Double-quantum filtered 1H MAS NMR spectra

It is shown that straightforward double-quantum filtered (1)H MAS NMR experiments yield spectral lineshapes that permit to estimate the minimum number of (1)H spins in a cluster. The approach may offer an alternative to multiple-quantum experiments for the characterisation of (1)H spin clusters of moderate size. The duration of the double-quantum excitation period has to be chosen suitably, it is necessary to find a practical compromise between optimum double-quantum filtration efficiency and optimum information content of the spectral lineshapes. Some (1)H MAS NMR experiments on partially deuterated maleic acid are reported as well as numerical simulations.     

Analysis of 1H NMR spectra with the combined use of zero- and double-quantum coherence

Both homonuclear zero-quantum and double-quantum coherence correlation experiments have been suggested previously as alternatives to the standard single-quantum coherence correlation experiment (COSY). However, both those experiments have a number of associated problems, including the difficulty in obtaining uniform excitation of coherence and the size of the data matrix which in both cases may be twice as large, and hence takes twice as long to acquire, as that of the corresponding COSY experiment. Both those problems are substantially alleviated in the approach demonstrated here, which combines the simultaneous acquisition of both types of correlation spectra and the most economical formatting of the data; this is shown to be particularly significant where the F₁ dimension is broadband decoupled. The method is demonstrated at 300 MHz for allyl bromide and dehydrotestosterone.     

Filtering of spinning sidebands in 1D MAS NMR spectra

Spinning sidebands (SSBs) in the MAS NMR spectrum of a polycrystalline solid are related to the principal values of the chemical shift or quadrupole coupling tensors. At present, 2D methods are widely used to sort out the SSBs for each isotropic peak. Here a simple and efficient method for separating the SSBs in 1D MAS NMR spectra is described. It is based on finding the optimal spinning rate with a mathematical algorithm and subsequently treating the spectra with filtering functions.     

1H MAS, 13C CP/MAS, and 2H NMR spectra studies of piperidinium {\emph{p}}-chlorobenzoate

Anomalous H/D isotope effects were detected in the ¹H MAS NMR spectra of piperidinium p-chlorobenzoate (C₅H₁₀NH $_{2}{^{+}}\cdot $ ClC₆H₄COO^???) upon deuterium substitution of hydrogen atoms which form two kinds of N-H?O H-bonds in the crystal; in contrast to these spectra, only slight chemical shifts were recorded in ¹³C CP/MAS NMR spectra. ²H NMR spectrum of the deuterated sample show quadrupole coupling constants of 148 and 108 kHz, and reveal that there are a few motions contributing to the electric-field modulation of the ²H nucleus. The ¹H MAS NMR spectra of piperidinium p-chlrobenzoate-d ₁₆ (C₅D₁₀ND $_{2}{^{+}}\cdot $ ClC₆D₄COO^???) and -d ₁₄ (C₅D₁₀NH $_{2}{^{+}}\cdot $ ClC₆D₄COO^???) revealed that the change in the envelope is caused by chemical shifts of each signal upon deuteration. Calculations based on the density-functional-theory showed that the N-H distance along the crystallographic a-axis mainly contributes to the anomalous isotope effects on ¹H MAS NMR envelopes.     

1H-1H double-quantum CRAMPS NMR at very-fast MAS (nuR=35 kHz): a resolution enhancement method to probe 1H-1H proximities in solids

A High-resolution two-dimensional (2D) (1)H double-quantum (DQ) homonuclear recoupling experiments, combined with smooth amplitude-modulation (SAM) homonuclear decoupling is presented. The experiment affords highly resolved and clean (1)H DQ-SQ 2D spectra at very-fast MAS rates (nu(R)=35 kHz). The method is well suited to probe (1)H-(1)H distances in powdered solids and demonstrations are applied on a NaH(2)PO(4) powdered sample, an inorganic compound having hydrogen bonding networks.     

Inverse detection and heteronuclear editing in 1H-15N correlation and 1H-1H double-quantum NMR spectroscopy in the solid state under fast MAS

Signal enhancement in heteronuclear correlation spectra as well as signal selection in 1H experiments can be achieved through inverse, i.e., 1H, detection in the solid state under fast MAS conditions. Using recoupled polarization transfer (REPT), a heteronuclear 1H-15N single-quantum correlation (HSQC) experiment is presented whose symmetrical design allows the frequency dimensions to be easily interchanged. By observing the 15N dimension indirectly and detecting on 1H, the sensitivity is experimentally found to be increased by factors between 5 and 10 relative to conventional 15N detection. In addition, the inverse 1H-15N REPT-HSQC scheme can be readily used as a filter for the 1H signal. As an example, we present the combination of such a heteronuclear filter with a subsequent 1H-1H DQ experiment, yielding two-dimensional 15N-edited 1H-1H DQ MAS spectra. In this way, specific selection or suppression of 1H resonances is possible in solid-state MAS experiments, by use of which the resolution can be improved and information can be unravelled in 1H spectra.     

Magnetic susceptibility effects on 13C MAS NMR spectra of carbon materials and graphite

13C high-resolution solid-state nuclear magnetic resonance (NMR) was employed to study carbon materials prepared through the thermal decomposition of four different organic precursors (rice hulls, endocarp of babassu coconut, peat, and PVC). For heat treatment temperatures (HTTs) above about 600 C, all materials presented 13C NMR spectra composed of a unique resonance line associated with carbon atoms in aromatic planes. With increasing HTT a continuous broadening of this resonance and a diamagnetic shift in its central frequency were verified for all samples. The evolution of the magnitude and anisotropy of the magnetic susceptibility of the heat-treated carbon samples with HTT explains well these findings. It is shown that these results are better understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite, for which the magnetic susceptibility effect is also present and is much more pronounced.     

MAS double-quantum filtered dipolar shift correlation spectroscopy

A carbon-13 magic angle spinning double-quantum filtered dipolar shift correlation NMR experiment which can be used to establish through-space connectivities in solids is analyzed. The main advantage of the double-quantum filtered approach is the removal of intensity arising from natural abundance background signals. The variation in intensity of the cross and diagonal peaks observed in the two-dimensional spectrum as a function of mixing time is investigated experimentally for model systems. The observed behavior is compared with analytical expressions derived for three coupled spins, as well as with simulations based on average Hamiltonian theory. Copyright 2000 Academic Press.     

Supercycled symmetry-based double-quantum dipolar recoupling of quadrupolar spins in MAS NMR: I. Theory

Using average Hamiltonian (AH) theory, we analyze recently introduced homonuclear dipolar recoupling pulse sequences for exciting central-transition double-quantum coherences (2QC) between half-integer spin quadrupolar nuclei undergoing magic-angle-spinning. Several previously observed differences among the recoupling schemes concerning their compensation to resonance offsets and radio-frequency (rf) inhomogeneity may qualitatively be rationalized by an AH analysis up to third perturbation order, despite its omission of first-order quadrupolar interactions. General aspects of the engineering of 2Q-recoupling pulse sequences applicable to half-integer spins are discussed, emphasizing the improvements offered from a diversity of supercycles providing enhanced suppression of undesirable AH cross-terms between resonance offsets and rf amplitude errors.     

含硼分子筛的11B VAS NMR和1H MAS NMR研究

以变角旋转(VAS) NMR方法考察了三配位硼的存在状态,得到其四极作用常数C_q为2.4MHz,电场梯度不对称系数η为0.0.硼引入骨架形成的酸性羟基质子的信号位于2.3ppm,¹H{¹¹B}双共振实验证实其与硼原子的关联很弱.与6.5ppm分子筛吸水峰同时出现的2.7ppm的信号也被认为是吸水的信号,与非骨架铝羟基质子的信号在同一位置,但可通过¹H{²⁷Al}双共振实验加以判别.     

Very high-resolution 1H MAS NMR of a natural polymeric material

The use of ultrafast magic angle spinning (> 30 kHz) in tandem with delayed echo acquisition is shown to yield very high-resolution lH MAS NMR spectra of complex natural organic materials. For the first time, very high-resolution 1H MAS NMR spectra are reported for cork and wood components, two natural materials with great economic importance. The effect of the spinning rate on the 1H NMR spectra was evaluated with single-pulse acquisition and delayed-echo acquisition. The delayed-echo acquisition spectra presented linewidths as sharp as 67 and 25 Hz. The narrow peaks, characterised by proton spin-spin and spin-lattice relaxation, were assigned to the isotropic chemical shifts and the general spectral features were shown to correlate with the sample chemical structure. The tentative assignments of cork 1H MAS NMR signals were presented.     

H/D isotope effect of 1H MAS NMR spectra and 79Br NQR frequencies of piperidinium p-bromobenzoate and pyrrolidinium p-bromobenzoate

H/D isotope effects onto ⁷⁹Br NQR frequencies of piperidinium p-bromobenzoate were studied by deuterium substitution of hydrogen atoms which form two kinds of N–H?O type hydrogen bonds, and the isotope shift of ca. 100 kHz were detected for a whole observed temperature range. In addition, ¹H MAS NMR spectra measurements of piperidinium and pyrrolidinium p-bromobenzoate were carried out and little isotope changes of NMR line shape were detected. In order to reveal effects of molecular arrangements into the obtained isotope shift of NQR frequencies, single-crystal X-ray measurement of piperidinium p-bromobenzoate-d2 and density-functional-theory calculation were carried out. Our estimation showed the dihedral-angle change between piperidine and benzene ring contributes to isotope shift rather than those of N–H lengths by deuterium substitution.     

Solid-state variable-temperature H MAS NMR studies on deuterated polyethylene

Solid-state varible-temperature/magic angle spinning(VT/MAS) ¹H NMR measurements were carried out on deuterated polyethylene. From these experimental results it was found that the ¹H chemical shift induced by conformational and morphological changes of the polyethylene sample is within the linewidth of ≈ 0.5 ppm. Furthermore, from MAS/dipolar decoupling experiments it was found that the resonance frequency of the proton varies linearly with the inverse square of the deuterium decoupling power. This experimental finding is discussed theoretically.     

High-field high-speed MAS resolution enhancement in 1H NMR spectroscopy of solids

Resolution in ¹H NMR spectra of solids can be significantly enhanced with fast magic-angle spinning and high magnetic fields. A variable field and spinning speed study up to 25 T and 40 kHz shows that the homogeneous line broadening is inversely proportional to the product of magnetic field strength and spinning speed. The combination of high field and fast speed yields a ¹H linewidth approaching the intrinsic limit determined by anisotropy of magnetic susceptibility. An analysis of the anisotropic magnetic susceptibility line broadening is presented.     

Rotor-synchronized acquisition of quadrupolar satellite-transition NMR spectra: practical aspects and double-quantum filtration

The very broad resonances of quadrupolar (spin I > 1/2) nuclei are resolved by magic angle spinning (MAS) into a large number of spinning sidebands, each of which often remains anisotropically broadened. The quadrupolar interaction can be removed to a first-order approximation if the MAS NMR spectrum is acquired in a rotor-synchronized fashion, aliasing the spinning sidebands onto a centreband and thereby increasing the signal-to-noise ratio in the resulting, possibly second-order broadened, spectrum. We discuss the practical aspects of this rotor-synchronization in the direct (t(2)) time domain, demonstrating that the audiofrequency filters in the receiver section of the spectrometer have a significant impact on the precise timings needed in the experiment. We also introduce a novel double-quantum filtered rotor-synchronized experiment for half-integer spin quadrupolar (spin I = 3/2, 5/2, etc.) nuclei that makes use of central-transition-selective inversion pulses to both excite and reconvert double-quantum coherences and yields a simplified spectrum containing only the ST(1) (m(I) = +/-1/2 <--> +/-3/2) satellite-transition lineshapes. For spin I = 5/2 nuclei, such as (17)O and (27)Al, this spectrum may exhibit a significant resolution increase over the conventional central-transition spectrum.     

1H and 13C NMR studies of glycine in anisotropic media: double-quantum transitions and the effects of chiral interactions

The (1)H NMR spectrum of glycine in stretched gelatin gel and in cromolyn liquid crystal displays a well-resolved doublet due to (1)H-(1)H dipolar interaction. Multiple spectra were obtained within a wide range of offset frequencies of partially saturating radio-frequency (RF) radiation to generate steady-state irradiation envelopes or z-spectra of glycine. Maximal suppression of the doublet occurred when the irradiation was applied exactly at the centre frequency, between the two glycine peaks. This phenomenon is due to double-quantum transitions and is similar to our previous work on quadrupolar nuclei (2)H (HDO) and (23)Na(+). When the (13)C isotopomer glycine-2-(13)C was used, the same effect was found in twice, split by (1)J(CH)+2D(CH). Additional signals in (1)H and (13)C NMR due to prochiral-chiral interactions were found when glycine-2-(13)C was dissolved in chiral anisotropic gelatin and κ-carrageenan gels. The NMR spectra were successfully simulated assuming a (2)J(HH) coupling constant of -16.5Hz and two distinct dipolar coupling constants for the -(13)CH(2)- group (D(C,HA), and D(C,HB)).     

2H T2rho relaxation dynamics and double-quantum filtered NMR studies

In this study 2H T2rho DQF NMR spectra of water in MCM-41 were measured. The T2rho double-quantum filtered (DQF) NMR signal is generated by applying a radio frequency (RF) field for various durations and then observed after a monitor RF pulse. It was found that the transfer between different quantum coherences by the couplings during long-duration RF fields (i.e., soft pulses) and that residual quadrupolar interaction dominates the signal decay. Knowledge of coherence transfer during long-RF pulses has special significance for the development of sophisticated multi-quantum NMR experiments especially multi-quantum MRI applications.