Efficiency at high spinning frequencies of heteronuclear decoupling methods designed to quench rotary resonance

The performance of two recently developed heteronuclear decoupling schemes designed to quench rotary resonance, phase-inverted supercycled sequence for attenuation of rotary resonance (PISSARRO) and high-phase two-pulse phase modulation (high-phase TPPM), are probed at high spinning frequencies. High-phase TPPM may be useful at the n=1 rotary resonance condition while PISSARRO permits efficient decoupling over a broad commonly used range of rf amplitudes, even at very high spinning frequencies. New insights into the response of spin systems to both decoupling schemes have been gained. High-phase TPPM is sensitive to the offsets of remote protons, their chemical shift anisotropies, and the relative orientations of the heteronuclear dipolar and proton chemical shift tensors. Since PISSARRO is virtually immune against such effects, the method is especially suited for very high magnetic fields.     

Heteronuclear decoupling under fast MAS by a rotor-synchronized Hahn-echo pulse train

A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-gamma nuclei ((13)C, (15)N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small (1)H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger (1)H chemical shifts.     

High-resolution solid-state NMR of anisotropically mobile molecules under very low-power H decoupling and moderate magic-angle spinning

We show that for observing high-resolution heteronuclear NMR spectra of anisotropically mobile systems with order parameters less than 0.25, moderate magic-angle spinning (MAS) rates of 11 kHz combined with ¹H decoupling at 1–2 kHz are sufficient. Broadband decoupling at this low ¹H nutation frequency is achieved by composite pulse sequences such as WALTZ-16. We demonstrate this moderate MAS low-power decoupling technique on hydrated POPC lipid membranes, and show that 1 kHz ¹H decoupling yields spectra with the same resolution and sensitivity as spectra measured under 50 kHz ¹H decoupling when the same acquisition times (50 ms) are used, but the low-power decoupled spectra give higher resolution and sensitivity when longer acquisition times (>150 ms) are used, which are not possible with high-power decoupling. The limits of validity of this approach are explored for a range of spinning rates and molecular mobilities using more rigid membrane systems such as POPC/cholesterol mixed bilayers. Finally, we show ¹⁵N and ¹³C spectra of a uniaxially diffusing membrane peptide assembly, the influenza A M2 transmembrane domain, under 11 kHz MAS and 2 kHz ¹H decoupling. The peptide ¹⁵N and ¹³C intensities at low-power decoupling are 70–80% of the high-power decoupled intensities. Therefore, it is possible to study anisotropically mobile lipids and membrane peptides using liquid-state NMR equipment, relatively large rotors, and moderate MAS frequencies.     

Solid-state F NMR investigation of poly(vinylidene fluoride) with high-power proton decoupling

The use of high-power proton decoupling has enabled highly-resolved spectra of fluorine polymers to be recorded, as is exemplified herein for semicrystalline poly(vinylidene fluoride) (PVDF). By means of high MAS speeds (up to 17 kHz), the spinning sidebands are removed from the whole of the relevant chemical shift range. For spectra of the crystalline regions of the polymer, the high-power decoupling is necessary, though its effect is not large. Various relaxation techniques have been used to examine the semicrystallinity and the polymorphism of PVDF, with special pulse sequences used to discriminate between the various domains. Different chemical shifts have been observed for the signals of the amorphous and crystalline phases. Those of the more immobile parts cover a substantial range.     

Double-quantum homonuclear NMR correlation spectroscopy of quadrupolar nuclei subjected to magic-angle spinning and high magnetic field

We present a new application of the symmetry-based dipolar recoupling scheme, for exciting directly double-quantum (2Q) coherences between the central transition of homonuclear half-integer quadrupolar nuclei. With respect to previously published 2Q-recoupling methods (M. Eden, D. Zhou, J. Yu, Chem. Phys. Lett. 431 (2006) 397), the sequence is used without π/2 bracketing pulses and with an original super-cycling. This leads to an improved efficiency (a factor of two for spin-5/2) and to a much higher robustness to radio-frequency field inhomogeneity and resonance offset. The 2Q-coherence excitation performances are demonstrated experimentally by ²⁷Al NMR experiments on the aluminophosphates berlinite, VPI5, AlPO₄-14, and AlPO₄-CJ3. The two-dimensional 2Q–1Q correlation experiments incorporating these recoupling sequences allow the observation of 2Q cross-peaks between central transitions, even at high magnetic field where the difference in offset between octahedral and tetrahedral ²⁷Al sites exceeds 10 kHz.     

Heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance under ultra-high magic-angle spinning

We compare in this communication several heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance experiments under a magic-angle spinning frequency of 60 kHz. The decoupling radiofrequency field amplitudes considered are 190 and 10 kHz. No substantial difference was found among the sequences considered here in performance barring the difference in the optimisation protocol of the various schemes, an aspect that favours the use of swept-frequency two pulse phase modulation (SW(f)-TPPM).     

Recoupling of heteronuclear dipolar interactions with rotational-echo double-resonance at high magic-angle spinning frequencies

Heteronuclear dipolar recoupling with rotational-echo double-resonance (REDOR) is investigated in the rapid magic-angle spinning regime, where radiofrequency irradiation occupies a significant fraction of the rotor period (10-60%). We demonstrate, in two model (13)C-(15)N spin systems, [1-(13)C, (15)N] and [2-(13)C, (15)N]glycine, that REDOR DeltaS/S(0) curves acquired at high MAS rates and relatively low recoupling fields are nearly identical to the DeltaS/S(0) curve expected for REDOR with ideal delta-function pulses. The only noticeable effect of the finite pi pulse length on the recoupling is a minor scaling of the dipolar oscillation frequency. Experimental results are explained using both numerical calculations and average Hamiltonian theory, which is used to derive analytical expressions for evolution under REDOR recoupling sequences with different pi pulse phasing schemes. For xy-4 and extensions thereof, finite pulses scale only the dipolar oscillation frequency by a well-defined factor. For other phasing schemes (e.g., xx-4 and xx-4) both the frequency and amplitude of the oscillation are expected to change.     

Insights into homonuclear decoupling from efficient numerical simulation: techniques and examples

A combination of techniques, including rational number synchronisation and pre-diagonalisation of the time-dependent periodic Hamiltonian, are described which allow the efficient simulation of NMR experiments involving both magic-angle spinning (MAS) and RF irradiation, particularly in the important special case of phase-modulated decoupling sequences. Chebyshev and conventional diagonalisation approaches to calculating propagators under MAS are also compared, with Chebyshev methods offering significant advantages in cases where the Hamiltonian is large and time-dependent but not block-diagonal (as is the case for problems involving combined MAS and RF). The ability to simulate extended coupled spin systems efficiently allows 1H spectra under homonuclear decoupling to be calculated directly and compared to experimental results. Reasonable agreement is found for the conditions under which homonuclear decoupling is typically applied for rigid solids (although the increasing deviation of experimental results from the predictions of theory and simulation at higher RF powers is still unexplained). Numerical simulations are used to explore three features of these experiments: the interaction between the magic-angle spinning and RF decoupling, the effects of tilt pulses in acquisition windows and the effects of "phase propagation delays" on tilted axis precession. In each case, the results reveal features that are not readily anticipated by previous analytical studies and shed light on previous empirical observations.     

Magnetic resonance experiments in NiBr2 at high frequencies and high magnetic fields

By performing magnetic resonance experiments in NiBr₂ at high frequencies and magnetic fields, we have observed a resonance line which we relate to a helimagnetic phase, and an antiferromagnetic resonance line corresponding to an antiferromagnetic phase with strong out-of-plane anisotropy and small in-plane anisotropy.     

Why does PMLG proton decoupling work at 65 kHz MAS?

Schemes such as phase-modulated Lee–Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution ¹H spectra at high magic-angle spinning (MAS) frequencies of 50–70 kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here, a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be successful at high MAS frequencies.     

Multiple resonance heteronuclear decoupling under MAS: dramatic increase of spectral resolution at moderate magnetic field and MAS frequencies

The effects of multiple-resonance heteronuclear decoupling under magic angle spinning (MAS) on the resolution of one-dimensional ¹⁹F and ³¹P and various two-dimensional MAS NMR spectra and on the residual non-refocusable coherence lifetimes in fluorinated aluminophosphate AlPO₄-CJ2, i.e. a compound that contains numerous highly abundant nuclei but no homonuclear spin bath, has been investigated. The design of the four-channel (¹H, ¹⁹F, ²⁷Al, ³¹P) MAS probe used for this study is first described. ¹H and ¹H–²⁷Al double-resonance decouplings allows lengthening the optimized transverse relaxation and increasing the resolution in the ¹⁹F and ³¹P dimensions. Under the application of multi-nuclear decoupling, a two-dimensional ¹⁹F–³¹P CP-HETCOR correlation spectrum for AlPO₄-CJ2 is recorded with unprecedented high-resolution in the two dimensions. Moreover, because ¹H-decoupling increases the ¹⁹F , it has been applied during the entire duration of the 2D NMR experiments, allowing the direct use of residual small interactions to generate ¹⁹F–¹⁹F and ¹⁹F–²⁷Al 2D NMR correlation spectra in AlPO₄-CJ2.     

Electron spin resonance of magnetic materials in high fields and high frequencies

Observation and analysis of electron spin resonance of many kinds of magnetic materials have been performed with high frequencies provided by Gunn oscillators, backward traveling wave tubes and far-infrared lasers and also with a vector network analyzer in both pulsed high magnetic fields up to 40 T and steady field up to 20 T. The magnetic behavior of quantum spin systems and metallic materials has been investigated.     

Fast EPR imaging at 300 MHz using spinning magnetic field gradients

Electron paramagnetic resonance imaging (EPRI) technology has rapidly progressed in the last decade enabling many important applications in the fields of biology and medicine. At frequencies of 300-1200 MHz a range of in vivo applications have been performed. However, the requisite imaging time duration to acquire a given number of projections, limits the use of this technique in many in vivo applications where relatively rapid kinetics occur. Therefore, there has been a great need to develop approaches to accelerate EPRI data acquisition. We report the development of a fast low-frequency EPRI technique using spinning magnetic field gradients (SMFG). Utilizing a 300 MHz CW (continuous wave) EPRI system, SMFG enabled over 10-fold accelerated acquisition of image projections. 2D images with over 200 projections could be acquired in less than 3s and with 20s acquisitions good image quality was obtained on large aqueous free radical samples. This technique should be particularly useful for in vivo studies of free radicals and their metabolism.     

Using a cross-coil to reduce RF heating by an order of magnitude in triple-resonance multinuclear MAS at high fields

Four different coil designs for use with MAS in triple-resonance multi-nuclear experiments at high fields are compared, using a combination of finite element analysis (FEA) software and NMR experiments, with respect to RF field strength per unit power and relative sample heating, as governed by mean E/B(1) within the sample region. A commercial FEA package, Microwave Studio 5.1 by Computer Simulation Technology (CST) is shown to obtain remarkably accurate agreement with the experiments in Q(L), L, B, E, and mode frequencies in all cases. A simplified treatment of RF heating in NMR MAS samples is derived and shown to agree with the NMR experimental results within about 10% for two representative stator designs. The coil types studied include: (1) a variable-pitch solenoid outside a ceramic coilform, (2) a conventional solenoid very closely spaced to the MAS rotor, (3) a scroll coil, and (4) a segmented saddle cross coil (XC) for (1)H with an additional solenoid over it for the two lower-frequency channels. The XC/solenoid is shown to offer substantial advantages in reduced decoupler heating, improved S/N, and improved compatibility with multinuclear tuning and high-power decoupling. This seems largely because the division of labor between two orthogonal coils allows them each, and their associated circuitry, to be separately optimized for their respective regimes.     

Broad-band homo-nuclear correlations assisted by 1H irradiation for bio-molecules in very high magnetic field at fast and ultra-fast MAS frequencies

We propose a new broadband second-order proton-assisted ¹³C–¹³C correlation experiment, SHANGHAI. The ¹³C–¹³C magnetization transfer is promoted by ¹H irradiation with interspersed four phases super-cycling. This through-space homo-nuclear sequence only irradiates on the proton channel during the mixing time. SHANGHAI benefits from a large number of modulation sidebands, hence leading to a large robustness with respect to chemical shift differences, which permits its use in a broad MAS frequency range. At ultra-fast MAS (ν_R ? 60 kHz), SHANGHAI is only efficient when the amplitude of ¹H recoupling rf-field is close to half the spinning speed (ν₁ ≈ ν_R/2). However, at moderate to fast MAS (ν_R = 20–35 kHz), SHANGHAI is efficient at any rf-power level larger than ν₁ ≈ 10 kHz, which simultaneously permits avoiding excessive heating of bio-molecules, and using large sample volumes. We show that SHANGHAI can be employed at the very high magnetic field of 23.5 T and then allows the observation of correlation between ¹³C nuclei, even if their resonance frequencies differ by more than 38 kHz.     

Ionization of Rydberg atoms in THz-laser fields at the transition from low to high scaled frequencies

We have studied the ionization of Rydberg-excited xenon atoms in THz-laser fields and by quantum dynamical calculations. The experimental threshold laser field strength for 10% ionization probability follows an n^*-1.68 (1.04 THz) dependence (n^* effective principal quantum number) with additional weak resonance structures and shows that ionization does not occur by a Landau-Zener mechanism. At scaled frequencies of to 5.6 the simulated threshold fields for ionization in oscillatory fields show a dependence on the principal quantum number n of n^-4.1 to n^-1.35. Received: 17 February 1998 / Revised: 20 April 1998 / Accepted: 21 April 1998     

Successive oscillatory fields at radio to optical frequencies

The methods of separated or successive oscillatory fields (often called Ramsey methods) are described for radio, microwave and optical coherent radiation. The methods are initially described qualitatively. The quantitative theories of the methods are also given. For a two-level system at radio frequencies exact expressions for the transition probability amplitudes for an arbitrary number of successive oscillatory regions can be found in terms of products of successive transformation matrices. The expressions are greatly simplified if the oscillatory-field regions are very short. If there are only two oscillatory-field regions the situation is similar to a Young's two-slit interference pattern except that the two paths are separated in spin space rather than normal space. Some results from more than two oscillatory regions are discussed. The extensions of the methods and the theories to optical frequencies are described. The advantages and disadvantages of the methods are given as are the many extensions of the method beyond the original concept.I am honored to write an article in celebration of the sixtieth birthday of Herbert Walther. He has established one of the world's best atomic-physics laboratories and has made great contributions to physics, ranging from precision spectroscopy and laser cooling to measurements of fundamental atomic and quantum properties. He has also been a helpful consultant, adviser, and friend to many of us in the field. Happy birthday!!     

Thermal insulating behavior in crystals at high frequencies

When solving heat-conduction problems with periodic temperature perturbations, the thermal conductivity is assumed to remain frequency independent. We, however, show by using the molecular dynamics technique and the fluctuation-dissipation theorem a decrease of the effective thermal conductivity of 2 orders of magnitude when the excitation frequency approaches or exceeds the reverse of the phonon mean relaxation time. Most of the dielectric and semiconductor materials have to be considered as strongly insulating in those conditions. The comparison between molecular-dynamics simulations performed in Si crystals and theoretical predictions reveals a clear agreement.     

Design and performance of a high pressure insert for use in a standard magic angle spinning NMR probe

In this paper we describe the construction and performance of high pressure magic angle inserts made from the polymer PEEK. The inserts were designed to fit inside standard commercial 7 mm magic angle spinning rotors and spin at the maximum frequency of the probe. The sample volume of the inserts was 100 microL. A gas loading chamber that operates at room temperature is described. The performance of the inserts is discussed for a number of gases in terms of resolution as a function of spinning speed and leakage of the gas due to permeation through the polymer. Finally, some preliminary results are shown in relation to complex food materials.     

Piezoelectric effect in polyvinylidene fluoride at high frequencies

A strong piezoelectric effect in polyvinylidene fluoride was observed at frequencies as high as 500 MHz using polarized films as ultrasonic transducers. In contrast to the low frequency behaviour, the transducer efficiency showed a remarkable increase with decreasing temperature.