Heteronuclear Spin Decoupling Research Articles

A unified heteronuclear decoupling picture in solid-state NMR under low radio-frequency amplitude and fast magic-angle-spinning frequency regime.

Heteronuclear spin decoupling is a highly important component of solid-state NMR experiments to remove undesired coupling interactions between unlike spins for spectral resolution. Recently, experiments using a unification strategy of standard decoupling schemes were presented for high radio-frequency (RF) amplitudes and slow-intermediate magic-angle-spinning (MAS) frequencies, in the pursuit of deeper understanding of spin decoupling under phase-modulated RF irradiation [A. Equbal et al., J. Chem. Phys. 142, 184201 (2015)]. The approach, unified two-pulse heteronuclear decoupling (UTPD), incorporates the simultaneous time- and phase-modulation strategies, commonly used in solid-state NMR. Here, the UTPD based decoupling scheme is extended to the experimentally increasingly important regime of low RF amplitudes and fast MAS frequencies. The unified decoupling approach becomes increasingly effective in identifying the deleterious dipole-dipole and, in particular, J recoupling conditions which become critical for the low-amplitude RF regime. This is because J coupling is isotropic and therefore not averaged out by sample spinning unlike the anisotropic dipole-dipole coupling. Numerical simulations and analytic theory are used to understand the effects of various nuclear spin interactions on the decoupling performance of UTPD, in particular, the crucial difference between the low-phase and high-phase UTPD conditions with respect to J coupling. In the UTPD scheme, when the cycle-frequency of the pulse-sequence is comparable to the RF nutation frequency, the existence of a non-zero effective rotation in the basic two-pulse scheme becomes an essential feature for the efficient and robust averaging out of the scalar J coupling. This broad viewpoint is expected to bring different optimum low-power decoupling pulse schemes under a common footing.

Simultaneous homonuclear and heteronuclear spin decoupling in magic-angle spinning solid-state NMR

We show here an effective way of implementing simultaneously homonuclear and heteronuclear dipolar decoupling in magic-angle spinning (MAS) solid-state NMR. Whilst the homonuclear spin decoupling is applied on the 1H channel, heteronuclear spin decoupling is applied on the 13C channel. The 1H spins are observed in a windowed fashion in this case. The resultant 1H spectrum has higher resolution due to the attenuation of broadening arising from both homonuclear 1H-1H and heteronuclear 1H-13C interactions, with the latter normally leading to additional line broadening in 13C labelled samples. The experiments are performed at MAS frequencies of ca. 60 kHz.

Systematic evaluation of heteronuclear spin decoupling in solid-state NMR at the rotary-resonance conditions in the regime of fast magic-angle spinning

The performance of heteronuclear spin decoupling sequences in solid-state NMR severely degrades when the proton radiofrequency (RF) nutation frequencies (ν1) are close to or at multiples of magic-angle spinning (MAS) frequency (νr) that are referred to as rotary-resonance recoupling conditions (ν1=n·νr). Recently, two schemes, namely, PISSARRO and rCWApA, have been shown to be less affected by the problem of MAS and RF interference, specifically at the n=2 rotary-resonance recoupling condition, especially in the fast MAS regime. Here, we systematically evaluate the loss in intensity of several heteronuclear spin decoupling sequences at the n=1, 2 conditions compared to high-power decoupling in the fast-MAS regime. We propose that in the fast-MAS regime (above 40kHz) the entire discussion about RF and MAS interference can be avoided by using appropriate low-power decoupling sequences which give comparable performance to decoupling sequences with high-power 1H irradiation of ca.195kHz.

Heteronuclear decoupling in MAS NMR in the intermediate to fast sample spinning regime

Heteronuclear spin decoupling in solid-state magic-angle spinning NMR is investigated to present methods overcoming interferences between rf irradiation and sample spinning in the intermediate to fast spinning regime. We demonstrate that a recent phase-alternated variant of refocused CW irradiation (rCWApA) provides efficient and robust decoupling in this regime. An extensive experimental and numerical comparison is presented for rCWApA and PISSARRO (phase-inverted supercycled sequence for attenuation of rotary resonance), previously introduced to quench rotary-resonance recoupling effects, under conditions with spinning frequencies between 30 and 60kHz. Simulations are used to identify the effect of decoupling for various nuclear spin interactions.

Sensitivity improvement during heteronuclear spin decoupling in solid-state nuclear magnetic resonance experiments at high spinning frequencies and moderate radio-frequency amplitudes

Searching for optimal conditions during one- and multi-dimensional solid-state NMR experiments in high static fields may require spinning the sample at frequencies above 40kHz. This implies challenging requirements for heteronuclear spin decoupling. We have compared the performance of the latest heteronuclear decoupling schemes at high magic-angle spinning frequencies. The results demonstrate that at commonly used rf amplitudes between 80 and 120kHz, PISSARRO decoupling provides substantial sensitivity improvement. The performance of low-amplitude decoupling at different spinning speeds is also compared and its dependence on the inherent inhomogeneity of the rf field is probed by numerical simulations.

ChemInform Abstract: Heteronuclear Spin Decoupling in Solid‐State Nuclear Magnetic Resonance: Overview and Outlook

AbstractReview: 52 refs.

Frequency-swept pulse sequences for 19F heteronuclear spin decoupling in solid-state NMR

Heteronuclear spin decoupling pulse sequences in solid-state NMR have mostly been designed and applied for irradiating 1H as the abundant nucleus. Here, a systematic comparison of different methods for decoupling 19F in rigid organic solids is presented, with a special emphasis on the recently introduced frequency-swept sequences. An extensive series of NMR experiments at different MAS frequencies was conducted on fluorinated model compounds, in combination with large sets of numerical simulations. From both experiments and simulations it can be concluded that the frequency-swept sequences SW f -TPPM and SW f -SPINAL deliver better and more robust spin decoupling than the original sequences SPINAL and TPPM. Whereas the existence of a large chemical shift anisotropy and isotropic shift dispersion for 19F does compromise the decoupling efficiency, the relative performance hierarchy of the sequences remains unaffected. Therefore, in the context of rigid organic solids under moderate MAS frequencies, the performance trends observed for 19F decoupling are very similar to those observed for 1H decoupling.

Swept‐frequency two‐pulse phase modulation (SWf‐TPPM) sequences with linear sweep profile for heteronuclear decoupling in solid‐state NMR

Recently, a pulse scheme for heteronuclear spin decoupling in solid-state NMR, called swept-frequency two-pulse phase modulation (SW(f)-TPPM), was introduced which outperforms the standard TPPM and small phase incremental alteration (SPINAL) schemes. It has also been shown that the frequency-sweep profile can be varied to achieve optimal efficiency for crystalline and liquid-crystalline samples, respectively. Here we present a detailed comparison of the proton decoupling performance for SW(f)-TPPM sequences with tangential sweep profiles (SW(f) (tan)-TPPM) and linear sweep profiles (SW(f) (lin)-TPPM). Using the (13)CH(2) resonance of crystalline tyrosine as a model system, it is shown that linear profiles have a decoupling performance which is at least as good and in some instances slightly better than that obtained from tangential sweep profiles. While tangential sweep profiles require a tangent cut-off angle as an additional parameter, the lack of that parameter makes linear sweep profiles easier to implement and optimise.

Floquet–van Vleck analysis of heteronuclear spin decoupling in solids: The effect of spinning and decoupling sidebands on the spectrum

We investigate the effect of magic angle spinning on heteronuclear spin decoupling in solids. We use an analytical Floquet–van Vleck formalism to derive expressions for the powder-averaged signal as a function of time. These expressions show that the spectrum consists of a centerband at the isotropic frequency of the observed spin, ω 0 , and rotational decoupling sidebands at ω 0± ω 1± mω r , where ω 1 is the decoupling field strength and ω r is the rotation frequency. Rotary resonance occurs when the rotational decoupling sidebands overlap with the centerband. Away from the rotary resonance conditions, the intensity of the centerband as a function of ω r / ω 1 is simply related to the total intensity of the rotational decoupling sidebands. Notably, in the absence of offset terms it is shown that as ω 1 decreases, the centerband intensity can decrease without any associated broadening. Furthermore, the centerband width is shown to be independent of spinning speed, to second order for the effects we consider. The effects of I spin chemical shift anisotropy and homonuclear dipolar couplings are also investigated. The analytical results are compared to simulations and experiments.

Decoupling and recoupling using continuous-wave irradiation in magic-angle-spinning solid-state NMR: A unified description using bimodal Floquet theory

The application of two or more different time-dependent coherent perturbations with, in general, incommensurable frequencies occurs quite commonly in NMR experiments. Here we develop a unified description of the entire class of experiments using bimodal Floquet theory and van Vleck-Primas perturbation theory. This treatment leads to a time-independent effective Hamiltonian in Hilbert space and can be looked at as a generalization of average Hamiltonian theory to several incommensurate time dependencies. As a prototype experiment we treat the application of continuous-wave (cw) radio-frequency irradiation in combination with magic-angle sample spinning. Practically relevant examples of this type of experiments are heteronuclear spin decoupling and recoupling experiments using cw irradiation, e.g., rotary-resonance recoupling. Perturbations up to the third order must be taken into account to explain all experimentally observed resonance conditions.

Heteronuclear spin decoupling in solid-state NMR under magic-angle sample spinning

Achieving high spectral resolution is an important prerequisite for the application of solid-state NMR to biological molecules. Higher spectral resolution allows to resolve a larger number of resonances and leads to higher sensitivity. Among other things, heteronuclear spin decoupling is one of the important factors which determine the resolution of a spectrum. The process of heteronuclear spin decoupling under magic-angle sample spinning is analyzed in detail. Continuous-wave RF irradiation leads only in a zeroth-order approximation to a full decoupling of heteronuclear spin systems in solids under magic-angle spinning (MAS). In a higher-order approximation, a cross-term between the dipolar-coupling tensor and the chemical-shielding tensor is reintroduced, providing a scaled coupling term between the heteronuclear spins. In strongly coupled spin systems this second-order recoupling term is partially averaged out by the proton spin-diffusion process, which leads to exchange-type narrowing of the line by proton spin flips. This process can be described by a spin-diffusion type superoperator, allowing the efficient simulation of strongly coupled spin systems under heteronuclear spin decoupling. Low-power continuous-wave decoupling at fast MAS frequencies offers an alternative to high-power irradiation by reversing the order of the averaging processes. At fast MAS frequencies low-power continuous-wave decoupling leads to significantly narrower lines than high-power continuous-wave decoupling while at the same time reducing the power dissipated in the sample by several orders of magnitude. The best decoupling is achieved by multiple-pulse sequences at high RF fields and under fast MAS. Two such sequences, two-pulse phase-modulated decoupling (TPPM) and X-inverse-X decoupling (XiX), are discussed and their properties analyzed and compared.

Low-power decoupling in fast magic-angle spinning NMR

We investigate the possibility of a low-power rf-irradiation approach to heteronuclear spin decoupling in solid-state NMR under high-frequency magic-angle sample spinning. Decoupling is achieved by applying an rf-field with an amplitude corresponding to a precession frequency much lower than the spinning frequency. This leads to a reversal of the averaging processes compared to normal high-power continuous-wave decoupling. Such an approach becomes increasingly interesting with increasing MAS spinning frequency. In rigid solids, low-power decoupling becomes competitive above about 40 kHz MAS frequency.

A simple model for heteronuclear spin decoupling in solid-state NMR

We propose a simple model to describe heteronuclear spin decoupling in solid-state NMR under magic-angle sample spinning (MAS). It is based on a coherent description of two heteronuclear dipolar-coupled spins (I and S) and an incoherent description of the interaction of the I-spin with a large number of other I-spins. The abundant and strongly coupled I-spins are irradiated. The selected I-spin is coupled by a spin-diffusion type superoperator to the I-spin bath, and this coupling is described by a single spin-diffusion rate constant. Such a model allows us to simulate efficiently the behavior of a spin system under heteronuclear decoupling in the case of CW irradiation as well as in the case of phase-modulated sequences such as TPPM.

Efficient Simulation of Periodic Problems in NMR. Application to Decoupling and Rotational Resonance

Abstract A numerical algorithm is described for efficient calculation of the dynamics of nuclear spin systems under a periodic Hamiltonian. The method employs time-domain integration of the quantum evolution over a single modulation period. This information is used to construct the NMR spectrum, effectively with infinite frequency resolution. The computation procedure is more rapid than Floquet simulations in the frequency domain and explicit calculation over long times in the time domain. Furthermore, it avoids the ambiguity in the eigenvalues associated with the effective Hamiltonian approach. The method is demonstrated by calculating the NMR spectrum in two different circumstances: (i) heteronuclear spin decoupling by periodic radiofrequency irradiation, and (ii) recoupling of homonuclear dipolar interactions in rotating solids by spinning at the rotational-resonance condition.

NMR spectroscopic properties of heptakis(2,6‐di‐O‐pentyl)‐β‐cyclodextrin: Two‐dimensional NMR spectra of a key intermediate in preparing chiral stationary phases for enantioselective capillary gas chromatography

AbstractThe 1H and 13C NMR spectra of heptakis(2,6‐di‐O‐pentyl)‐β‐cyclodextrin in deuteriochloroform have been fully and unambiguously assigned. Several methods, including homo‐ and heteronuclear spin decoupling, two‐dimensional homo‐ and hetero‐nuclear correlation NMR spectroscopy, spectral simulation and the measurement of relaxation times were used for chemical shift assignments.

Off resonance heteronuclear spin-decoupling in solids

It is shown that, under what may be called an “intermediate” decoupling condition, off-resonance heteronuclear spin decoupling of an abundant spin causes not only a line broadening, but also a line splitting, and a frequency shift of the NMR spectra of the dilute spin in solids. Typical examples of the fineshape behavior as a function of off-resonance decoupling frequency are presented and analyzed for the CH, CH 2, and CH 3 carbon nuclei in a single crystal of dimedone. The quadratic dependence of the linewidth on the off-resonance decoupling frequency, under the “strong near resonance” decoupling condition, is examined for both the single-crystal case, and for the CP-MAS spectrum of dimedone.

Full assignment of the proton and carbon-13 nmr spectra of 2,3,6 -tri-o-methyl-β-cyclodextrin

The proton and carbon-13 NMR spectra of 2,3,6 -tri-O-methyl-β-cyclodextrin in deuteriochloroform have been fully and unambiguously assigned using homonuclear and selective heteronuclear spin decoupling and two-dimensional homo- and heteronudear correlation NMR spectroscopy. Corrections are made to some earlier literature assignments.

Utilization of chirp frequency modulation with 180°-phase modulation for heteronuclear spin decoupling

Heteronuclear decoupling with Chirp modulation in conjunction with 180°-phase alternation is demonstrated to be a more efficient decoupling method than 180°-phase alternation alone. A theory that predicts the results of Chirp modulation alone is developed, and it is used to explain the results of decoupling with Chirp modulation and 180°-phase alternation. This method allows for efficient spin decoupling with as little as 2 W of decoupling power.

Quantitative aspects of deuteron (spin 1) spin decoupling in solid-state N M R

The dynamics of heteronuclear spin decoupling in solids is treated rigorously in the case of deuterons ($^{2}\mathrm{D}$) decoupled from protons ($^{1}\mathrm{H}$). Dipole-dipole interaction among each spin species is neglected. Deuteron decoupling in the presence of strong quadrupolar interaction ${\ensuremath{\omega}}_{Q}$ is governed by a double-quantum process, which is demonstrated by experiments and by double-quantum-limit calculations as compared with the rigorous treatment. Double-quantum satellites are observed in the proton NMR spectra due to coherent double-quantum motion of the deuteron spins.

Dynamics of heteronuclear spin coupling and decoupling in solids

Dynamics of heteronuclear spin coupling and decoupling in solids