New Solid-State NMR Methods For Homo-/Hetero-Nuclear Correlations And Spin=1 Nucleus

The main aim of this thesis is to develop new methods of Homo- and hetero-nuclear Correlation Spectroscopy that allows probing proximities between active nuclei in Solid-State NMR. Minor parts of this thesis concern the broadband excitation of 2H nuclei with composite pulses.In Chapter 1, a brief introduction of spin Hamiltonians in solids is given. Common methods of Homo- and Hetero-nuclear Correlation Spectroscopy are also explained in detail. Some of these methods are employed and improved in the following chapters.In Chapter 2, a broadband first-order finite-pulse radio-frequency-driven recoupling (fp-RFDR) NMR sequence with a nested (XY8)41 super-cycling for very high magnetic fields is presented. At fast to ultra-fast magic-angle spinning (MAS), this super-cycling, formed by combining phase inversion and a global four-quantum phase cycle, improves the robustness of fp-RFDR to (i) chemical shift anisotropy (CSA), (ii) spread in isotropic chemical shifts, (iii) rf-inhomogeneity and (iv) hetero-nuclear dipolar couplings for long recoupling times. The performances of fp-RFDR-XY8, fp-RFDR-XY16 and fp-RFDR-(XY8)41 have been compared using simulations and experiments with very large offsets and/or CSAs. In all cases, rp-RFDR-(XY8)41 is superior to the two other types of sequences, especially in case of very large CSAs and long recoupling times. The robustness of fp-RFDR-(XY8)41 to offset is demonstrated by the analysis of 13C-13C proximities in a solid-state tetra-peptide sans 1H decoupling at MAS frequency of 60 kHz and the high magnetic field of 21.1 T, for which the difference in 13C resonance frequencies can differ by 36 kHz. The increased efficiency of this super-cycling on phosphorous atoms submitted to very large CSAs of ca.80 kHz at 21.1 T is also demonstrated. The stability of the transfer efficiency for long recoupling times in fp-RFDR-(XY8)41 facilitates the enhancement of 13C-13C long-range correlations in uniformly labeled samples, especially at very high magnetic fields.In Chapter 3, modulation-sideband recoupling conditions of the 13C-13C Second-order Hamiltonian among Analogous nuclei plus (SHA+) is discussed, and it is shown that this sequence can be used in two different recoupling regimes, depending on the ratio between the spinning speed and the carbon frequency range, △visomax. The first regime, vR>△visomax, is recommended for broad-band recoupling to avoid any rotational resonance broadening. In this regime, the spinning speed should be only slightly larger than △visomax, to obtain the best transfer efficiency. This condition could be used in most cases. The second regime, vR<△visomax, benefits from higher transfer efficiency and S/N ratio owing to the use of slower spinning speed. It can be used selectively to observe long-range constraints with lower spinning speed, which increases the transfer efficiency, and may allow using bigger rotors to increase the S/N ratio. It also leads to lower proton rf-power, which facilitates the use of longer mixing time, and avoids the denaturation of temperature-sensitive proteins.In Chapter 4, a systematic comparison of the performances of various decoupling schemes during the indirect t1 evolution period of dipolar-mediated HMQC (D-HMQC) experiment is presented. It is shown that the spectral resolution along the indirect dimension of proton-detected HMQC spectra can be enhanced by applying decoupling schemes during the t1 period to a level close to best directly observed decoupled CP spectra. It is also demonstrated that 1H-1H dipolar decoupling sequences during t1, such as symmetry-based schemes, phase-modulated Lee-Goldburg (PMLG) and Decoupling Using Mind-Boggling Optimization (DUMBO), provide better resolution than continuous wave 1H irradiation. When observing indirectly broad spectra presenting numerous spinning sidebands, the D-HMQC sequence must be fully rotor-synchronized owing to the rotor-synchronized indirect sampling and dipolar recoupling sequence employed. In this case, a solution to reduce artefact sidebands caused by the modulation of window delays before and after the decoupling application during the t1 period is proposed. The performances of the various decoupling schemes are assessed via numerical simulations and compared to 2D 1H-{13C} D-HMQC experiments on [U-13C]-L-histidine.HCl.H2O at various magnetic fields and Magic Angle spinning (MAS) frequencies. At vR≈32 kHz, similar spectral resolutions were obtained using C1472 (aoaiso), SAM3.5 and SAM4 since all decoupling schemes were applied at optimal rf field value. Hardware limitations of Avance-Ⅱ consoles did not allow using non rotor synchronized sequences like PMLG or DUMBO. At vR= 62 kHz, we recorded a D-HMQC spectrum using PMLG sequence, the resolution of which matches the resolution of directly observed 13C. Resolution improvement for SAM sequences was limited by the specifications of the 1.3 mm probe, since the optimal peak rf field amplitude for SAM3.5 and SAM4 exceeds the maximal rf field the probe can deliver. Thanks to transient free pulses, Smoothed Amplitude Modulation (SAM) generally produced spectra with reduced ti noise levels, which is beneficial for the observation of weak long-range correlations. Experimentally, the independent execution of pulse sequences on each channel using Bruker Avance Ⅲ spectrometer greatly facilitates the implementation of decoupling schemes, especially the unsynchronized ones. Great resolution and sensitivity enhancements resulting from decoupling during t1 period enable the detection of hetero-nuclear correlation between aliphatic protons and ammonium 14N sites in L-histidine.HCl.H2O.In Chapter 5, we demonstrate how frequency-selective radio-frequency (rf) long pulses allow one to achieve an efficient excitation of nuclei experiencing large anisotropic NMR interactions. It is shown that these rf pulses can be applied on the indirect channel of Hetero-nuclear Multiple-Quantum Correlation (HMQC) experiments, which facilitate the detection of nuclei exhibiting wide spectra via spin-1/2 isotopes. Selective excitation is achieved using long pulses as well as trains of long pulses in the manner of Delays Alternating with Nutation for Tailored Excitation (DANTE). Numerical simulations show that this indirect excitation method is applicable to spin-1/2 nuclei experiencing large chemical shift anisotropy, as well as to spin-1 isotopes subject to large quadrupolar interaction, such as 14N. The performances of selective pulses are analyzed by numerical simulations of scalar-mediated HMQC experiments indirectly detecting spin-1/2 or spin-1 nuclei, as well as by dipolar-mediated HMQC experiments between 1H and 14N nuclei in solid-state a-glycine amino-acid at 21.1 T and Magic Angle Spinning (MAS) frequency of 60 kHz. It is shown that the efficiency of selective excitation is comparable to that of broadband excitation given the rf field delivered by common solid-state NMR probes. Furthermore, selective excitation:(ⅰ) requires moderate rf field, (ⅱ) can be easily optimized, and (ⅲ) displays high robustness to offset, rf field inhomogeneity, and fluctuations in MAS frequencies. The choice in between the selective excitation with two long pulses or two DANTE trains depends on a compromise in between the sensitivity to offsets and the required rf field strength. However, it must be noted that even with long pulse selective excitations the required rf field strength is weak and smaller than the spinning speed. More globally, the selective excitation with two long pulses looks to be the most appropriate way to perform 1H-{14N} D-HMQC experiments, except when the 14N frequency range is small or moderate where DANTE trains provide a slightly larger efficiency than selective long pulses.In Chapter 6, a revision of four well-known composite pulses (COM-Ⅰ, Ⅱ, Ⅲ, and Ⅳ) for broadband excitation in deuterium quadrupolar echo (solid-echo) spectroscopy is presented. These composite pulses are combined with several phase cycling schemes that were previously shown to decrease finite pulse width distortions in deuterium solid-echo experiments performed with two single pulses. The simulations and experiments shown COM-Ⅱ and -Ⅳ are superior to the other composite 90° pulses studied as they reduce the requisite RF fields for uniform excitation, but also give undistorted spectra without baseline artifacts. COM-Ⅲ was found to yield a distorted powder pattern due to its much longer pulse duration. Our results show that the full 8-step phase cycling is robust in mitigating undesired finite pulse width effects that result in spectral distortions and should be applicable for quadrupolar echo spectroscopy based on composite pulses.In Chapter 7, a theoretical analysis of COM-Ⅱ with 8-step phase cycling by average Hamiltonian theory is given, which allows us to understand the experiment results we obtained in Chapter 6. Analytical results, to first order of the Magnus expansion, highlight the performance of this composite pulse with an 8-step phase cycling scheme. By applying the fictitious spin-1 operators, this chapter highlights the mechanism of the 8-step phase cycling that minimizes spectral distortions.