Synthesis of a polyrotaxane-based macroporous polymer as stationary phase for capillary electrochromatography via host-guest complexation of N,N '-ethylenedianilinediacrylamide with statistically methylated beta-cyclodextrin.

A synthetic route to acrylamide-based monolithic stationary phases for CEC with rotaxane-type immobilized derivatized beta-CD was explored. N,N'-Ethylenedianilinediacrylamide was synthesized as the water-insoluble crosslinker forming water-soluble inclusion complexes with statistically methylated beta-CD. Mixed-mode stationary phases were synthesized by free radical copolymerization of the bisacrylamide-CD host-guest complex with water-soluble monomers and an additional water-soluble crosslinker in aqueous solution. Complex formation in solution and inclusion of the pseudorotaxane into the polymeric network (formation of a polyrotaxane architecture) were studied by means of (1)H-NMR chemical shift analysis, CD modified micellar EKC (CD-MEKC), 2D-NOESY spectroscopy, and solid state( 13)C-NMR spectroscopy. The presence of a mixed-mode selectivity of the stationary phase based on hydrophobic and hydrophilic interaction was confirmed by CEC with neutral polar and nonpolar solutes.

Self-assembly of semifluorinated dendrons attached to electron-donor groups mediates their pi-stacking via a helical pyramidal column.

Semifluorinated first-generation self-assembling dendrons attached via a flexible spacer to electron-donor molecules induce pi-stacking of the donors in the center of a supramolecular helical pyramidal column. These helical pyramidal columns self-organize in various columnar liquid crystal phases that mediate self-processing of large single crystal liquid crystal domains of columns and self-repair their intracolumnar structural defects. In addition, all supramolecular columns exhibit a columnar phase at lower temperatures that maintains the helical pyramidal columnar supramolecular structure and displays higher intracolumnar order than that in the liquid crystals phases. The results described here demonstrate the universality of this concept, the power of the fluorous phase or the fluorophobic effect in self-assembly and the unexpected generality of pyramidal liquid crystals.

Self-assembly of programmed building blocks into structurally uniform dendrimers.

Selective and independent dimerization of tri- and tetraurea derivatives was used to build up dendritic assemblies which are uniform in size and structure. Dendrimers with the total molecular masses of about 25 000 g/mol were obtained. The existence of uniform assemblies was proved by 1H and 1H DOSY NMR experiments and also by dynamic light scattering.

Determining the geometry of hydrogen bonds in solids with picometer accuracy by quantum-chemical calculations and NMR spectroscopy.

The structure of multiply hydrogen-bonded systems is determined with picometer accuracy by a combined solid-state NMR and quantum-chemical approach. On the experimental side, advanced 1H-15N dipolar recoupling NMR techniques are capable of providing proton-nitrogen distances of up to about 250 pm with an accuracy level of +/-1 pm for short distances (i.e., around 100 pm) and +/-5 pm for longer ones (i.e., 180 to 250 pm). The experiments were performed under fast magic-angle spinning, which ensures sufficient dipolar decoupling and spectral resolution of the 1H resonance lines. On the quantum-chemical side, the structures of the hydrogen-bonded systems were computationally optimised, yielding complete sets of nitrogen-proton and proton-proton distances, which are essential for correctly interpreting the experimental NMR data. In this way, nitrogen-proton distances were determined with picometer accuracy, so that vibrational averaging effects on dipole-dipole couplings need to be considered. The obtained structures were finally confirmed by the complete agreement of computed and experimental 'H and '5N chemical shifts. This demonstrates that solid-state NMR and quantum-chemical methods ideally complement each other and, in a combined manner, represent a powerful approach for reliable, high-precision structure determination whenever scattering techniques are inapplicable.

Two-dimensional double-quantum 2H NMR spectroscopy in the solid state under OMAS conditions: correlating 2H chemical shifts with quasistatic line shapes.

Solid-state 2H NMR spectroscopy is a well-established and versatile method to study molecular orientation and dynamics in selectively deuterated samples. Herein, we introduce a 2D 2H double-quantum (DQ) NMR experiment performed under fast magic-angle spinning with a slight offset of the magic angle (OMAS). The experiment combines 2H chemical-shift resolution with DQ-filtered quasistatic 2H line shapes. In this way, it is possible to separate 2H resonances and to independently determine 2H quadrupole couplings at multiple sites. While 2H chemical shifts are resolved in the 2H DQ dimension, the quadrupole parameters can be obtained from characteristic line shapes which are reintroduced in the second dimension by the magic-angle offset. The 2D 2H DQ OMAS experiment is demonstrated on L-histidine which was deuterated at multiple sites by recrystallisation from D2O.

Rotor modulations and recoupling strategies in 13C solid-state magic-angle-spinning NMR spectroscopy: probing molecular orientation and dynamics.

Recoupling strategies for anisotropic interactions enable the investigation of molecular structure, order and dynamics in a sensitive and site-specific fashion by solid-state NMR spectroscopy. Whereas magic-angle spinning (MAS) efficiently averages anisotropic interactions and enhances spectral resolution, recoupling pulse sequences selectively restore certain parts of rotor-modulated dipole-dipole couplings or chemical shift anisotropies (CSA). More specifically, it is possible to recouple either the omegaR- or the 2omegaR-modulated terms of an interaction Hamiltonian, which exhibit different orientation dependencies and, in this way, provide a means of distinguishing whether the observed NMR spectra are affected by molecular motion or by molecular orientation. Sideband patterns generated by reconversion rotor encoding allow for a precise and selective determination of coupling constants and anisotropies, which contain site-specific information on structure, orientation and/or dynamics of individual molecular segments. Corresponding recoupling schemes are presented in a common context, and the possibilities of exploiting these effects for the determination of order parameters of oriented materials, such as oriented polymer chains or extruded fibres of a discotic mesogen, are discussed. The obtained orientational order parameters are compared to results from two-dimensional wide angle X-ray scattering (WAXS).

Shape-persistent macrocycles with intraannular polar groups: synthesis, liquid crystallinity, and 2D organization.

The synthesis of macrocycles with intraannular polar ester groups and extraannular oligo-alkyl groups is described. The compounds exhibit stable liquid crystalline phases showing fan-shaped textures under the polarizing microscope, typical for a columnar order of the molecules. X-ray powder diffraction data of the LC phase indicate that the unit cell contains two symmetry-related units, a feature pointing most probably to a restricted rotation of the macrocycles within a stack. The X-ray data were further supported by solid-state NMR experiments, showing that the rigid core of the compounds does not rotate with kHz or higher frequencies within the column in the LC phase. Apart from the organization of the molecules in the LC phase, the 2D organization of the macrocycles at the solvent-highly oriented pyrolytic graphite (HOPG) interface was investigated and showed that these compounds are capable of nanofunctionalizing the HOPG surface in the multinanometer regime.

Determination of long-range distances and dynamic order parameters by dipolar recoupling in high-resolution magic-angle spinning NMR spectroscopy.

By introducing dipolar recoupling methods to high-resolution magic-angle spinning (HRMAS) NMR spectroscopy, a class of experiments has been delevoped that allows the measurement of residual dipole-dipole couplings of approximately 1 Hz in weakly immobilized molecules. Using homonuclear 1H-1H recoupling, distances of up to approximately 8 A can be selectively determined, while heteronuclear 1H-13C recoupling provides access to dynamic order parameters of individual molecular segments on the order of approximately 10-3. The experiments are demonstrated on functionalized oligopeptides that are attached to polymer resins.

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy.

Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.

PFG-assisted selection and suppression of 1H NMR signals in the solid state under fast MAS.

Under fast MAS conditions, techniques for 1H signal selection and suppression, which have originally been developed for solution-state NMR, become applicable to solids. In this work, we describe how WATERGATE and DANTE pulse sequences can be used under MAS to selectively excite or suppress peaks in 1H solid-state spectra. As known from the liquid-state analogues, signal selection and/or suppression is supported by pulsed-field gradients which selectively dephase and rephase transverse magnetisation. Under MAS, the required field gradients are provided by a simple pair of coils which have been built into a standard fast-MAS probe. PFG-assisted techniques enable efficient selection or suppression of 1H peaks in a single transient of the pulse sequence without the need for phase cycles. Therefore, these tools can readily be incorporated into solid-state MAS NMR experiments, which is demonstrated here for 1H-1H double-quantum NMR spectra of supramolecular systems. In the examples presented here, the 1H signals of interest are relatively weak and need to be observed despite the presence of the strong 1H signal of long alkyl sidechains. PFG-assisted suppression of this strong perturbing signal is shown to be particularly useful for obtaining unambiguous results.

Benzoxazine oligomers: evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations.

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.

REDOR-based heteronuclear dipolar correlation experiments in multi-spin systems: rotor-encoding, directing, and multiple distance and angle determination.

We review a variety of recently developed 1H-X heteronuclear recoupling techniques, which rely only on the homonuclear decoupling efficiency of very-fast magic-angle spinning. All these techniques, which are based on the simple rotational-echo, double-resonance (REDOR) approach for heteronuclear recoupling, are presented in a common context. Advantages and possibilities with respect to the complementary application of conventionally X and 1H-inversely detected variants are discussed in relation to the separability and analysis of multiple couplings. We present an improved and more sensitive approach to the determination of 1H-X dipolar couplings by spinning-sideband analysis, termed REREDOR, which is applicable to XHn groups in rigid and mobile systems and bears some similarity to more elaborate separated local-field methods. The estimation of medium-range 1H-X distances by analyzing signal intensities in two-dimensional REDOR correlation spectra in a model-free way is also discussed. More specifically, we demonstrate the possibility of combined distance and angle determination in H-X-H or X-H-X three-spin systems by asymmetric recoupling schemes and spinning-sideband analysis. Finally, an 1H-X correlation experiment is introduced which accomplishes high sensitivity by inverse (1H) detection and is therefore applicable to samples with 15N in natural abundance.

(15)N-(1)H bond length determination in natural abundance by inverse detection in fast-MAS solid-state NMR spectroscopy.

A solid-state 15N-1H correlation NMR experiment is presented, which provides a substantial gain in signal sensitivity by 1H inverse detection under fast MAS conditions and allows for the precise determination of NH bond lengths via heteronuclear 1H-15N dipole-dipole couplings on samples naturally abundant in 15N. Pulsed-field gradients or, alternatively, radio frequency pulses ensure suppression of unwanted 1H signals. In this way, natural-abundance 15N-1H correlation NMR spectroscopy becomes feasible in the solid state with experiment times of a few hours. The dipole-dipole coupling constants are extracted from spinning sideband patterns generated by recently developed recoupling strategies. The information on 15N/1H chemical shifts and quantitative 15N-1H couplings can readily be combined in a single two-dimensional spectrum using a split-t1 approach.

Dipolar recoupling in NOESY-type 1H-1H NMR experiments under HRMAS conditions.

[structure: see text]. The concept of dipolar recoupling is introduced to 1H-1H NOESY experiments performed under HRMAS conditions. Dipole-dipole couplings are selectively recoupled during the mixing period, while MAS ensures high resolution in the spectral dimensions. Incoherent dipolar exchange is replaced by amplified coherent processes, such that time scales for polarization transfer are shortened, and dipolar double-quantum techniques become applicable. In this way, dipole-dipole couplings, as well as J-couplings, can be individually measured.