Solid-state 2H NMR Studies Research Articles

Encasing Triaryltriazine with a Bulky Chiral Cap: Luminescent Chiral Crystalline Molecular Rotors with Modulation of Solid-State Chiroptical Properties Mediated by Molecular Rotation.

A novel structural motif for luminescent chiral crystalline molecular rotors with chiroptical properties correlated with the rotational motion in crystalline media is presented. This scaffold incorporates bulky chiral caps consisting of a homochiral binaphthyl moiety with a triisopropylsilyl (TIPS) group into triaryltriazine, as confirmed by single-crystal X-ray diffraction (XRD) analysis. Variable-temperature solid-state 2H NMR studies revealed a 4-fold rotation of the phenylenes occurred in the rotor crystal between 263 and 333 K, while a steric rotor analogue shows no rotational motion. Notably, a reduction in the dihedral angle of the binaphthyl moiety upon heating was observed in the chiral rotors, and a corresponding alteration of the circular dichroism (CD) signal was detected in the solid-state, while those of the steric rotors showed no alteration by the temperature change. We propose that the fast rotation of the phenyl rings affects the motion of neighboring isopropyl groups, leading to steric repulsion with the binaphthyl moieties and thereby inducing its conformational change. Furthermore, the chiral rotors exhibited circularly polarized phosphorescence in the solid-state at low temperature, originating from rotational displacement of the phenylene on triphenyltriazine during structural relaxation in the excited state. Meanwhile, the steric rotors showed significant circularly polarized fluorescence induced by the suppressed molecular motion via a sterically hindered lattice environment in the excited state. These results indicate that the bulky chiral cap introduced into the triaryltriazines, acting as a luminescent chiral crystalline molecular rotor, can be a useful scaffold for the modulation of solid-state chiroptical properties via molecular rotational motions.

Solid-State 2H NMR Study for Deuterated Phenylene Dynamics in a Crystalline Gyroscope-Like Molecule

Molecular rotors have earned substantial popularity in recent times, owing to the unique dependence of its crystalline properties on the rotational dynamics of the rotor. We have recently reported the synthesis and crystal structure of a phenylene-bridged macrocage as a gyroscope-like molecule in the crystalline state. The dynamics of the phenylene moiety was probed by solid-state 13C CP/MAS proton dipolar dephasing NMR spectroscopy. Herein, solid-state 2H NMR studies were performed to study the dynamics of the gyroscope-like molecule with a deuterated rotor in the crystalline state. A spectrum with a narrow line shape was obtained at 300 K. The facile exchange among three stationary states, which was observed by X-ray crystallography, was clearly confirmed. Additionally, a crystal-to-crystal phase transition that switches the motion of the rotor was observed in the DSC analysis of the powdered sample.

A Small Lattice Change Induces Significant Dynamic Changes of CH3NH3+ Caged in Hybrid Perovskite Crystals: Toward Understanding the Interplay between Host Lattices and Guest Molecules.

Two perovskite-type compounds, (MA)2[B'Co(CN)6] (MA = methylammonium, B' = K(I) and Na(I)), have very similar structures, but exhibit marked differences in the phase and dielectric transitions. Solid state 2H NMR studies reveal the detailed dynamic changes of the caged methylammonium (MA) cations before and after the phase transitions, which are correlated with the different dielectric states of the compounds. Using solid state 59Co NMR, the dynamic changes of the host lattices before and after the transitions, which accompany the changes in the dynamics of the caged MA cations, are unveiled, demonstrating the intriguing interplay between the MA cations and the host lattices. On the basis of these observations, the molecular origins of the dielectric transitions are discussed in detail.

Solid State 2H NMR Studies of the Disordering of Raft-Like Domains by n-3 PUFA

The health benefits of omega-3 polyunsaturated fatty acids (n-3 PUFA) contained in fish oils continue to be investigated intensively in pre-clinical and clinical studies. The major bioactive components are docosahexaenoic acid (DHA, 22:6) with 22 carbons and 6 double bonds and eicosapentaenoic acid (EPA, 20:5) with 20 carbons and 5 double bonds. An emerging hypothesis is that n-3 PUFA are incorporated into membrane phospholipids and modify the structure and organization of lipid rafts, thus affecting cell signaling. We used solid-state 2H NMR spectroscopy to compare molecular organization in mixtures of 1-palmitoyl-2-docosahexaenoylphosphatidylcholine (PDPC) and 1-palmitoyl-2-eicosapentaenoylphosphatidylcholine (PEPC) with the raft-stabilizing molecules sphingomyelin (SM) and cholesterol. Spectra for PDPC-d31 and PEPC-d31, analogs of PDPC and PEPC with a perdeuterated palmitoyl sn-1 chain, revealed that DHA and EPA incorporate into raft-like domains enriched in SM and cholesterol. Greater incorporation was seen for DHA than EPA. We used PSM-d31, an analog of SM with a perdeuterated N-palmitoyl chain, as a probe to directly investigate molecular order within raft-like domains. Our initial experiments looked at mixtures of (POPC) with SM and cholesterol as a control. In POPC/SM (1:1 mol) mixtures, the 2H NMR spectra revealed segregation into POPC-rich (less ordered) and SM-rich (more ordered) domains that are nano-scale in size and contain < 180 lipids. When cholesterol (1:1:1 mol) was added, both domains became more ordered and greater mixing of POPC and SM was observed. These results will be discussed together with the results from experiments on PSM-d31 in mixtures with DHA- and EPA-containing phospholipids and cholesterol.

On the Orientation of a Designed Transmembrane Peptide: Toward the Right Tilt Angle?

The orientation of the transmembrane peptide WALP23 under small hydrophobic mismatch has been assessed through long-time-scale molecular dynamics simulations of hundreds of nanoseconds. Each simulation gives systematically large tilt angles (>30 degrees). In addition, the peptide visits various azimuthal rotations that mostly depend on the initial conditions and converge very slowly. In contrast, small tilt angles as well as a well-defined azimuthal rotation were suggested by recent solid-state 2H NMR studies on the same system. To optimally compare our simulations with NMR data, we concatenated the different trajectories in order to increase the sampling. The agreement with 2H NMR quadrupolar splittings is spectacularly better when these latter are back-calculated from the concatenated trajectory than from any individual simulation. From these ensembled-average quadrupolar splittings, we then applied the GALA method as described by Strandberg et al. (Biophys J. 2004, 86, 3709-3721), which basically derives the peptide orientation (tilt and azimuth) from the splittings. We find small tilt angles (6.5 degrees), whereas the real observed tilt in the concatenated trajectory presents a higher value (33.5 degrees). We thus propose that the small tilt angles estimated by the GALA method are the result of averaging effects, provided that the peptide visits many states of different azimuthal rotations. We discuss how to improve the method and suggest some other experiments to confirm this hypothesis. This work also highlights the need to run several and rather long trajectories in order to predict the peptide orientation from computer simulations.

Solid-state 2H NMR Study of Nanocrystal Formation of D2O and Their Dynamic Aspects in ACF Hydrophobic Nanospaces

Abstract Molecular motion of heavy water confined in activated carbon fiber (ACF) was investigated using solid-state 2H NMR. The 2H NMR spectrum indicated that π-flip motion as well as tetrahedral jump of D2O was active below 190 K, suggesting the formation of nanocrystal of D2O in the ACF nanoslits.

Solid triphenylmethanol: A molecular material that undergoes multiple internal reorientational processes on different timescales

In solid triphenylmethanol, the molecules are arranged in hydrogen-bonded tetramers, and it is already well established that the hydrogen bonding in this material undergoes a dynamic switching process between different hydrogen bonding arrangements. In addition to this motion, we show here, from solid-state 2H NMR studies of the deuterated material (C 6D 5) 3COH, that each phenyl ring in this material undergoes a 180°-jump reorientation about the C 6D 5–C(OH) bond, with an activation energy of ca. 50 kJ mol −1. The timescale for the phenyl ring dynamics is several orders of magnitude longer than the timescale for the hydrogen bond dynamics in this material, and is uncorrelated with the dynamics of the hydrogen bonding arrangement.

Compositional and2H NMR Studies of Bis(benzene)chromium Composites of Mesoporous Vanadium−Niobium Mixed Oxides

New mesoporous niobium oxides with 5, 10, and 15 mol% vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(l) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.

Local Dynamics of Poly(ethylene oxide) Confined in 1nm Slits

ABSTRACTMolecular Dynamics simulations are used to explore the short-time dynamics of nanoscopically confined poly(ethylene oxide). Both bulk and confined systems have been studied using an atomistically detailed force field so as to comparatively illustrate their differences and complement experimental results. Our aim is to elucidate the origins of the counter intuitive distribution of relaxation times for C-H bond reorientation for PEO in severe confinements, as experimentally observed in solid state 2H NMR studies. In contrast with the respective bulk PEO system, where a transition from distinct solid to liquid like dynamics is seen with increasing temperature, for the confined chains there is a coexistence of fast and slow segmental dynamics over a wide temperature range. Our studies have revealed that factors such as local density inhomogeneities, proximity of Li+, and translational motion, synergistically contribute to the generation of fast PEO segmental dynamics in 1nm confinements.

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.

Solid state 2H NMR study of the orientation and dynamics of cobaltocenium intercalated in the layered silicate LaponiteRD

Ion-exchange intercalation of [Co(η-C 5 D 5 ) 2 ] + ions in the smectite clay LaponiteRD [Na 0.46 Mg 5.42 Li 0.46 Si 8 (OH) 4 O 20 ] yields [Mg 5.42 Li 0.46 Si 8 (OH) 4 O 20 {Co(η-C 5 D 5 ) 2 } 0.4 Na 0.1 ]. Structural and dynamic information about the material was obtained with the benefit of 2 H NMR spectroscopic measurements, performed on powder and specially prepared oriented samples, combined with comparisons to 2 H NMR lineshape simulations. The cobaltocenium cations adopt an ordered arrangement in the host lattice with their principal molecular axes parallel to the silicate layers. In addition to rotation about the principal molecular axis, previously observed for similar cobaltocenium layered intercalates, the emergence of an inner component with a splitting of 22.3 kHz over the temperature range 220-300 K strongly suggests the molecule is undergoing n-fold reorientations about the C 2 axis of symmetry. Concurrent to this dynamic motion is an additional vibrational motion in and out of the plane parallel to the silicate layers. Coupled with rotation about the C 2 axis, this gives rise to a gyratory motion in the slow-intermediate regime which increases with temperature until the fast regime is reached at 330 K.

Solid-state 2H NMR study of methyl-d3-cobalamin

A solid-state 2H NMR study of methyl-d3-cobalamin has been performed as a function of temperature to provide information concerning the character and energetics of the motion performed by this unique bioorganometallic methyl group. Analysis of the 2H NMR line shape indicates that the methyl group undergoes rapid three-fold rotation, and that the Co-C-2H angle lies between 105.9 and 109.5 degrees. Determination of the spin-lattice relaxation times T1 shows that the relaxation is anisotropic, consistent with a "jumping" motion of the methyl group rather than rotational diffusion. This also provides the activation energy to methyl jumps as 8.3 +/- 1.3 kJ/mol. It is proposed that this energetic barrier may be a useful probe of changes in the electronic character of the Co-C bond that accompany the biological role of this molecule in such enzymes as methionine synthase.

Dynamics of benzene and pyridine guest molecules in their tri-ortho-thymotide inclusion compounds. Solid-state 2H NMR Studies

Dynamic properties of perdeuteriated benzene (C6D6) and perdeuteriated pyridine (C5D5N) guest molecules in their tri-ortho-thymotide (TOT) inclusion compounds have been studied via variable-temperature wide-line solid-state 2H NMR spectroscopy. The temperature dependences of the quadrupole echo 2H NMR lineshape and the 2H NMR spin–lattice relaxation time for C6D6/TOT in the temperature range 96–293 K are interpreted in terms of in-plane rotation (nearest-neighbour 2π/6 jump model) of the benzene molecule, together with restricted wobbling of the molecular plane. The 2H NMR spin–lattice relaxation time measurements suggest that there are two dynamically inequivalent types of benzene molecule (included in structurally different types of tunnel within the TOT host structure), with their activation energies for in-plane rotation estimated to be 4.0 and 5.9 kJ mol–1. For C5D5N/TOT in the temperature range 90–363 K, three types of motion have been identified: (i) in-plane libration; (ii)π jumps of each pyridine molecule about its two-fold symmetry axis; (iii) restricted wobbling of the molecular plane. On the basis of 2H NMR lineshape analysis of 2H NMR spin–lattice relaxation time measurements, the activation energy for motion (ii) is estimated to be ca. 17-20 kJ mol–1.

Natural Abundance Solid State 2H NMR Studies of Phase Transitions in Rotator Phase Solids

Variable temperature natural abundance 2 H NMR studies of solid state phase transitions in adamantane, tetrakis(trimethylsilyl)methane (TTMSM), tetrakis(trimethylsilyl)silane (TTMSS) and 2,2-dimethyl-1,3-propanediol (DMPD) are reported; in spite of the low sensitivity associated with natural abundance 2 H NMR, this technique is demonstrated as a feasible approach for elucidating changes in molecular dynamic properties associated with the phase transitions in these materials.

Dynamic properties of p-diiodobenzene investigated by solid-state2H and13C nuclear magnetic resonance spectroscopy

p-Diiodobenzene exists in two crystalline phases, both of which are orthorhombic, with space groups Pbca(α phase) and Pccn(β phase) respectively. The α phase is the stable form at ambient temperature, and a phase transition to the β phase occurs at ca. 326 K. In this paper, we present variable-temperature solid-state 2H NMR studies of perdeuteriated p-diiodobenzene. Over the temperature range 293–320 K, the 2H NMR spectrum of the α phase is a rigid lattice powder pattern, with static quadrupole coupling constant 176 ± 1 kHz and static asymmetry parameter 0.04 ± 0.01. Over the temperature range 350–370 K, the 2H NMR spectrum of the β phase shows features characteristic of 180° jumps of the aromatic ring about the C(1)–C(4) axis, and the jump frequency (κ) for this motion has been determined as a function of temperature. On the assumption of Arrhenius behaviour for the temperature dependence of κ, the activation energy for this jump motion is estimated to be (90 ± 10) kJ mol–1. The temperature-dependent structural properties of perdeuteriated p-diiodobenzene have also been investigated by high-resolution solid-state 13C NMR spectroscopy, and results from these investigations are reported and discussed in the light of the dynamic information determined from the 2H NMR studies.

Structure of cobaltocene in SnS2: a single crystal solid state 2H NMR study

Solid state 2H NMR spectra of single crystals of 2H-SnS2 intercalated with cobaltocene [Co (η-C5D5)2] indicate that the guest molecules are ordered within the host layers with their principal axes parallel to the host layers.

ChemInform Abstract: Solid-State 2H NMR Study of Thymidine. Base Rigidity and Ribose Ring Flexibility in Deoxynucleosides.

ChemInform Abstract: Solid-State 2H NMR Study of Thymidine. Base Rigidity and Ribose Ring Flexibility in Deoxynucleosides.