Host–guest complexes between cryptophane‐C and chloromethanes revisited

Cryptophane‐C is composed of two nonequivalent cyclotribenzylene caps, one of which contains methoxy group substituents on the phenyl rings. The two caps are connected by three OCH₂CH₂O linkers in an anti arrangement. Host–guest complexes of cryptophane‐C with dichloromethane and chloroform in solution were investigated in detail by nuclear magnetic resonance techniques and density functional theory (DFT) calculations. Variable temperature proton and carbon‐13 spectra show a variety of dynamic processes, such as guest exchange and host conformational transitions. The guest exchange was studied quantitatively by exchange spectroscopy measurements or by line‐shape analysis. The conformational preferences of the guest‐containing host were interpreted through cross‐relaxation measurements, providing evidence of the gauche+2 and gauche?2 conformations of the linkers. In addition, the mobility of the chloroform guest inside the cavity was studied by carbon‐13 relaxation experiments. Combining different types of evidence led to a detailed picture of molecular recognition, interpreted in terms of conformational selection. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamics of chloromethanes in cryptophane-E inclusion complexes: a 2H solid-state NMR and X-ray diffraction study

In this paper, we present a variable temperature (2)H solid-state NMR investigation of cryptophane-E:chloroform and cryptophane-E:dichloromethane inclusion complexes. The (2)H line shapes and nuclear spin relaxation rates were analyzed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad (2)H spectra, and that its reorientation is relatively slow with a correlation time of approximately 0.17 mus at 292 K. In contrast, the (2)H line shapes of encaged dichloromethane are narrow and the motion of this guest molecule is fast with a correlation time of approximately 1.4 ps at 283 K. The (2)H NMR data were complemented by an X-ray diffraction study of the cryptophane-E:dichloromethane structure, which was utilized in the analysis of the NMR parameters.     

Exclusively Heteronuclear NMR Experiments to Obtain Structural and Dynamic Information on Proteins

Provided that ¹³C‐detected NMR experiments are either preferable or complementary to ¹H detection, we report here tools to determine C^α? C′, C′? N, and C^α? H^α residual dipolar couplings on the basis of the CON experiment. The coupling constants determined on ubiquitin are consistent with the subset measured with the ¹H‐detected HNCO sequences. Since the utilization of residual dipolar couplings may depend on the mobility of the involved nuclei, we also provide tools to measure longitudinal and transverse relaxation rates of N and C′. This new set of experiments is a further development of a whole strategy based on ¹³C direct‐detection NMR spectroscopy for the study of biological macromolecules.     

NMR investigation of guest–host complex between chloroform and cryptophane C

Guest–host complex between cryptophane C, possessing two non‐equivalent caps, and chloroform is investigated by NMR spectroscopy. The kinetics of the chloroform exchange between the bound and free sites is determined by ¹H exchange spectroscopy. Moreover, the preferential orientation of chloroform molecule with respect to the cryptophane C frame is examined by the NOESY and ROESY experiments. The experimental findings are compared to the results of quantum chemical calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Systematic Dielectric and NMR Study of the Ionic Liquid 1‐Alkyl‐3‐Methyl Imidazolium

The dynamic behaviors of ionic liquid samples consisting of a series of 1‐alkyl‐3‐methylimidazolium cations and various counteranionic species are investigated systematically over a wide frequency range from 1 MHz to 20 GHz at room temperature using dielectric relaxation (DR) and nuclear magnetic resonance (NMR) spectroscopies. DR spectra for the ionic liquids are reasonably deconvoluted into two or three relaxation modes. The slowest relaxation times are strongly dependent upon sample viscosity and cation size, whereas the relaxation times of other modes are almost independent of these factors. We attribute the two slower relaxation modes to the rotational relaxation modes of the dipolar cations because the correlation times of the cations evaluated using longitudinal relaxation time (T₁ ¹³C NMR) measurements corresponded to the dielectric relaxation times. On the other hand, the fastest relaxation mode is presumably related to the inter‐ion motions of ion‐pairs formed between cationic and anionic species. In the case of the ionic liquid bis(trifluoromethanesulfonyl)imide, the system shows marked dielectric relaxation behavior due to rotational motion of dipolar anionic species in addition to the relaxation modes attributed to the dipolar cations.     

NMR Structure Determination of Saccharose and Raffinose by Means of Homo‐ and Heteronuclear Dipolar Couplings

Residual dipolar couplings have dramatically improved the accuracy and precision of high‐resolution NMR structures during the last years. This was first demonstrated for proteins. In this article, we describe, with raffinose and saccharose as examples, that dipolar couplings improve the precision of structures of carbohydrates for which usually very few structural parameters are available. The relative orientation as well as the dynamics of the monosaccharide moieties with respect to each other can be determined with the help of ¹³C,¹H and ¹H,¹H dipolar couplings, which can easily be measured. Significant differences between the solution and the X‐ray crystal structure exist. These results indicate that residual dipolar‐coupling data may provide a more complete and dynamic model of carbohydrates in particular, and small molecules in general.     

Solid‐State NMR Correlation Experiments and Distance Measurements in Paramagnetic Metalorganics Exemplified by Cu‐Cyclam

We show how to record and analyze solid‐state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu‐cyclam complex as an example. Assignment of the ¹³C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the ¹H signals was done by means of ¹H–¹³C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct ¹H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such paramagnetic ¹H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording ¹H–¹H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the ¹H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu‐cyclam by means of ¹³C R₂ relaxation experiments combined with the electronic relaxation determined by EPR.     

Solid‐state 11B and 13C NMR,IR, and X‐ray crystallographic characterization of selected arylboronic acids and their catechol cyclic esters

Nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (boroxines) are investigated using ¹¹B solid‐state NMR spectroscopy at three different magnetic field strengths (9.4, 11.7, and 21.1 T). Through the analysis of spectra of static and magic‐angle spinning samples, the ¹¹B electric field gradient and chemical shift tensors are determined. The effects of relaxation anisotropy and nutation field strength on the ¹¹B NMR line shapes are investigated. Infrared spectroscopy was also used to help identify peaks in the NMR spectra as being due to the anhydride form in some of the arylboronic acid samples. Seven new X‐ray crystallographic structures are reported. Calculations of the ¹¹B NMR parameters are performed using cluster model and periodic gauge‐including projector‐augmented wave (GIPAW) density functional theory (DFT) approaches, and the results are compared with the experimental values. Carbon‐13 solid‐state NMR experiments and spectral simulations are applied to determine the chemical shifts of the ipso carbons of the samples. One bond indirect ¹³C‐¹¹B spin‐spin (J) coupling constants are also measured experimentally and compared with calculated values. The ¹¹B/¹⁰B isotope effect on the ¹³C chemical shift of the ipso carbons of arylboronic acids and their catechol esters, as well as residual dipolar coupling, is discussed. Overall, this combined X‐ray, NMR, IR, and computational study provides valuable new insights into the relationship between NMR parameters and the structure of boronic acids and esters. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of New 1,2,3‐Triazole Derivatives Possessing 4H‐Pyran‐4‐one Moiety by 1,3‐Dipolar Cycloaddition Reaction of Azidomethyl Phenylpyrone with Various Alkynes

A series of new 1,2,3‐triazole derivatives were synthesized by 1,3‐dipolar cycloaddition reaction of 2‐(4‐azidomethylphenyl)‐6‐phenyl‐4H‐pyran‐4‐one with different alkynes in 40–71% yields. In the case of terminal alkynes, the reaction was proceeded in the presence of Cu(I) catalyst. The structure of the synthesized compounds were confirmed by FTIR, ¹H‐NMR, and ¹³C‐NMR spectroscopy and elemental analysis.     

The structural and dynamic properties of 1‐bromodecane in urea inclusion compounds investigated by solid‐state 1H, 13C and 2H NMR spectroscopy

For asymmetric guest molecules in urea, the end‐groups of two adjacent guest molecules may arrange in three different ways: head–head, head–tail and tail–tail. Solid‐state ¹H and ¹³C NMR spectroscopy is used to study the structural properties of 1‐bromodecane in urea. It is found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1‐bromodecane in urea inclusion compounds are probed by variable‐temperature solid‐state ²H NMR spectroscopy (line shapes, spin–spin relaxation: T₂, spin‐lattice relaxation: T_1Z and T_1Q) between 120 K and room temperature. The comparison between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in a quantitative way using a non‐degenerate three‐site jump process in the low‐temperature phase and a degenerate three‐site jump in the high‐temperature phase, in combination with the small‐angle wobbling motion. The kinetic parameters can be derived from the simulation. Copyright © 2011 John Wiley & Sons, Ltd.     

Iodo cyclofunctionalization of 2,4‐dialkenyl‐1,3‐dicarbonyl compounds: Synthesis of substituted 3‐(2‐furylidene)‐2‐furanones

A facile method for the synthesis of substituted 3‐(2‐furylidene)‐2‐furanones has been developed using cyclofunctionalization reactions of 2,4‐dialkenyl‐1,3‐dicarbonyl compounds and iodine as electrophile in the presence of Na₂CO₃, in refluxing chloroform. Compounds 4 are obtained in modest to good yields and their structural identification was established by ¹H NMR, ¹H COSY, ¹³C NMR and ¹H‐¹³C COSY. A mechanism has been proposed to rationalize the formation of the ylidene furanone.     

Host–Guest Chemistry of a Water‐Soluble Pillar[5]arene: Evidence for an Ionic‐Exchange Recognition Process and Different Complexation Modes

The complexation of an anionic guest by a cationic water‐soluble pillararene is reported. Isothermal titration calorimetry (ITC), ¹H NMR, ¹H and ¹⁹F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and ¹H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.     

Studies on the phase structure of ethylene‐vinyl acetate copolymers by solid‐state 1H and 13C NMR

The phase structure of a series of ethylene‐vinyl acetate copolymers has been investigated by solid‐state wide‐line ¹H NMR and solid‐state high‐resolution ¹³C NMR spectroscopy. Not only the degree of crystallinity but the relative contents of the monoclinic and orthorhombic crystals within the crystalline region varied with the vinyl acetate (VA) content. Biexponential ¹³C NMR spin–lattice relaxation behavior was observed for the crystalline region of all samples. The component with longer ¹³C NMR spin–lattice relaxation time (T₁) was attributed to the internal part of the crystalline region, whereas the component with shorter ¹³C NMR T₁ to the mobile crystalline component was located between the noncrystalline region and the internal part of the crystalline region. The content of the mobile crystalline component relative to the internal part of the crystalline region increased with the VA content, showing that the ¹³C NMR spin–lattice relaxation behavior is closely related to the crystalline structure of the copolymers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2199–2207, 2002     

1H‐19F REDOR‐filtered NMR spin diffusion measurements of domain size in heterogeneous polymers

Solid state NMR spectroscopy is inherently sensitive to chemical structure and composition and thus makes an ideal method to probe the heterogeneity of multicomponent polymers. Specifically, NMR spin diffusion experiments can be used to extract reliable information about spatial domain sizes on multiple length scales, provided that magnetization selection of one domain can be achieved. In this paper, we demonstrate the preferential filtering of protons in fluorinated domains during NMR spin diffusion experiments using ¹H‐¹⁹F heteronuclear dipolar dephasing based on rotational echo double resonance (REDOR) MAS NMR techniques. Three pulse sequence variations are demonstrated based on the different nuclei detected: direct ¹H detection, plus both ¹H?¹³C cross polarization and ¹H?¹⁹F cross polarization detection schemes. This ¹H‐¹⁹F REDOR‐filtered spin diffusion method was used to measure fluorinated domain sizes for a complex polymer blend. The efficacy of the REDOR‐based spin filter does not rely on spin relaxation behavior or chemical shift differences and thus is applicable for performing NMR spin diffusion experiments in samples where traditional magnetization filters may prove unsuccessful. This REDOR‐filtered NMR spin diffusion method can also be extended to other samples where a heteronuclear spin pair exists that is unique to the domain of interest.     

Heteronuclear dipolar decoupling in liquid‐crystal NMR using supercycled SWf‐TPPM sequences

Heteronuclear dipolar decoupling is an essential requirement for extracting structural information from the ¹³C NMR spectra of liquid crystals. Efficient schemes for heteronuclear dipolar decoupling in such systems are formulated here by supercycling SW_f‐TPPM, a sequence introduced recently for this purpose in rotating solids. These sequences are compared with two other commonly used decoupling schemes in liquid‐crystal NMR, SPINAL‐64 and SW_f‐TPPM, by analyzing the intensities of various resonances in the proton decoupled ¹³C spectrum of the liquid‐crystal 4‐n‐pentyl‐4′‐cyanobiphenyl (5CB). The effectiveness of the decoupling programs with respect to experimental parameters such as RF field strength, decoupler offset frequency and phase angle is also presented. Copyright © 2010 John Wiley & Sons, Ltd.     

“Direct” 13C Hyperpolarization of 13C‐Acetate by MicroTesla NMR Signal Amplification by Reversible Exchange (SABRE)

Herein, we demonstrate “direct” ¹³C hyperpolarization of ¹³C‐acetate via signal amplification by reversible exchange (SABRE). The standard SABRE homogeneous catalyst [Ir‐IMes; [IrCl(COD)(IMes)], (IMes=1,3‐bis(2,4,6‐trimethylphenyl), imidazole‐2‐ylidene; COD=cyclooctadiene)] was first activated in the presence of an auxiliary substrate (pyridine) in alcohol. Following addition of sodium 1‐¹³C‐acetate, parahydrogen bubbling within a microtesla magnetic field (i.e. under conditions of SABRE in shield enables alignment transfer to heteronuclei, SABRE‐SHEATH) resulted in positive enhancements of up to ≈100‐fold in the ¹³C NMR signal compared to thermal equilibrium at 9.4 T. The present results are consistent with a mechanism of “direct” transfer of spin order from parahydrogen to ¹³C spins of acetate weakly bound to the catalyst, under conditions of fast exchange with respect to the ¹³C acetate resonance, but we find that relaxation dynamics at microtesla fields alter the optimal matching from the traditional SABRE‐SHEATH picture. Further development of this approach could lead to new ways to rapidly, cheaply, and simply hyperpolarize a broad range of substrates (e.g. metabolites with carboxyl groups) for various applications, including biomedical NMR and MRI of cellular and in vivo metabolism.     

Dipolar 1,3‐cycloaddition of arylnitriloxides on 1,2‐dihydroisoquinolines in a two‐phase medium

The 1,3‐dipolar cycloaddition of arylnitriloxides on 1,2‐dihydroisoquinoline derivatives led to new 3‐aryl, 3a‐8,9,9a‐tetrahydro[5,4‐c]‐isoxazoloisoquinoline adducts. The regioselectivity of the cycloaddition reactions is discussed on the basis of ¹H and ¹³C NMR data.     

Rotational Dynamics of Adamantanecarboxylic Acid in Complex with β-cyclodextrin

The reorientation of 1-adamantanecarboxylic acid (AdCA) within the β-cyclodextrin (β-CD) cavity is investigated by means of multiple-field ¹³C NMR relaxation. The dissociation constant describing the complexation equilibrium is determined using translational diffusion measurements for the guest during a titration by the host in D₂O/DMSO solvent mixture. The changes in apparent diffusion properties of AdCA during the titration are at 25 °C well described assuming the formation of a 1:1 complex, whereas at 0 °C the data indicate the presence of a 2:1 (guest:host) complex. The ¹³C NMR relaxation parameters for the AdCA molecule bound inside the β-CD cavity are extracted. Despite the high association constant, indicating a strong interaction between the two molecules, the guest molecule is quite mobile. The reorientation of the bound AdCA at 25 °C can be described by either the Lipari–Szabo or the axially symmetric rotational diffusion model. The motion is extremely anisotropic: the adamantyl group rotates fast around the β-CD symmetry axis, inside its cylindrical cavity. At lower temperature, the relaxation properties are no longer possible to explain using these models. Instead, the data are analyzed using extended, three-step spectral density of Clore et al. [J. Am. Chem. Soc. 112, 4989 (1990)].     

A Red‐Emissive Sextuple Hydrogen‐Bonding Self‐Assembly Molecular Duplex Bearing Perylene Diimide Fluorophores for Warm‐White Organic Light‐Emitting Diode Application

A novel sextuple hydrogen‐bonding (HB) self‐assembly molecular duplex bearing red‐emitting perylene diimide (PDI) fluorophores, namely PDIHB , was synthesized, and its molecular structure was confirmed by ¹H NMR, ¹³C NMR, TOF‐MS and 2D NMR. Compared with the small molecular reference compound PDI , PDIHB shows one time enhanced fluorescence efficiency in solid state (4.1% vs. 2.1%). More importantly, the presence of bulky HB oligoamide strands in PDIHB could trigger effective spatial separation between guest and host fluorophores in thin solid film state, hence inefficient energy transfer occurs between the blue‐emitting host 2TPhNIHB and red guest PDIHB in the 2 wt% guest/host blending film. As a result, a solution‐processed organic light‐emitting diode (OLED) with quite simple device structure of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/ PDIHB (2 wt%): 2TPhNIHB (50 nm)/LiF (0.8 nm)/Al (100 nm) could emit bias‐independent warm‐white electroluminescence with stable Commission Internationale de L'Eclairage coordinates of (0.42, 0.33), and the maximum brightness and current efficiency of this device are 260 cd·m^?2 and 0.49 cd·A^?1, respectively. All these results indicated that HB self‐assembly supramolecular fluorophores could act as prospective materials for white OLED application.     

One‐Pot Synthesis of Zeolitic Strong Solid Bases: A Family of Alkaline‐Earth Metal‐Containing Silicalite‐1

Fabricating stable strong basic sites in well‐preserved crystallized zeolitic frameworks still remains a difficult issue. Here, we reported a family of MFI‐type metallosilicate zeolites, AeS‐1 (Ae: alkaline‐earth metal ions of Mg, Ca, Sr or Ba; S‐1: silicalite‐1) through a direct one‐pot hydrothermal method involving the acidic co‐hydrolysis/condensation of the silica precursor with the Ae salts. Step‐by‐step full characterizations were designed and conducted for in‐depth discussion of the Ae status in AeS‐1. Strong basicity (H_≈22.5–26.5) was detected in AeS‐1. The basicity was further confirmed by CO₂ sorption measurements, ¹³C NMR spectra of chloroform‐adsorbed samples, and ¹H→¹³C and ¹H→²⁹Si cross‐polarization magic‐angle spinning NMR spectra of ethyl cyanoacetate‐adsorbed samples. The results of Knoevenagel condensations demonstrated the excellent solid base catalysis of AeS‐1, which showed high activity, reusability, and shape‐selectivity, all of which are explained by Ae‐derived zeolitic intracrystalline strong basic sites.