On the 13C CP/MAS spectra of leucine.

The CP/MAS 13C NMR spectra of crystalline L-leucine and DL-leucine at 7 T are compared with previously reported spectra at lower field strengths. An increasing dominance of chemical shift effects over residual 14N-13C dipolar interactions is observed on the C alpha and C beta splittings with increasing field strength. A new structure is observed in the 25 ppm region of both samples. The spectra in this region were assigned by application of the depolarisation-repolarisation method. The assignment showed differences in the ordering of peaks between solid state and liquid state chemical shifts.     

Modified spectral editing methods for (13)C CP/MAS experiments in solids

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH(2), and CH(3) types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH(3) peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH(3) carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH(2) only spectra, respectively, can be acquired directly from combining so called (+) and (-) sequences using different spectral delay periods and pulse parameters. The (+) and the (-) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH(3) and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.     

Quantitative characterization of coal structure by high-resolution CP/MAS 13C solid-state NMR spectroscopy

A method for quantitatively characterizing the carbon skeletal structure of coal by variable contact time experiment using high-resolution CP/MAS ¹³C solid-state NMR spectroscopy is proposed in this paper. The initial polarization transfer intensity from protons directly bonded with carbons, instead of dipolar-dephasing techniques which had to run on a lower frequency NMR spectrometer (100.02 MHz for proton), was used to divide the bridgehead and protonated aromatic carbons, making all the NMR data in this paper obtained on a high frequency NMR spectrometer (500.12 MHz for proton). On this basis, the fractions of different carbons in coal were further divided by the initial polarization transfer intensity from spin diffusion of protons unbonded with carbons. The structure of Naomaohu coal, a subbituminous coal from China, was measured. The change of polarization transfer with contact time was analyzed quantitatively. The fractions of aromatic, aliphatic, carboxyl and carbonyl carbons, and corrective aromaticity are 0.61, 0.39, 0.1 and 0.51, respectively. The fractions of protonated and bridgehead aromatic carbons are 0.22 and 0.09, respectively. These results agreed with literatures, and bond concentration calculated by the carbon skeletal structure distribution of coal was reasonable.     

~(13)C CP/MAS OF MOTIONS IN SOLID POLYMERS

High-resolution 13C spectra of solid polymers are often acquired with cross polarization with magic angle spinning (CP/MAS) to take advantage of the abunda ntly available protons in the polymer chains. The sequence for transferring mag netization from 1H to13C using Hartmann-Hahn spin locking in the rotating frame is now a standard in commercial solidstate NMR spectrometers.     

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.     

A 4-mm probe for (13)C CP/MAS NMR of solids at 21.15 T

The design of a broadband 4-mm magic-angle spinning (MAS) X-(1)H/(19)F double resonance probe for cross-polarization (CP)/MAS NMR studies at 21.15 T ((1)H at 900 MHz) is described. The high-frequency (1)H/(19)F channel employs a new and efficient transmission line tuning design. The first (13)C CP/MAS NMR spectra recorded at 21.15 T have been obtained with this probe and exhibit the best S/N per milligram sample of hexamethylbenzene achieved so far for a 4-mm rotor.     

Structural and 14N EFG Information on Solid Imidazole by 13C CP/MAS NMR Data

Residual dipolar coupling between carbons and ¹⁴N nuclei in the ¹³C CPMAS NMR spectrum of solid imidazole is studied. Calculations of expected splittings with a previously reported equation leads to the complete assignment of the solid state carbon chemical shifts. Additionally, information is provided on the location of ¹⁴N electric field gradient axes at the N-H site.     

13C CP MAS NMR and GIAO-CHF calculations of coumarins

13C cross-polarization magic-angle spinning NMR spectra were recorded for a series of solid coumarins. Ab initio calculations of shielding constants were performed with the use of GIAO-CHF method. The combined CPMAS NMR and theoretical approach was successful in characterizing solid-state conformations of coumarins; a relationship sigma (ppm) = -1.032 xdelta + 205.28 (R(2) = 0.9845) can be used to obtain structural information for coumarins, for which solid-state NMR or crystal structure data are not available.     

Selective saturation and inversion of multiple resonances in high-resolution solid-state 13C experiments using slow spinning CP/MAS and tailored DANTE pulse sequences.

Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.     

Experimental method for low-temperature reaction studies in sealed samples by 13C CP/MAS NMR

A new, simple, and inexpensive technique is presented for monitoring high-resolution solid-state NMR of ¹³C at temperatures ranging between 85 and 450 K. In this procedure, the reaction conditions are controlled by preparing samples at 77 K in 5 mm NMR tubes, while attached to a vacuum system. The NMR tubes are prefitted with a rotor for spinning. After preparation, the samples are sealed, transferred to the double-resonance MAS NMR probe, and analyzed, all while the sample temperature is maintained as low as 85 K. The spinning rates vary from 3.0 kHz at 85 K to 5.2 kHz at 300 K using nitrogen drive gas. Probe design and performance, sample-preparation procedure, and details of the low-temperature experiment are described. In general, the technique may be applied in studies of low-temperature reaction mechanisms and kinetics. ¹³C CP/MAS spectra of ethylene adsorbed on silica-supported ruthenium catalyst are presented to illustrate its performance and possible application.     

Crystal structure and 13C CP/MAS NMR of the p-xylene clathrate of Dianin's compound

Single-crystal X-ray diffraction and ¹³C solid state NMR spectroscopy were used to characterize the structure and dynamics of the p-xylene clathrate of Dianin's compound. In contrast to conclusions obtained from single-crystal ²H NMR and modelling, the diffraction results suggest there is a single (symmetry disordered) guest site without any significant distortion of the host framework. A single xylene guest statistically disordered over six overlapping equivalent positions can account for the ¹³C NMR spectrum at room temperature. The high crystal-lographic symmetry arises from space averaging. At high temperatures the ¹³C spectrum is consistent with the onset of dynamic processes that result in higher effective symmetry.     

MAS NMR studies of carbon-13 spin exchange in durene

One- (1-D) and two-dimensional (2-D) carbon-13 NMR exchange measurements in powder samples of isotopically normal durene under magic angle spinning (MAS) are reported. The experiments include rotor synchronized 2-D exchange (RS2DE), 1-D magnetization transfer (MT) and time reverse ODESSA (tr-ODESSA). The latter two experiments were performed as a function of several external parameters, including proton decoupling field during mixing time, sample spinning rate and partly, of temperature. The effects of these parameters on the spin exchange induced by spin diffusion and by chemical, or physical exchange, is discussed. Spin exchange between all types of carbons in the durene molecules occurs on the time scale of seconds. From the dependence of the spin exchange rate on the external parameters it is concluded that the process is dominated by spin diffusion. On the basis of these results an upper limit of 10(-16) cm2 s(-1) can be set for the self-diffusion constant in crystalline durene.     

Cross-polarisation to low-gamma nuclei: the first 183W CP/MAS spectra.

The possibility to obtain 183W CP/MAS spectra has been explored. The choice of suitable set-up compounds, convenient set-up procedures and experimental considerations with respect to contact times, TIS and T1 rho interplay, are discussed.     

Efficient spin-spin scalar coupling mediated C-13 homonuclear polarization transfer in biological solids without proton decoupling

We demonstrate that an efficient C′↔C polarization transfer based on J-coupling can be realized under fast magic-angle spinning (MAS) condition without ¹H decoupling. Experimental results are presented for model crystalline compounds as well as a non-crystalline 17-residue polypeptide MB(i+4)EK. Measurements on MB(i+4)EK demonstrate that 53% of the initial C′ polarization was transferred to the cross peaks at 7.05 T under 25 kHz MAS spinning.     

固体中13C CP/MAS谱编辑一些改进

本文将基本的谱编辑脉冲序列与消旋转边带的TOSS脉冲序列联用,可以实现强磁场下¹³C CP/MAS谱编辑.通常,固体中非质子化碳与甲基碳的自旋晶格弛豫时间差别很大,此差别可作为谱编辑过程中区分它们的一条附加判据.对具有复杂分子结构的体系,此判据的作用尤其重要.文中的实例将表明:一个完整的¹³C CP/MAS谱编辑一般可通过SCPTOSS谱(短接触时间交叉极化谱);SCPPITOSS (短接触时间交叉极化极化反转)谱,LCPDTOSS (长接触时间交叉极化及去极化)谱及部分弛豫的LCPDTOSS谱适当组合而得到.     

CP MAS 13C spectral editing and relative quantitation of a soil sample

A hydrofluoric acid (HF)-treated soil sample was studied by 13C NMR spectroscopy. Cross polarization (CP) Magic Angle Spinning (MAS) 13C spectral editing and relative CP peak quantitation, obtained through variable-contact-time experiments, were used to aid the interpretation of the spectrum. The combination of these two types of experiment allowed to obtain a higher degree of detail on the composition of the sample with respect to a standard CP MAS experiment.     

Recoupled polarization transfer heteronuclear 1H-13C multiple-quantum correlation in solids under ultra-fast MAS.

A new approach for high-resolution solid-state heteronuclear multiple-quantum MAS NMR spectroscopy of dipolar-coupled spin-12 nuclei is introduced. The method is a heteronuclear chemical shift correlation technique of abundant spins, like 1H with rare spins, like 13C in natural abundance. High resolution is provided by ultra-fast MAS and high magnetic fields, high sensitivity being ensured by a direct polarization transfer from the abundant protons to 13C. In a rotor-synchronized variant, the method can be used to probe heteronuclear through-space proximities, while the heteronuclear dipolar coupling constant can quantitatively be determined by measuring multiple-quantum spinning-sideband patterns. By means of recoupling, even weak heteronuclear dipolar interactions are accessible. The capabilities of the technique are demonstrated by measurements on crystalline L-tyrosine hydrochloride salt.     

Effect of strains on the indirect nuclear spin-spin coupling in platinum

Using c.w. and field-cycling NMR methods we show that cold working of platinum metal results in a substantial increase in the Pt¹⁹⁵ linewidth δH with increasing magnetic field and in its zero field intercept. The latter effect is deduced to be caused by a change in the non scalar part of the indirect spin-spin interaction. A tight-binding approach is used to estimate the magnitude of the indirect couplings. One can only surmise that strain induced symmetry lowering removes destructive interference between terms contributing to the non scalar interaction and produces the observed increase in δH at H = 0.     

Modified Spectral Editing Methods for C CP/MAS Experiments in Solids

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH₂, and CH₃ types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH₃ peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH₃ carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH₂ only spectra, respectively, can be acquired directly from combining so called (+) and (−) sequences using different spectral delay periods and pulse parameters. The (+) and the (−) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH₃ and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.