Hartmann-Hahn Mismatch Research Articles

Composite-pulse and partially dipolar dephased multiCP for improved quantitative solid-state 13C NMR

Improved multiple cross polarization (multiCP) pulse sequences for quickly acquiring quantitative 13C NMR spectra of organic solids are presented. Loss of 13C magnetization due to imperfect read-out and storage pulses in multiCP has been identified as a significant mechanism limiting polarization enhancement for 13C sites with weak couplings to 1H. This problem can be greatly reduced by composite 90° pulses with non-orthogonal phases that flip the magnetization onto the spin-lock field and back to the longitudinal direction for the 1H repolarization period; the observed loss is <3% for over ±10 kHz resonance offset and up to 20% flip-angle error. This composite-pulse multiCP (ComPmultiCP) sequence consistently provides performance superior to that of conventional multiCP, without any trade-off. The longer total CP time enabled by the composite pulses allows for a wider amplitude ramp during CP, which decreases the sensitivity to Hartmann-Hahn mismatch by a factor of two, with a <7% root-mean-square deviation within a 1-dB range for Boc-alanine. In samples with very short T1ρ, under-polarization of non-protonated carbons can be compensated by slight dipolar dephasing of CHn signals resulting from relatively weak decoupling during the Hahn spin echo period before detection. Quantitative spectra have been obtained by ComPmultiCP for low-crystallinity branched polyethylene at 4.5 kHz MAS, and in combination with partial dipolar dephasing for soil organic matter at 14 kHz MAS.

A cross-polarization based rotating-frame separated-local-field NMR experiment under ultrafast MAS conditions

Rotating-frame separated-local-field solid-state NMR experiments measure highly resolved heteronuclear dipolar couplings which, in turn, provide valuable interatomic distances for structural and dynamic studies of molecules in the solid-state. Though many different rotating-frame SLF sequences have been put forth, recent advances in ultrafast MAS technology have considerably simplified pulse sequence requirements due to the suppression of proton–proton dipolar interactions. In this study we revisit a simple two-dimensional 1H–13C dipolar coupling/chemical shift correlation experiment using 13C detected cross-polarization with a variable contact time (CPVC) and systematically study the conditions for its optimal performance at 60kHz MAS. In addition, we demonstrate the feasibility of a proton-detected version of the CPVC experiment. The theoretical analysis of the CPVC pulse sequence under different Hartmann–Hahn matching conditions confirms that it performs optimally under the ZQ (w1H−w1C=±wr) condition for polarization transfer. The limits of the cross polarization process are explored and precisely defined as a function of offset and Hartmann–Hahn mismatch via spin dynamics simulation and experiments on a powder sample of uniformly 13C-labeled L-isoleucine. Our results show that the performance of the CPVC sequence and subsequent determination of 1H–13C dipolar couplings are insensitive to 1H/13C frequency offset frequency when high RF fields are used on both RF channels. Conversely, the CPVC sequence is quite sensitive to the Hartmann–Hahn mismatch, particularly for systems with weak heteronuclear dipolar couplings. We demonstrate the use of the CPVC based SLF experiment as a tool to identify different carbon groups, and hope to motivate the exploration of more sophisticated 1H detected avenues for ultrafast MAS.

COMPOZER-based longitudinal cross-polarization via dipolar coupling under MAS

We propose a cross polarization (CP) sequence effective under magic-angle spinning (MAS) which is tolerant to RF field inhomogeneity and Hartmann–Hahn mismatch. Its key feature is that spin locking is not used, as CP occurs among the longitudinal (Z) magnetizations modulated by the combination of two pulses with the opposite phases. We show that, by changing the phases of the pulse pairs synchronized with MAS, the flip–flop term of the dipolar interaction is restored under MAS.

Volume localized shift selective13C spectroscopy using pulsed rotating frame transfer sequences with windows (PRAWN)

Some novel techniques for volume localized, chemical shift selective (13) C spectroscopy are described in this work. These techniques are based on rotating frame J cross polarization and are reported for both direct and indirect modes of (13) C detection. The performance of two selective mixing sequences, viz., pulsed rotating frame transfer sequences with windows (PRAWN) and PRAWN-π has been studied systematically with different liquid and gel phantoms. Two different front-end modules are used for volume localization, viz., point resolved spectroscopy (PRESS) and localized distortionless enhancement by polarization transfer (LODEPT). It is shown experimentally that both the selective J cross polarization sequences can operate efficiently with very low radiofrequency duty cycle; further, they have considerable tolerance to Hartmann-Hahn mismatch. A simple theoretical analysis is also presented to understand J cross-polarization dynamics at low RF field amplitudes. Finally, the performance of LODEPT-PRAWN-π is demonstrated for the selective detection of saturated fat in pigeon egg in indirect detection mode.

2 n -SEMA -a robust solid state nuclear magnetic resonance experiment for measuring heteronuclear dipolar couplings in static oriented systems using effective transverse spin-lock

Separated local field (SLF) spectroscopy is a powerful technique to measure heteronuclear dipolar couplings. The method provides site-specific dipolar couplings for oriented samples such as membrane proteins oriented in lipid bilayers and liquid crystals. A majority of the SLF techniques utilize the well-known Polarization Inversion Spin Exchange at Magic Angle (PISEMA) pulse scheme which employs spin exchange at the magic angle under Hartmann-Hahn match. Though PISEMA provides a relatively large scaling factor for the heteronuclear dipolar coupling and a better resolution along the dipolar dimension, it has a few shortcomings. One of the major problems with PISEMA is that the sequence is very much sensitive to proton carrier offset and the measured dipolar coupling changes dramatically with the change in the carrier frequency. The study presented here focuses on modified PISEMA sequences which are relatively insensitive to proton offsets over a large range. In the proposed sequences, the proton magnetization is cycled through two quadrants while the effective field is cycled through either two or four quadrants. The modified sequences have been named as 2(n)-SEMA where n represents the number of quadrants the effective field is cycled through. Experiments carried out on a liquid crystal and a single crystal of a model peptide demonstrate the usefulness of the modified sequences. A systematic study under various offsets and Hartmann-Hahn mismatch conditions has been carried out and the performance is compared with PISEMA under similar conditions.

Efficient cross-polarization using a composite 0° pulse for NMR studies on static solids

In most solid-state NMR experiments, cross-polarization is an essential step to detect low-γ nuclei such as 13C and 15N. In this study, we present a new cross-polarization scheme using spin-locks composed of composite 0° pulses in the RF channels of high-γ and low-γ nuclei to establish the Hartmann–Hahn match. The composite 0° pulses with no net nutation-angle{(2 π) X − (2 π) − X − (2 π) Y − (2 π) − Y −} n applied simultaneously to both high-γ ( I) and low-γ ( S) nuclei create an effective heteronuclear dipolar Hamiltonian H d ( 0 ) = d 2 ( 2 I Z S Z + I X S X + I Y S Y ) , which is capable of transferring the Z-component of the I spin magnetization to the Z-component of the S spin magnetization. It also retains a homonuclear dipolar coupling Hamiltonian that enables the flip–flop transfer among abundant spins. While our experimental results indicate that the new pulse sequence, called composite zero cross- polarization (COMPOZER-CP) performs well on adamantane, it is expected to be more valuable to study semi-solids like liquid crystalline materials and model lipid membranes. Theoretical analysis of COMPOZER-CP is presented along with experimental results. Our experimental results demonstrate that COMPOZER-CP overcomes the RF field inhomogeneity and Hartmann–Hahn mismatch for static solids. Experimental results comparing the performance of COMPOZER-CP with that of the traditional constant-amplitude CP and rampCP sequences are also presented in this paper.

Efficient cross polarization with simultaneous adiabatic frequency sweep on the source and target channels

In this work, we propose a new and efficient heteronuclear cross polarization scheme, in which adiabatic frequency sweeps from far off-resonance toward on-resonance are applied simultaneously on both the source and target spins. This technique, which we call as Simultaneous ADIabatic Spin-locking Cross Polarization (SADIS CP), is capable of efficiently locking both the source and target spins with moderate power even in the presence of large spectral distribution and fast relaxation. It is shown that by keeping the time-dependent Hartmann–Hahn mismatch minimal throughout the mixing period, polarization transfer can be accelerated. Experiments are demonstrated in a powder sample of l-alanine.

Selective cross-polarization in solution state nuclear magnetic resonance of scalar coupled spin 12 and quadrupolar nuclei

Semiselective heteronuclear cross-polarization for achieving coherence transfer between a spin I=12 scalar coupled to a spin S⩾12 in isotropic solution is considered. The expansion of the density operator as products of irreducible tensor operators provides a compact formalism for describing cross-polarization involving scalar coupled quadrupolar nuclei. An analytical description of cross-polarization is presented in the limit of strong radio-frequency (rf) fields, with respect to the scalar-coupling constant. Numerical simulations show that reducing the rf field amplitudes does not have a detrimental effect on the efficiency of the transfer provided they are comparable to or greater than the scalar-coupling constant. The use of weak rf fields largely circumvents the reduced efficacy due to Hartmann–Hahn mismatch. Applications of the method for selective observation of scalar-coupled quadrupolar nuclei are considered and experimental results are presented for a mixture of beryllium fluoride complexes.

The effect of Hartmann–Hahn mismatching on polarization inversion spin exchange at the magic angle

The effect of the Hartmann–Hahn mismatch Δ= ω eff− ω 1S during polarization inversion spin exchange at the magic angle (PISEMA) has been investigated, where ω eff and ω 1S represent the amplitudes of the 1H effective spin-locking field at the magic angle and the 15N RF spin-locking field, respectively. During the PISEMA evolution period, the exact Hartmann–Hahn match condition (i.e., Δ=0) yields a maximum dipolar scaling factor of 0.816 for PISEMA experiments, while any mismatch results in two different effective fields for the first and second half of each frequency switched Lee–Goldburg (FSLG) cycle. The mismatch effect on the scaling factor depends strongly on the transition angle from one effective field to the other within each FSLG cycle as well as on the cycle time. At low RF spin-lock amplitudes in which the FSLG cycle time is relatively long, the scaling factor rapidly becomes smaller as ω 1S becomes greater than ω eff. On the other hand, when ω 1S< ω eff, there is relatively little effect on the scaling factor with variation in Δ. As a result, the presence of RF inhomogeneities may significantly broaden the line-width in the dipolar dimension because of the mismatch effect. Higher RF spin-lock amplitudes result in a relatively small variation for the scaling factor. Furthermore, ramped amplitude of the 15N RF spin-lock field in synchronization with the flip-flop of the FSLG sequence minimizes the transition angle between the two effective fields within the FSLG cycle. It is shown experimentally that such a ramped amplitude not only gives rise to the same scaling factor but also results in a narrower dipolar line-width in comparison with the rectangular amplitude.

1H Detected 13C Echo Planar Imaging

Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of 13C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected 13C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of 13C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B1 field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant (13C/1H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64×64 axial field maps to be generated in about a minute.

PRAWN: Mixing Sequences for Selective HeteronuclearJCross Polarization

In this work, we present a family of pulse sequences for selective heteronuclearJcross-polarization (JCP), which we have developed especially for indirect13C imaging usingJCP, for example in the CYCLCROP environment. The sequences are straightforward to implement and operate reliably. Results of an average Hamiltonian analysis are given for the basic sequence, which we term PRAWN (pulsed rotating frame transfer sequence with windows). It is shown experimentally that the pulse sequence, which operates efficiently with low RF duty cycles down to a few percent, has a useful tolerance range to absolute Hartmann–Hahn mismatch and generates coherence transfer spectra in close correspondence with theJCP average Hamiltonian. Computer simulation of the performance of the basic sequence on a heteronuclear spin-12 AX system is also presented. The mismatch compensation of PRAWN may be markedly enhanced further by issuing a π pulse to each spin halfway through the basic PRAWN train and in phase quadrature to it. A simple analysis of this modified sequence, PRAWN-π, is given under conditions of mismatch and off-resonance irradiation.

Spinning-Sideband-Pattern Deviations in Cross-Polarization MAS NMR Spectra

Cross-polarization (CP) MAS spectra show deviations in the intensity distribution of the sideband pattern in comparison with experiments done without CP. Therefore, the evaluation of the principal values of the CSA tensor by use of the method of Herzfeld and Berger leads to a systematic error. The reason for the sideband-intensity deviations by using CP is the anisotropic magnetization of the S spins caused by the Hartmann-Hahn mismatch anisotropy, as well as the anisotropic CP rate, and the anisotropic relaxation rate of the I spins in the rotating frame. A proposed multiple-contact pulse sequence partially diminishes the sideband-pattern deviations. A further reduction of the deviations by a simple empirical correction function is possible. The combination of these two methods is appropriate to reduce errors in the CSA-tensor principal values evaluated from the sideband pattern of CP MAS spectra.

Heteronuclear cross polarization in liquid-state nuclear magnetic resonance: Mismatch compensation and relaxation behavior

Nuclear spin dynamics in liquid-state heteronuclear cross-polarization experiments were explored by experiments on di-15N -labeled histidine in DMSO-d6. Compensation for Hartmann–Hahn mismatch was achieved by simultaneously shifting the phases of both spin-locking fields by π at intervals short compared to the spin–spin coupling. With rf compensation, slightly more 15N polarization was achieved than with the conventional selective INEPT scheme. This was attributed to relaxation by slowly fluctuating random fields associated with proton exchange. Theory for the rf mismatch compensation and relaxation behavior of the Hartmann–Hahn method is presented for the case of a heteronuclear 2-spin system.

Effect of sample spinning on detection of carbon-13 NMR in coals

The effects of sample spinning and of Hartmann-Hahn mismatch on detection of 13 C for a Pittsburgh no 8 coal and coronene (as a model compound) by CP/MAS NMR have been studied. The results on the coal were compared with the «true» 13 C intensities under a Bloch decay, indicating that reasonable 13 C intensities may be obtained by using cross-polarization and variable contact time. It is suggested that when CP/MAS is used to obtain NMR of 13 C in coals, the most reliable results are obtained at low enough static fields such that at moderate spinning speeds production of sidebands and the interference of spinning with the cross-polarization process are avoided

Heteronuclear isotropic mixing in liquids

We report on a new experimental NMR technique for heteronuclear polarization transfer in the rotating frame which utilizes a WALTZ-16 decoupling sequence applied synchronously to both nuclear species to effect magnetization transfer among J-coupled spins. The technique increases the tolerance to Hartmann-Hahn mismatch between coupled spins and provides easily phased one-bond correlation spectra. Long-range connectivities can also be elucidated with longer mixing times.

Zero quantum NMR in the rotating frame: J cross polarization in AXN systems

An analysis of liquid state cross polarization in AXN spin systems is presented. A two-level geometrical formalism derived from symmetry considerations provides a closed form description of the quantum dynamics for arbitrary N. A parallel discussion using a classical vector model explains the significance of the quantum mechanical solutions and illustrates the nature of the spin–spin correlations accompanying magnetization transfer. General expressions for the J cross polarization A signal with and without X decoupling are given. A discussion of RJCP and PCJCP pulse sequences introduced previously shows how manipulation of Hartmann–Hahn mismatch can advantageously modify cross-polarization dynamics. The quantum formalism demonstrates that efficient polarization transfer is equivalent to population inversion of an ensemble of fictitious spins subjected to different rf field intensities, and that pulse sequences for cross polarization can be constructed by analogy with conventional spin inversion techniques.