Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy

¹⁴N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C₂H₅– to n-C₄H₉–) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of ¹⁴N chemical shift anisotropies (CSAs) on the order of |δ_σ|=10–30 ppm along with ¹⁴N quadrupole coupling constants in the range of 10–70 kHz. The magnitude and sign of the CSAs determined from ¹⁴N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500–650 Hz) ¹⁵N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the ¹⁴N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding ¹⁵N CSAs from the ssb pattern in the ¹⁵N CP/MAS spectra.     

In vivo 1H NMR spectroscopy of individual human brain metabolites at moderate field strengths

This article reviews spectral editing techniques for in vivo ¹H NMR spectroscopy of human brain tissue at moderate field strengths of 1.5–3 Tesla. Various aspects of ¹H NMR spectroscopy are discussed with regard to in vivo applications. The parameter set [δ, J, n] (δ being the relative chemical shift, J the scalar coupling constant and n the number of coupled spins) is used to characterize the spin systems under investigation and to classify the editing techniques that are used in in vivo ¹H NMR spectroscopy.     

93Nb NMR chemical shift scale for niobia systems

⁹³Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the ⁹³Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the ⁹³Nb NMR chemical shift scale for NbO_x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980 ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360 ppm; the shifts from −1200 to −1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO_x–MO_y polyhedra association has been suggested. The magnitude of the ⁹³Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.     

Proton dynamics in letovicite, (NH4)3H(SO4)2: a 1H and 14N NMR spectroscopic study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.     

高转速下采用S-RESIDOR方法测量1H-27Al偶极偶合常数

提出利用S-RESIDOR(Symmetry-based Resonance Echo Saturation Irradiation DOuble Resonance)方法测量¹H-²⁷Al偶极偶合常数. 通过理论模拟,讨论了四极作用常数、饱和照射脉冲功率、照射时间对¹H/²7Al S-RESIDOR实验的影响. 发现四极作用常数对¹H/²⁷Al S-RESIDOR偶极去相曲线影响较小,中等强度的饱和照射功率即能满足实验要求. 高转速下,在氢型丝光沸石上进行了¹H/²⁷Al S-RESIDOR实验,测量得到Br-nsted酸位上的 ¹H与骨架上²⁷Al的偶极偶合常数为874 Hz.     

Expanding the frequency range of the solid-state T1rho experiment for heteronuclear dipolar relaxation

Solid-state spin–lattice relaxation in the rotating frame permits the investigation of dynamic processes with correlation times in the range of microseconds. The relaxation process in organic solids is driven by the fluctuation of the local magnetic field due to the dipole–dipole interaction of the probe nuclei (¹³C,¹⁵N) with ¹H in close proximity. However, its effect is often hidden by a competing relaxation process due to the contact between the rotating frame ¹³C/¹⁵N Zeeman and ¹H dipolar reservoirs. In most cases the latter process becomes superior for the commonly applied low and moderate spin-lock fields and practically does not provide information about the molecular dynamics. To suppress this undesired process and to expand the dynamic range of T_{1 ρ} experiments, we present two approaches. The first one uses a resonance offset of the frequency of the spin-lock irradiation, which leads to a significant enhancement of the effective spin-lock frequency without the application of destructive high transmitter powers. We derive the theory and demonstrate the applicability of the method on various model compounds. The second approach utilizes heteronuclear ¹H decoupling during the ¹³C/¹⁵N spin-lock irradiation which disrupts the contact between the ¹³C/¹⁵N Zeeman and ¹H dipolar reservoirs. We demonstrate the method and discuss the results qualitatively.     

27Al NMR study in ZrNiAl

We have studied the microscopic properties of the hexagonal ZrNiAl, a model compound for a wide family of intermetallic compounds crystallizing in this type of structure, by using ²⁷Al NMR spectroscopy. We have investigated the lineshape of static and MAS NMR spectra as a function of magnetic field strength (4.7–9.4 T) and temperature (5–300 K). Our data indicate that the ²⁷Al NMR spectra result from a combined effect of quadrupole and anisotropic shift interactions. The ²⁷Al nuclei are in an environment characterized by the quadrupole coupling constant e²qQ/h of 3.3 MHz, asymmetry parameter η_Q of 0.42, isotropic shift δ_iso of 393 ppm, shift anisotropy δ_anis = δ_zz − (δ_xx + δ_yy)/2 of 150 ppm, and asymmetry factor η_S of 0.5. They are found to be temperature independent. The spin–lattice relaxation rate measured at 7.05 T is proportional to the temperature with T₁T = 135 s K. The mechanisms responsible for observed values of δ_iso, δ_anis, T₁T, and the enhanced Korringa constant are discussed.     

H2O/OH ratio determination in hydrous aluminosilicate glasses by static proton NMR and the effect of chemical shift anisotropy

Static ¹H NMR spectra of hydrous NaAlSi₃O₈ glasses have been acquired at low temperature (140 K) in order to quantitatively determine OH and H₂O concentrations. Since both components overlap in the spectra, an unambiguous determination of the line shapes is required. The structurally bonded hydroxyl groups are well described by a Gaussian line and the water molecules exhibit a Pake doublet-like line shape due to the strong proton–proton dipolar interaction. However, at proton resonance frequencies used in this study (360 MHz), the Pake doublet has an asymmetric line shape due to chemical shift anisotropy (CSA), which is significant and must be included in any simulation in order to reproduce the experimental line shape successfully. The simulations for rigid water molecules dissolved in our hydrous aluminosilicate glasses result in a CSA of 30±5 ppm and a dipolar interaction constant of 63.8±2.5 kHz (i.e., dipolar coupling constant (DCC) of 42.6±1.7 kHz), corresponding to a proton–proton distance of r_ij=154±2 pm. In contrast to earlier work, water speciation obtained from the simulations of our ¹H NMR spectra are in excellent agreement with those obtained from infrared (IR) spectroscopy.     

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in ¹H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between ¹H spins is strong. ²⁹Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the ²⁹Si relaxation of relatively pure samples under static and slow-spinning conditions. ²⁷Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between ²⁷Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.     

Quantitative Comparison of REAPDOR and TRAPDOR Experiments by Numerical Simulations and Determination of H–Al Distances in Zeolites

Quantitative H–Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of ¹H{²⁷Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A ²⁷Al offset-dependent data set yields for each resolved ¹H NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring ²⁷Al site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the ¹H–²⁷Al dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of ²⁷Al with respect to the Al–H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them.     

C–H dipolar recoupling under very fast magic-angle spinning using virtual pulses

A new solid-state NMR pulse sequence for recoupling ¹³C–¹H dipolar interactions under magic-angle spinning is proposed, which works under a spinning speed of a few to several tens kilohertz. The sequence is composed of two different frequency switched Lee–Goldburg sequences, and the modulation of the spin part of the ¹³C–¹H dipolar interaction is introduced by a virtual pulse sequence consisting of unitary operators connecting the rotating frame and the tilted rotating frame. When the cycle time of the spinning is equal to or twice the cycle time of the sequence, the ¹³C–¹H dipolar interactions can be recoupled. The sequence is insensitive to experimental imperfections such as rf inhomogeneity or frequency offset, and the resulting lineshape can be represented by a simple analytical equation based on the zeroth-order average Hamiltonian. Experimental results for [2-¹³C] -valine·HCl are reported.     

A microscopic three-cluster model in the hyperspherical formalism

The hyperspherical formalism is applied in the framework of the microscopic generator-coordinate method. Three-cluster systems are described with a single generator coordinate, the hyperradius. This model is a natural extension of previous microscopic theories, and can be applied to bound states and resonances in a range of nuclei exhibiting a cluster structure. As an illustrative example, the model is applied to ⁶He and ⁶Li. Density distributions and electric dipole transition strengths are calculated, including B(E2;0⁺→2⁺) in ⁶He. The results are consistent with a neutron halo structure for the ground state of ⁶He, and with a proton–neutron halo structure of its isobaric analog state in ⁶Li. No 1⁻ “soft dipole” resonance is found in ⁶He.     

应用E.COSY型的13C-1H相关谱准确测定果糖-1,6-二磷酸根离子中异核31P-1H和31P-13C偶合常数

建议了一种E.COSY型的¹³C-¹H相关实验.在相应的¹³C-¹H相关谱中,³¹P核对¹³C,¹H核的被动偶合给出E.COSY型的谱峰裂分,可用于准确测定含磷化合物中的³¹P-¹H和³¹P-¹³C偶合常数及其相对符号.测定了果糖-1,6-二磷酸根离子的³¹P-¹H和³¹P-¹³C偶合常数。     

1,2-二氢-4-苯基(2H)二氮杂萘酮的NMR谱研究

报道了1,2-二氢-2-(4-氨基苯基)-4-[4-(4-氨基苯氧基)苯基](2H)二氮杂萘-1-酮的合成,并用2D同核化学位移相关谱(¹H,¹H-COSY),2DROESY谱,2D异核¹³C-¹H相关谱和2D异核¹³C-¹H多键相关谱(HMBC)对该化合物的¹H和¹³C NMR谱进行了完整归属,证明了此化合物是含二氮杂萘酮结构,而不是二氮杂萘醚结构.     

S-烷基-O-(邻烷氧基羰基苯基)-硫(赶)代磷酰胺的NMR研究

本工作记录了六种通式为H₂N RS P=O-O-COOR'(R=Me,Et,i-Pr,n-Pr;R'=Me,Et,i-Pr)的有机磷农药的¹H、¹³C、³¹P NMR谱及¹³C,¹H异核化学位移相关谱。确定了各¹H、¹³C、³¹P的化学位移值和¹H-¹H、³¹P-¹H、³¹P-¹³C的偶合常数。讨论了核磁参数与分子结构和运动以及³¹P化学位移与生物活性之间的关系。     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

J-modulated ADEQUATE (JM-ADEQUATE) experiment for accurate measurement of carbon-carbon coupling constants

A new method for the accurate determination of carbon-carbon coupling constants is described. The method is based on a modified ADEQUATE experiment, where a J-modulated spin-echo sequence precedes the ADEQUATE pulse scheme. The J-modulation and scaling of carbon-carbon couplings is based on simultaneous incrementation of 13C chemical shift and coupling evolution periods. The time increment for the homonuclear carbon-carbon coupling evolution can be suitably scaled with respect to the corresponding increment for the chemical shift evolution. Typically a scaling factor of 2 to 3 is employed for the measurement of one-bond coupling constants, while multiplication by a factor of 10 to 15 is applied when small long-range couplings are determined. The same pulse scheme with coupling evolution period optimized for one-bond or long-range couplings allows the determination of the corresponding carbon-carbon coupling constants. The splittings of the ADEQUATE crosspeaks in the F1 dimension yield the appropriately multiplied coupling constants.     

H NMR study of YT2-hydride (T = Fe, Mn)

In the hydrides YMn₂H_3.0, the nuclear magnetic resonance of ¹H nuclei has been measured under external fields up to 50 kOe at 4.2 K and three ¹H NMR signals have been observed. The ¹H hyperfine fields corresponding to these NMR signals can be discussed in terms of the dipole interaction between the H nuclei and the localized moments of the Mn(or Fe) atoms surrounding H atoms.     

一个A环开环型芳基萘内酯类木脂素2D NMR全归属

采用二维核磁共振技术(¹H-¹H COSY, DEPT, HSQC, HMBC, NOESY)和CD光谱,确定了A环开环型芳基萘内酯类木脂素3′, 4′-O, O-demethylene-4-O- demethylpodophyllotoxin (1)的结构. 首次对化合物1的¹³C NMR数据进行了全归属.     

59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO2 and HCoO2

⁵⁹Co and ²³Na NMR has been applied to the layered cobalt oxides NaCoO₂ and HCoO₂ at three different magnetic field strengths (4.7, 7.1 and 11.7 T). The ⁵⁹Co and ²³Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large ⁵⁹Co quadrupole interaction in NaCoO₂, a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the ⁵⁹Co and ²³Na shift and quadrupole coupling tensors in NaCoO₂ are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the ⁵⁹Co NMR spectra clearly show the presence of structural disorder in HCoO₂. The ²³Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO₂ is due to Van Vleck paramagnetism for Co³⁺. Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.