 K. Over the temperature range 180 to 128 K, 2H NMR line shape analysis yields an activation energy of 30.1 ± 1.5 kJ mol-1 for the in-plane jumps of the borazine ring. Although a precise experimental determination of boron chemical shift anisotropy was impeded by intramolecular and intermolecular boron−boron dipolar interactions and heteronuclear nitrogen−boron dipolar interactions, simulations of high-field 11B NMR spectra of a stationary sample of I suggest a value of 55 ± 15 ppm for the motionally averaged span of the chemical shift tensor. Lastly, high-level ab initio and density functional theory calculations provide values of the boron EFG tensor and the boron and nitrogen magnetic shielding tensors for a rigid molecule of I.

Hydrogen Bond Dynamics in Solid Triphenylsilanol

Dynamic properties of the hydroxyl groups in a selectively deuterated polycrystalline sample of triphenylsilanol (Ph3SiOD) have been studied using variable-temperature solid-state 2H NMR spectroscopy. The crystal structure of triphenylsilanol contains eight crystallographically independent molecules, which are arranged in two tetrameric building units. Within each of these tetrameric units, the four silicon atoms are arranged in the form of a slightly distorted square, with the O atoms of the four hydroxyl groups involved in O−H···O hydrogen bonding. The temperature dependence of the quadrupole echo 2H NMR line shape in the temperature range 213−358 K and 2H NMR spin−lattice relaxation time measurements at 368 K demonstrate that the hydrogen-bonding arrangement is dynamic. From the 2H NMR line-shape analysis, the dynamic process is interpreted as interconversion between “clockwise” and “anticlockwise” hydrogen-bonding arrangements within each tetrameric unit, via a two-site jump motion of each hydroxyl de...

X-ray and 2H NMR studies of structure and dynamics in the Hofmann-type and the Hofmann-(en)-type pyrrole clathrates

Structure and dynamics in the Hofmann and Hoffmann-en type pyrrole clathrates, Cd(NH 3) 2Ni(CN) 4 ·2C 4H 4NH and Cd(en)Ni(CN) 4 ·2C 4H 4NH, have been studied by single crystal X-ray crystallography, solid state 2H NMR, PXRD, TGA and DSC. Both show in-plane reorientation of the pyrrole in an inequivalent six-site potential, with two low energy minima where the N of the pyrrole is oriented up or down with respect to the crystal c axis, followed by out-of-plane 4-fold motion about an axis parallel to the crystal c axis. There is also evidence for an intermediate dynamic state where there is 2-fold rather than 4-fold motion. The NH 3 or en groups of the host frameworks rotate about the crystal c direction. The Hofmann-en pyrrole clathrate shows two phase transitions at 196 and 259 K, and the dynamics of the host en and pyrrole guest appear to be intimately linked to the latter structural transition. The results for these pyrrole clathrates have been compared with those for the two equivalent benzene clathrates, obtained previously and augmented with new 2H NMR lineshape data for Cd(en)Ni(CN) 4 ·2C 6D 6 above 300 K.

Effects of Polymorphism on Functional Group Dynamics: Solid State 2H NMR Studies of the Dynamic Properties of the α and β Phases of l-Glutamic Acid

l-Glutamic acid (LGA) exists as two polymorphic solid phases (denoted the α and β phases) with different molecular conformations. This paper investigates the contrasting dynamic properties of the α and β phases of LGA, from measurements of solid state 2H NMR line shapes and spin−lattice relaxation times for the −ND3+ groups in selectively deuterated samples. For both phases, the 2H NMR line shapes have been simulated successfully in terms of a three-site 120° jump motion of the −ND3+ group, with the jump frequency varying from 1.5 × 107 to 1.5 × 103 s-1 within the temperature range 373 to 198 K. The activation energy for the reorientation of the −ND3+ group is higher for the α phase [(47 ± 2) kJ mol-1] than for the β phase [(34 ± 3) kJ mol-1]. The 2H NMR spin−lattice relaxation time data are also consistent with the three-site 120° jump motion of the −ND3+ group and are in good agreement with the activation energies [α phase, (48 ± 1) kJ mol-1; β phase, (39 ± 2) kJ mol-1] determined from our 2H NMR line s...

Dynamic Properties of the Tetrahydrofuran Clathrate Hydrate, Investigated by Solid State 2H NMR Spectroscopy

Clathrate hydrates are solid inclusion compounds in which cages formed by a water host structure accommodate guest molecules of appropriate size and shape. Dynamic properties of the clathrate hydrate containing tetrahydrofuran (THF) guest molecules have been investigated using solid-state 2H NMR techniques, including line shape analysis and spin−lattice relaxation time (T1) measurements. 2H NMR results for THF−D2O suggest that there are at least two dynamically distinguishable types of water molecule, both undergoing four-site tetrahedral jump motions but with different rates. The population ratio may be assigned as 3:1, consistent with the known crystal structure. Above 193 K, the dynamics of both components are in the intermediate motion regime (10-7 s ≲ τc ≲ 10-3 s). From analysis of the 2H NMR line shapes, the jump rates have been determined as a function of temperature, and activation parameters have been estimated. The 2H NMR spectrum for THF-d8−H2O comprises a single line, which broadens and changes shape on decreasing temperature. These observations are interpreted in terms of the THF-d8 guest molecules undergoing an approximately isotropic motion in the rapid regime (τc ≲ 10-7 s), but with a distribution of slightly anisotropic characteristics. As the guest molecules reorient rapidly in comparison with the dynamics of the water molecules, different guest molecules are effectively surrounded by structurally different water cages so that there is a distribution in terms of the anisotropic characteristics associated with the reorientational motions of different guest molecules. These conclusions are complemented by information determined from 2H NMR spin−lattice relaxation time measurements.

Solid State 2H NMR Studies of n-Alkanes Confined in Solid Matrices

2H NMR spectroscopy is used to study the molecular behaviour of selectively deuterated n-alkanes of variable length that serve as guest molecules in channel-forming inclusion compounds, made from urea, cyclophosphazene or α-cyclodextrin. In the present study 2H NMR lineshape and relaxation experiments are performed which provide a deeper insight into the motional characteristics as well as the ordering behaviour of such spatially confined guest molecules. The analysis of these experiments reveals the presence of chain motions that are found to depend strongly on the particular host matrix.

Monohalocyclohexanes in liquid crystalline solution: the molecular behaviour of chloro- and iodo-cyclohexane

Dynamic 2H NMR spectroscopy is used to study selectively and perdeuteriated samples of chloro- and iodo-cyclohexane dissolved in a liquid crystalline solution. The equatorial and axial conformers are found to exist in a dynamic equilibrium with relative amounts of about pe = 0.7 and pa = 0.3, respectively, as taken from the 2H NMR lineshapes and 2D exchange spectra. The quantitative analysis of variable temperature 2H NMR spectra provides the kinetic parameters of the underlying ring inversion process. The activation enthalpies of this internal process are given by ΔH = 48.1 ± 1.0 and 40.6 ± 2.3 kJ mol-1 for chloro- and iodocyclohexane, respectively. These values resemble those reported from earlier studies of isotropic solutions. The molecular order parameters, as well as the orientation of the main ordering axis, for the axial and equatorial conformers are determined as functions of temperature. These molecular quantities exhibit a strong dependence on the actual conformational state and size of the substituent, while polar effects of the substituents seem to play no important role.

A dip in powder deuterium NMR lineshapes

An analysis of powder (2)H NMR lineshapes based on the Redfield theory reveal an interesting dip at the center of the powder spectra, that is, at the magic angle beta = 54.7. This extra feature, which has completely been ignored in the literature, has interesting dynamic implications. From its presence or absence it is possible to tell whether the dominant dynamic process causing relaxation is fast or slow relative to the inverse of the Larmor frequency, 1/omega(0). The angle-dependent lineshape function is derived in closed form and the powder spectra are displayed for two typical cases. Copyright 2000 Academic Press.

Molecular Dynamics of iso-Butyl Alcohol Inside Zeolite H-ZSM-5 as Studied by Deuterium Solid-State NMR Spectroscopy

The molecular mobility of iso-butyl alcohol, selectively deuterated in the methylene group (iBA[1-d2]) or in the methyl groups (iBA[3-d6]), adsorbed on zeolite H-ZSM-5 was studied with 2H NMR spectroscopy. At 115−293 K, the 2H NMR line shape for the adsorbed iBA[3-d6] represents a superposition of one solidlike and two liquidlike signals, whereas for iBA[1-d2], it is a superposition of the solidlike and the liquidlike lines. Two liquidlike signals are assigned to the alcohol molecules isotropically reorienting with correlation time τR ∼ 1 × 10-6 s by jumping among Al−OH−Si groups, which are located inside the channels and at channel intersections of the zeolite channel system. Being adsorbed on Al−OH−Si groups, these two types of alcohol molecules differ in the effective amplitude of libration γ0 (γ0 is a libration cone semiangle) of the methyl groups, which is large for both adsorption sites (γ0 ∼ 52° for one of the types, and γ0 ∼ 72° for the other). The solidlike signal with the observed quadrupole splitting of 38 kHz is assigned to the alcohol molecules located inside the zeolite channels. These alcohol molecules reorient with a correlation time τR > 4.2 × 10-6 s, and their methyl groups experience small librations with amplitude γ0 ∼ 19°. Methyl groups of the alcohol molecules located at channel intersections rotate about the CH3−CH axis with correlation time τP = (1.4−2.6) × 10-11 s at 293 K and activation energy EP = 11−12 kJ/mol, whereas those located inside the channel rotate with correlation time τj ∼ 2 × 10-10 s at 293 K and activation energy Ej = 10.5 kJ/mol. The difference in the rotation rates is attributed to the influence of the walls of the zeolite channel on dynamics of one-axis methyl group rotation, which is expected to be more profound in the confined area of a narrow channel than at channel intersections.

Rates of Acid-Catalyzed NH Proton Exchange of Enaminones, an 1H NMR Study

: 1H NMR spectra of a series of enaminones R-CO-CH=C(NHCH2Ph)-R [1, R=CH3; 2, R=C6H5; 3, R=CF3; 4, R=CH2CH2CH2; 5, R=CH2C(CH3)2CH2], were obtained in the presence of trifluoroacetic acid in CDCl3 or DMSO-d6 at 28 °C. Steric, solvent, electronic and hydrogen bonding effects on the rates of acid-catalyzed NH proton exchanges were investigated. Rate constants were calculated by 1H NMR line shape analysis.

Experimental 1H NMR and Computational Studies of Internal Rotation of Solvated Formamide

Solvent effects on the C−N bond rotation process of formamide (FA) are investigated experimentally and theoretically. Temperature-dependent exchange broadened 1H NMR line shapes of [15N]FA in water, dimethyl sulfoxide-d6, and tetrachloroethane-d2, and as neat solution are consistent with ΔG⧧298 values for FA internal rotation, in kilocalories per mole, of 18.2(0.1), 17.8(0.1), 17.2(0.1), and 18.0(0.1), respectively. These values are significantly higher than the gas-phase ΔG⧧298 value, 16.0(0.1) kcal mol-1, and they correlate well with empirical spectroscopic solvent polarity scales. Self-consistent isodensity polarizable continuum model (SC-IPCM) calculations at the HF/6-311++G** level of theory for FA in a series of dielectric constants ranging from 1 to 109 predict a complete reversal of the preferred internal rotation path for hydrated FA compared to gas-phase FA. Equal contributions to the internal rotation rate constant from both possible internal rotation paths is predicted to occur at a dielectric constant of 7.08. SC-IPCM calculations predict that ΔG⧧298 for hydrated FA is 1.99 kcal mol-1 higher than ΔG⧧298 for gas-phase FA. SC-IPCM calculations for a 1:1 FA−H2O complex were performed to allow for direct inclusion of intermolecular hydrogen bonding. The predicted ΔG⧧298 for the hydrated 1:1 FA−H2O complex is ca. 2.3 kcal mol-1 higher than the predicted gas-phase ΔG⧧298, in excellent agreement with experiment.

Temperature dependence of solid state 1H NMR line shapes and M2 in polycrystalline BF4− salts of 1-propyltetrazole complexes of iron(II) and zinc(II)

1H NMR spectra of the spin-crossover compound [Fe(ptz)6](BF4)2 and its zinc(II) analogue were recorded on polycrystalline samples as the function of decreasing temperature in order to study the connection of the change in the electronic state and the molecular motions present in both crystals. The temperature range for the zinc compound was 295–30K, with a cooling rate of ∼1Kmin−1, while in the case of the iron(II) compound this temperature range was 298–10K with the same cooling rate as in the case of [Zn(ptz)6](BF4)2. The analysis of the temperature dependence of the line shape parameters and calculations of 1H second moments showed the following. The temperature of the spin transition in the iron(II) compound is 132K on cooling. On the basis of the similarity of the temperatures at which the different molecular motions appear, the same motions are present in both compounds in the whole temperature range. The [Zn(ptz)6](BF4)2 probably goes through a structural transition between 90 and 60K, this transition manifests in the virtual increase of the average proton–proton distance, and the sign of a similar transition is also observed in [Fe(ptz)6](BF4)2.

2H MAS NMR of strongly dipolar coupled deuterium pairs in transition metal dihydrides: extracting dipolar coupling and quadrupolar tensor orientations from the lineshape of spinning sidebands

A selectively D2 labeled transition metal dihydride, Os(D2)(Cl)2(CO)(PiPr3)2, was synthesized and studied by 2H magic angle spinning (MAS) NMR spectroscopy. It was found that the interference between the quadrupolar and homonuclear dipolar interaction results in a characteristic lineshape of the MAS sidebands. The basic properties of the interference of homonuclear dipolar and quadrupolar coupling on the 2H NMR lineshape were elucidated, using average Hamiltonian theory, and exact simulations of the experiments were achieved by stepwise integration of the equation of motion of the density matrix. These simulations show that it is possible to determine the size of the dipolar interaction and thus the D–D distance from the lineshape of the sidebands.

Dynamics in the crystalline polymorphic forms I and II and form III of isotactic poly-1-butene

Following an earlier study of the 1H relaxation and NMR line shapes, we have carried out selective one-dimensional and two-dimensional 13C solid-state NMR studies that yield to detailed interpretation of the dynamics in form I, II, and III polymorphs of isotactic poly-1-butene. A specific defect diffusion along the side group is proposed to account for the temperature dependence of the 13C spectra in form I. The backbone of the helix in forms II and III is shown to undergo large angle motions above the glass-transition temperature. High-resolution solid-state 13C two-dimensional exchange NMR under magic-angle spinning with cross-polarization techniques demonstrates the existence of slow rotational jumps of the helices in form III with typical jump rates of about 10 s−1. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2611–2624, 2000

Characterization of ultradrawn polyethylene fibers by NMR: crystallinity, domain sizes and a highly mobile second amorphous phase

The structure of ultradrawn ultra-high molecular weight polyethylene fibers has been investigated by solid-state NMR. A crystallinity of (88±2)% was determined by traditional 1H NMR lineshape decomposition, and by a new adaptation of 13C NMR crystallinity determination for polyethylenes with extremely long crystalline T 1 relaxation times. 1H spin diffusion yields amorphous domain sizes of 10±5 nm , and crystalline regions of 100±50 nm diameters. A second, highly mobile, amorphous phase, making up (0.8±0.2)% of the sample, was detected by 1H NMR. In spite of its 1.8 kHz 1H line width, it shows little spin diffusion to the other phases, even on a 500-ms time scale; this suggests domains of more than 3 nm thickness or chains extending into voids. Being undetectable in the extruded precursor material and in the fibers after melting, this highly mobile phase must have been induced by the drawing process. 13C NMR confirms that no low-molecular-weight additives are present on a level above 0.01%. A similar highly mobile component has also been detected in drawn medium-molecular-weight polyethylenes. The fraction of partially mobile, oriented interfacial material or tie-molecules in the fiber was found to be ∼5%, while rigid gauche conformers could not be detected (concentration <1%). Altogether, five morphological components have been identified: 83% crystal core, of which 80% is orthorhombic and 3% monoclinic, with thickness of ∼100 nm; 5% disordered all- trans interfacial and/or tie molecules; 11% mobile amorphous regions, with diameters of ∼10 nm; and 1% highly mobile segments, probably at void surfaces. On this basis, a structural model for ultradrawn PE fibers is proposed.

Energy barrier to TTC–TTT isomerisation for the merocyanine of a photochromic spiropyran

The 1H NMR signal of a proton of a spiropyran photomerocyanine was resolved at low temperature into two signals attributed to the two isomers, TTT and TTC. The equilibrium distribution and free energy difference between the TTT and TTC isomers were calculated to be 0.15 and 4.6±0.7 kJ mol−1, respectively. By 1H NMR lineshape analysis at various temperatures, an activation barrier for the TTC→TTT isomerisation of 43.6±3 kJ mol−1 was calculated.

Dynamics of the Tyrosine Side Chain in Bombyx mori and Samia cynthia ricini Silk Fibroin Studied by Solid State 2H NMR

Solid-state deuterium NMR (2H NMR) was used to study the dynamics of the tyrosine (Tyr) residue in silk fibroin from Bombyx mori (B. mori) and Samia cynthia ricini (S. c. ricini). Specifically deuterated cocoon silk was obtained by feeding silk worms with Tyr, labeled either at the Cβ carbon ([3,3-2H2]Tyr) or at the aromatic ring ([3‘,5‘-2H2]Tyr). The 2H NMR spectra of the [3,3-2H2]Tyr-labeled silk fibroins showed typical rigid powder patterns, indicating that there is essentially no motion about the Cα−Cβ bond axis, both in B. mori and S. c. ricini. In contrast, the 2H NMR spectra of the [3‘,5‘-2H2]Tyr-labeled silk fibroins consisted of two dynamic components each: a rigid powder contribution, plus a motionally averaged contribution. Hence, some of the Tyr side chains are mobile at the phenolic ring. This motion was characterized to be a π-flip as is typical of aromatic rings. The corresponding 2H NMR line-shape of the B. mori sample could be successfully simulated by attributing 20% of the signal to a motionally averaged component with a fast rate (106 Hz) and the remaining 80% to a much slower component (<103 Hz). Likewise, the simulation of S. c. ricini silk fibroin indicated that 60% of the rings are engaged in fast motional averaging (107 Hz), while 40% undergo slow motion (104 Hz). Thus, the fraction of the fast component is considerably higher for S. c. ricini silk fibroin than for B. mori, which must be a consequence of their different amino acid sequences. It appears that the side-chain mobility depends on the local packing density around the Tyr residue. We conclude that the Gly-Ala repeats in B. mori silk fibroin are relatively tightly packed. In contrast, a large part of the Gly-rich regions in S. c. ricini are comparatively loosely packed.

Symmetry, disorder, and dynamics in solid crown ether complexes: 2H NMR studies of 15-crown-5·NaI, 15-crown-5·NaClO4, and 21-crown-7·KI

2H NMR line shapes have been obtained as a function of temperature for partially deuteriated 15-crown-5·NaI, 15-crown-5·NaClO4 and 21-crown-7·KI. Sudden changes in the line shapes above 330 K correspond to phase transitions: DSC shows transitions at 338, 347 K for 15-crown-5·NaI, and 356, 374, 383 K for 15-crown-5·NaClO4. The 2H NMR line shapes for the room temperature (RT) phases show the onset of a motion of the macrocycles above 200K, which is rapid a little above room temperature. Through detailed analysis of the complicated line shapes it has been shown that the most consistent interpretation of the dynamics in these RT phases is a "merry-go-round" type of motion similar to that found for 18-crown-6 and 12-crown-4 complexes in which O-CH2-CH2 units exchange sites around the ring, simultaneously adjusting their conformation to fit the site. In the high-temperature phases of all three complexes all the ring atoms are dynamically equivalent. This can only happen through a combination of increased symmetry and disorder for which possible models have been devised.Key words: 2H NMR, crown ether complexes, dynamics, disorder.