Measurement of carbon-proton dipolar couplings in liquid crystals using DAPT

Dipolar couplings provide valuable information on order and dynamics in liquid crystals. For measuring heteronuclear dipolar couplings in oriented systems, a new separated local field experiment is presented here. The method is based on the dipolar assisted polarization transfer (DAPT) pulse sequence proposed recently (Chem. Phys. Lett. 2007, 439, 407) for transfer of polarization between two spins I and S. DAPT utilizes the evolution of magnetization of the I and S spins under two blocks of phase shifted BLEW-12 pulses on the I spin separated by a 90 degree pulse on the S spin. Compared to the rotating frame techniques based on Hartmann-Hahn match, this approach is easy to implement and is independent of any matching conditions. DAPT can be utilized either as a proton encoded local field (PELF) technique or as a separated local field (SLF) technique, which means that the heteronuclear dipolar coupling can be obtained by following either the evolution of the abundant spin like proton (PELF) or that of the rare spin such as carbon (SLF). We have demonstrated the use of DAPT both as a PELF and as a SLF technique on an oriented liquid crystalline sample at room temperature and also have compared its performance with PISEMA. We have also incorporated modifications to the original DAPT pulse sequence for (i) improving its sensitivity and (ii) removing carrier offset dependence.     

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.     

Using switched angle spinning to simplify NMR spectra of strongly oriented samples

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.     

Orientational ordering of 4-substituted phenylcyclohexanes studied by carbon-13 two-dimensional N.M.R.

An N.M.R. method combining the techniques of separated local field spectroscopy (SLF) and variable angle spinning (VAS) is valuable in the investigtion of nematic liquid crystals. Rapid sample spinning causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 N.M.R. spectrum. SLF is a two-dimensional N.M.R. method which produces a first order splitting pattern for each carbon signal from which C-H dipolar coupling constants can be determined. The order parameters for all segments of the liquid crystal molecule can then be calculated. Results for three 4'-cyanophenylcyclohexanes are considered here. These compounds are trans-substituted at the 4 position of cyclohexane ring with n-pentane (PCH5), 1-pentene (3d₁CP) and 3-pentene (1d₃CP), respectively.     

Solid state separated-local-field NMR spectroscopy on half-integer quadrupolar nuclei: principles and applications to borane analysis

New multidimensional NMR methods correlating the quadrupolar and heteronuclear dipolar interactions affecting a half-integer quadrupolar spin in the solid state are introduced and exemplified. The methods extend separated-local-field magic-angle spinning (SLF MAS) NMR techniques that have been used successfully in spin-(1)/(2) spectroscopy to the study of S >/= (3)/(2) nuclei. In our implementation, these techniques avoid homonuclear proton decoupling requirements by relying on moderately fast MAS rates (6-15 kHz) and use rotor-synchronized constant-time pulse sequences to achieve nearly arbitrary amplifications of the apparent dipolar coupling strengths. The result is a suite of simple 2D NMR experiments, whose line shapes carry valuable information about the structure and dynamics of solids containing quadrupolar and proton nuclei. The potential of these sequences was exploited to gather new insight into the structure and dynamics of a variety of boron-containing samples. These experimental SLF schemes were also extended to 3D NMR experiments that incorporate multiple-quantum MAS, thus enabling the resolution needed to study multiple chemical sites in a solid and providing a useful tool for the assignment of inequivalent sites.     

Proton evolved local field solid-state nuclear magnetic resonance using Hadamard encoding: theory and application to membrane proteins

NMR anisotropic parameters such as dipolar couplings and chemical shifts are central to structure and orientation determination of aligned membrane proteins and liquid crystals. Among the separated local field experiments, the proton evolved local field (PELF) scheme is particularly suitable to measure dynamically averaged dipolar couplings and give information on local molecular motions. However, the PELF experiment requires the acquisition of several 2D datasets at different mixing times to optimize the sensitivity for the complete range of dipolar couplings of the resonances in the spectrum. Here, we propose a new PELF experiment that takes the advantage of the Hadamard encoding (HE) to obtain higher sensitivity for a broad range of dipolar couplings using a single 2D experiment. The HE scheme is obtained by selecting the spin operators with phase switching of hard pulses. This approach enables one to detect four spin operators, simultaneously, which can be processed into two 2D spectra covering a broader range of dipolar couplings. The advantages of the new approach are illustrated for a U-(15)N NAL single crystal and the U-(15)N labeled single-pass membrane protein sarcolipin reconstituted in oriented lipid bicelles. The HE-PELF scheme can be implemented in other multidimensional experiments to speed up the characterization of the structure and dynamics of oriented membrane proteins and liquid crystalline samples.     

Orientational ordering of 4-substituted phenylcyclohexanes studied by carbon-13 two-dimensional N.M.R.

Abstract

An N.M.R. method combining the techniques of separated local field spectroscopy (SLF) and variable angle spinning (VAS) is valuable in the investigtion of nematic liquid crystals. Rapid sample spinning causes the nematic director to align along the spinning axis, resulting in narrow peaks in the C-13 N.M.R. spectrum. SLF is a two-dimensional N.M.R. method which produces a first order splitting pattern for each carbon signal from which C–H dipolar coupling constants can be determined. The order parameters for all segments of the liquid crystal molecule can then be calculated. Results for three 4′-cyanophenylcyclohexanes are considered here. These compounds are trans-substituted at the 4 position of cyclohexane ring with n-pentane (PCH5), 1-pentene (3d ₁CP) and 3-pentene (1d ₃CP), respectively.     

Spin dynamics in the modulation frame: application to homonuclear recoupling in magic angle spinning solid-state NMR

We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms-cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) (13)C recoupling using >70 kHz omega(1,C)/2pi rf field for the spinning frequency omega(r)/2=10-30 kHz and (1)H Larmor frequency omega(0,H)/2pi up to 900 MHz. Importantly, for p>or=5, CMpRR recouples efficiently in the absence of (1)H decoupling. Extension to lower p values (3.5相似文献   

13C NMR investigations of molecular order of rod-like,bent-core,and thiophene mesogens

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one- and two-dimensional (2D) solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy are described. Besides ¹³C chemical shifts, the ¹³C─¹H dipolar couplings measured from 2D-separated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the ¹³C─¹H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B₁ and B₂ mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state ¹³C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.     

Orientational ordering of a bent-core mesogen by two-dimensional 13C NMR spectroscopy

Various two-dimensional (2D) NMR techniques are reported on a bent-core mesogen 4,6-dichloro-1,3-phenylenebis[4'-(9-decenyloxy)-1,1'-biphenyl] carboxylate in its nematic and solid phases in order to unambiguously assign its carbon-13 NMR spectrum. The (13)C chemical shifts from the molecular core were studied as a function of temperature to extract its molecular geometry and orientational order tensor. To this end, the chemical shift anisotropy tensors of some carbon sites were measured in the solid state of this mesogen using a recent method called the separation of undistorted powder patterns by effortless recoupling (SUPER). The average bending angle subtended by the two arms of the bent-core structure is determined to be 148.7 degrees. The C-H dipolar couplings obtained from the separated local field (SLF) experiment for the aromatic rings are used to find the local order parameter tensors.     

Orientational ordering in the nematic phase of 4-alkenyl-substituted bicyclohexylnitriles

The orientational ordering of a series of 4-alkenyl-substituted bicyclohexyl liquid crystals was studied by natural abundance ¹³C NMR spectroscopy. A combination of the methods of variable angle spinning (VAS) and separated local field spectroscopy (SLF) was used. Rapid sample spinning about an axis forming an angle of about 65° with respect to the magnetic field forces the nematic director to align parallel to the spinnin axis, leading to narrow peaks in the ¹³C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled ¹³C signals in the ω₂ dimension and first-order C-H splitting patterns in the ω₁ dimension, from which the C-H dipolar coupling constants for individual pairs of nuclei can be obtained. Then, the order parameters for different molecular segments can be calculated. The method was applied to five different 4-alkenyl-substituted cyanobicyclohexanes. For the compounds studied, the dependence of the ring order parameters on the alkenyl chains shows the same trend as the melting and clearing temperatures and the elastic constants.     

Orientational ordering of ferroelectric thiobenzoate liquid crystals

The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance ¹³C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle of c. 45° with respect to the magnetic field, the smectic director aligns parallel to the spinning axis, leading to narrow peaks in the ¹³C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled ¹³C signals in the ω2 dimension and first-order Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C-H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order parameters was observed.     

Numerical and experimental studies of long-range magnetic dipolar interactions

We describe several numerical methods developed to analyze the behavior of spin polarized liquids in the presence of long-range magnetic dipolar interactions and external field gradients. Two of the methods use a discrete lattice of spins. In the first we calculate the magnetic field from the lattice of spins directly, either in the rotating frame, or in the lab frame. In the second method we include the dipolar fields from linear magnetization gradients analytically and calculate the dipolar fields from higher order gradients in Fourier space, where they are a local function of the magnetization. In the third method the magnetization is expanded in a Taylor series and the dipolar fields are calculated analytically for each term. The results of these calculations are compared to experimental data, in which we use two superconducting quantum interference device magnetometers adjacent to a spherical sample of hyperpolarized liquid 129Xe to detect the evolution of magnetization gradients. In particular, we observe an increase by a factor of 100 of the spin dephasing time in a longitudinal magnetic field gradient due to dipolar interactions of the spins. While each of the numerical techniques has certain limitations, they are generally in agreement with each other and with experimental data.     

1H and 19F ultra-fast MAS double-quantum single-quantum NMR correlation experiments using three-spin terms of the dipolar homonuclear Hamiltonian

Measuring internuclear distances through dipolar interaction is a major challenge for solid-state nuclear magnetic resonance (NMR) spectroscopy. Obtaining reliable interatomic distances provides an access to the local structure in ordered or disordered solids. We show that at magic angle spinning (MAS) frequencies larger than ca. 50 kHz, some of the three-spin terms of the homogeneous homonuclear dipolar Hamiltonian can be used to promote the creation of double-quantum coherences between neighbouring (1)H or (19)F spins without using dipolar recoupling pulse sequences in the Dipolar Homonuclear Homogeneous Hamiltonian (DH(3)) double-quantum/single-quantum correlation experiment. This makes it possible to probe inter-nuclear spatial proximity with limited risk of probe or sample damage from radio-frequency (RF) irradiation, and is fully appropriate for fast repetition rate offering sensitivity gains in favourable cases. Experimental demonstrations are supported by multi-spin numerical simulations, which points to new possibilities for the characterization of spin-system geometries.     

An Efficient,Robust New Scheme for Establishing Broadband Homonuclear Correlations in Biomolecular Solid State NMR

An efficient mixing scheme is introduced for establishing two-dimensional (2D) homonuclear correlations based on dipolar couplings. This mixing scheme achieves broadband dipolar recoupling using remarkably low powers even under ultrafast magic-angle spinning (MAS) rates. This Adiabatic Linearly FREquency Swept reCOupling (AL FRESCO) method applies a series of weak frequency-chirped pluses on the ¹H channel, for performing efficient ¹³C−¹³C magnetization transfers leading to cross peaks between sites separated over small or large chemical shift differences. The mixing scheme is nearly free from dipolar truncation effects, and thanks to the low RF powers it involves it can act over long mixing times (≥1.5 sec). Key considerations required for optimizing this chirped pulse mixing scheme are discussed, and the new kind of correlations that can emerge from this method are demonstrated using uniformly ¹³C-labeled Barstar as test protein sample.     

A capacitively coupled temperature‐jump arrangement for high‐resolution biomolecular NMR

A simple design for performing rapid temperature jumps within a high‐resolution nuclear magnetic resonance (NMR) setting is presented and exemplified. The design is based on mounting, around a conventional NMR glass tube, an inductive radiofrequency (RF) irradiation coil that is suitably tuned by a resonant circuit and is driven by one of the NMR's console high‐power RF amplifiers. The electric fields generated by this coil's thin metal strips can lead to a fast and efficient heating of the sample, amounting to temperature jumps of ≈ 20 °C in well within a second—particularly in the presence of lossy dielectric media like those provided by physiological buffers. Moreover, when wound around a 4‐mm NMR tube, the resulting device fits a conventional 5‐mm inverse probe and is wholly compatible with the field homogeneities and sensitivities expected for high‐resolution biomolecular NMR conditions. The performance characteristics of this new system were tested using saline solutions, as well as on a lyotropic liquid crystal capable of undergoing nematic → isotropic transitions in the neighborhood of ambient temperature. These settings were then incorporated into the performance of a new kind of single‐scan 2D NMR spectroscopy acquisition, correlating the anisotropic and isotropic patterns elicited by solutes dissolved in such liquid‐crystalline systems, before and after a sudden temperature jump occurring during an intervening mixing period. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of 13C chemical shift anisotropy tensors and molecular order of 4-hexyloxybenzoic acid

4-Alkoxy benzoic acids belong to an important class of thermotropic liquid crystals that are structurally simple and often used as starting materials for many novel mesogens. 4-Hexyloxybenzoic acid (HBA) is a homologue of the same series and exhibits an enantiotropic nematic phase. As this molecule could serve as an ideal model compound, high resolution (13)C NMR studies of HBA in solution, solid, and liquid crystalline phases have been undertaken. In the solid state, two-dimensional separation of undistorted powder patterns by effortless recoupling (2D SUPER) experiments have been carried out to estimate the magnitude of the components of the chemical shift anisotropy (CSA) tensor of all the aromatic carbons. These values have been used subsequently for calculating the orientational order parameters in the liquid crystalline phase. The CSA values computed by density functional theory (DFT) calculations showed good agreement with the 2D SUPER values. Additionally, (13)C-(1)H dipolar couplings in the nematic phase have been determined by separated local field (SLF) spectroscopy at various temperatures and were used for computing the order parameters, which compared well with those calculated by using the chemical shifts. It is anticipated that the CSA values determined for HBA would be useful for the assignment of carbon chemical shifts and for the study of order and dynamics of structurally similar novel mesogens in their nematic phases.     

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

Phase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003     

A supramolecular and liquid crystalline water-based alignment medium based on azobenzene-substituted 1,3,5-benzenetricarboxamides

A supramolecular, lyotropic liquid crystalline alignment medium based on an azobenzene-containing 1,3,5-benzenetricarboxamide (BTA) building block is described and investigated. As we demonstrate, this water-based system is suitable for the investigation of various water-soluble analytes and allows for a scaling of alignment strength through variation of temperature. Additionally, alignment is shown to reversibly collapse above a certain temperature, yielding an isotropic solution. This collapse allows for isotropic reference measurements, which are typically needed in addition to those in an anisotropic environment, to be performed using the same sample just by varying the temperature. The medium described thus provides easy access to anisotropic NMR observables and simplifies structure elucidation techniques based thereon.     

Revisiting the mean-field picture of dipolar effects in solution NMR

For more than three decades, the classical or mean-field picture describing the distant dipolar field has been almost always simplified to an effective field proportional to the local longitudinal magnetization, differing only by a scale factor of 1.5 for homomolecular (identical resonance frequency) and heteromolecular interactions. We re-examine the underlying assumptions, and show both theoretically and experimentally that the mathematical framework needs to be modified for modern applications such as imaging. We demonstrate new pulse sequences which produce unexpected effects; for example, modulating an arbitrarily small fraction of the magnetization can substantially alter the frequency evolution. Thus, matched gradient pulse pairs (a seemingly innocuous module in thousands of existing pulse sequences) can alter the time evolution in highly unexpected ways, particularly with small flip angle pulses such as those used in hyperpolarized experiments. We also show that specific gradient pulse combinations can retain only dipolar interactions between unlike spins, and the dipolar field can generate a secular Hamiltonian proportional to I(x).