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1.
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.  相似文献   

2.
An improved spectral editing method for solids is described which allows one to obtain a set of subspectra in roughly two-thirds the amount of time as our original CPPI editing method for the same signal to noise. This improvement is afforded by a new pulse sequence that is used to acquire a 13CH + 13CH2 spectrum which has very little 13CH3 or nonprotonated carbon contamination. By using this new sequence the 13CH-only subspectrum is obtained much more efficiently. Criteria for optimizing the signal to noise in the edited subspectra are discussed.  相似文献   

3.
New pulse sequences for the selective excitation and editing of NMR spectra of protons directly bonded to carbon-13 in CH, CH2, and CH3 groups are described. Experimental results as well as theoretical calculations demonstrate that these sequences have excitation profiles of about ±0.5JCH or less and require no pulse shaping or special hardware to implement. After the isotope-directed excitation of the protons has been generated, the proton magnetization may be relayed to other protons via an isotropic mixing sequence to yield 1D HOHAHA subspectra or 2D COSY subspectra. Examples of applications to compounds containing carbon-13 either at natural abundance or with enrichment are shown.  相似文献   

4.
Novel procedures for the spectral assignment of peaks in high-resolution solid-state 13C NMR are discussed and demonstrated. These methods are based on the observation that at moderate and already widely available rates of magic-angle spinning (10–14 kHz MAS), CH and CH2 moieties behave to a large extent as if they were effectively isolated from the surrounding proton reservoir. Dipolar-based analogs of editing techniques that are commonly used in liquid-state NMR such as APT and INEPT can then be derived, while avoiding the need for periods of homonuclear 1H–1H multipulse decoupling. The resulting experiments end up being very simple, essentially tuning-free, and capable of establishing unambiguous distinctions among CH, CH2, and carbon sites. The principles underlying such sequences were explored using both numerical calculations and experimental measurements, and once validated their editing applications were illustrated on a number of compounds.  相似文献   

5.
The vibrational spectra of gaseous and liquid 2‐propanol in the C–H stretching region of 2800 ~ 3100 cm−1 were investigated by polarized photoacoustic Raman spectroscopy and conventional Raman spectroscopy, respectively. Using two deuterated samples, that is, CH3CDOHCH3 and CD3CHOHCD3, the overlapping spectral features between the CH and CH3 groups were identified. With the aid of depolarization ratio measurements and density functional theory calculations, a new spectral assignment was presented. In the gas phase, the band at 2884 cm−1 was assigned to the overlapping of one CH3 Fermi resonance mode and a CH stretching of gauche conformer. The bands at 2917 and 2933 cm−1 were assigned to another two CH3 Fermi resonance modes, but the latter includes weak contribution from CH stretching of trans conformer. The bands at 2950 and 2983 cm−1 were assigned to CH3 symmetric and antisymmetric stretching, respectively. The spectral features of liquid 2‐propanol are similar to those in the gas phase except for the blue shift of CH and the red shift of CH3 band positions, which can be attributed to the intermolecular interaction in the liquid state. The new assignments not only clarify the confusions in previous studies from different spectral methods but also provide the reliable groundwork on spectral application of 2‐propanol in the futures. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
偶合常数是一个重要的NMR参数,其数值与分子中化学键的二面角有关,可以为分子结构研究提供很重要的信息.多维NMR谱由于具有较大的化学位移分辨率,因此常常被用来测定同核或异核自旋-自旋偶合常数.本文介绍了利用最高量子相关技术(MAXY)测定同核偶合常数的方法.MAXY是最近发展的一种多维NMR谱编辑技术,可以使不同官能团(CH, CH  相似文献   

7.
Methods for editing spectra based upon maximum-quantum filtering in two-dimensional 1H NMR are presented (MAXY NMR). Separation of 1H resonances from 13CH, 13CH2, and 13CH3 groups is demonstrated, using the coherence of the attached natural-abundance 13C spin. Two-dimensional correlation pulse sequences based on J connectivity (MAXY-COSY), total J connectivity (MAXY-TOCSY), and NOE and exchange processes (MAXY-NOESY) are given and exemplified using dexamethasone as a model compound. In addition, an improved form of a 13CH2 only COSY spectrum (gem-COSY) is shown, and the application of z magnetic-field gradients is demonstrated as an alternative to phase cycling. The approach should have utility in the assignment of complex 1H NMR spectra which arise from peptides or complex mixtures such as biofluids.  相似文献   

8.
13C-NMR spectra of several 9-acridanones with different substituents both on the ring (R1 = CH3, OCH3, NH2, N(CH3)2, NO2) and at the nitrogen atom (R2 = H, CH3 C2H5, CH2-C6H5, C[tbnd]C-CH3, (CH2)2N(C2H2)2, CH=C=CH2) have been recorded. The C-NMR chemical shifts are discussed as a function of the nature of the substituent, the importance of peri steric interactions and the electronic structure of the acridanone ring. There is a good linear relationship between the total electronic density and the chemical shifts.  相似文献   

9.
Electron energy loss spectra of the methyl halides in the region of carbon K-shell excitation have been obtained at higher resolution than those previously reported. The existence of two electronic transitions between 288 and 290 eV in CH3F is demonstrated. This result conflicts with a recent SCF calculation which suggests that the σ* (C–F) level in CH3F is unbound and thus predicts the existence of only one electronic transition in this spectral region. Studies of the carbon K-shell spectra of CH3Br and CD3Br demonstrate the vibrational origin of some spectral features. These new results support our earlier tentative assignments for the carbon K-shell spectra of the methyl halides.  相似文献   

10.
The 200 MHz H NMR spectra of the herbicide, fenoxaprop-ethyl, 1, have been studied in CDCl3 solution at ambient temperatures as the racemic ester free base with the added chiral lanthanide shift reagent (LSR), tris [3 - (heptafluoropropylhydroxymethylene) - (+) - camphorato]europium(III), Eu(HFC)3, 2, with some additional runs using the achiral LSR, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium(III), Eu(FOD)3, 3, for supplemental spectral simplification. With 2, definite enantiomeric shift differences (ΔΔδ) were observed only for the CH3CH2O resonance. Although increased spectral complexity for the CH2O signal was seen with added 2, this could have resulted from anisochrony of the diastereotopic protons, Ha and Hb, of this methylene group rather than true ΔΔδ, since the achiral 3 led to near-baseline separation between the CHaHbO signals. Lanthanide-induced shift (LIS) magnitudes were interpreted as consistent with predominant LSR binding at the ester carbonyl.  相似文献   

11.
Effect of composition, temperature and radiation dose in gamma irradiated acrylamide-2-acrylamido-2-methyl propane sulphonic acid (AA) copolymer has been investigated by electron spin resonance (ESR) and fourier transform infrared (FTIR) techniques. ESR spectra of gamma irradiated AA copolymer have been recorded under different conditions. The observed ESR spectra are analysed by computer simulation techniques, to separate the constituent component spectra. Magnetic parameters employed to simulate the component spectra enabled the identification of corresponding free radicals. The AA copolymer with low acrylamide content composed of macroradicals of the type ?CH2?CH?CH2? and methyl radicals (CH3) whereas the copolymer with high acryl amide content possess methyl radicals and radicals of the type ?CH2?C(CONH2)?CH2?/CH3?C?CH3. Reasons for the variation in the formation of free radicals have been explained. The observed changes in ESR spectra of irradiated AA copolymer at higher temperatures are thought to be due to the recombination of free radicals. Formation of free radicals found to be enhanced with the increase in dose of irradiation. FTIR spectra of pure and irradiated copolymers have also confirmed the previous results.  相似文献   

12.
13C MRS studies at natural abundance and after intravenous 1-13C glucose infusion were performed on a 1.5-T clinical scanner in four subjects. Localization to the occipital cortex was achieved by a surface coil. In natural abundance spectra glucose C3β,5β, myo-inositol, glutamate C1,2,5, glutamine C1,2,5, N-acetyl-aspartate C1-4,C=O, creatine CH2, CH3, and CC=N, taurine C2,3, bicarbonate HCO3 were identified. After glucose infusion 13C enrichment of glucose C1α,1β, glutamate C1-4, glutamine C1-4, aspartate C2,3, N-acetyl-aspartate C2,3, lactate C3, alanine C3, and HCO3 were observed. The observation of 13C enrichment of resonances resonating at >150 ppm is an extension of previously published studies and will provide a more precise determination of metabolic rates and substrate decarboxylation in human brain.  相似文献   

13.
The 17O NMR spectra for a series of saturated diols were investigated. From these studies both hydroxyl induced substituent chemical shift (SCS) effects of hydroxyl oxygen 17O NMR chemical shifts were determined. In addition, linear correlations between the 17O chemical shift of the hydroxyl oxygen (ROH) and the 13C chemical shift for the methyl group in the corresponding hydrocarbon (RCH3) were obtained.

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14.
An improved spectral editing method for solids is described which allows one to obtain a set of subspectra in roughly two-thirds the amount of time as our original CPPI editing method for the same signal to noise. This improvement is afforded by a new pulse sequence that is used to acquire a (13)CH + (13)CH(2) spectrum which has very little (13)CH(3) or nonprotonated carbon contamination. By using this new sequence the (13)CH-only subspectrum is obtained much more efficiently. Criteria for optimizing the signal to noise in the edited subspectra are discussed. Copyright 2000 Academic Press.  相似文献   

15.
A pulse sequence DEPTQ yielding the signals and multiplicity information for all carbon types including the signals of quaternary carbons and encompassing all the known advantages of the basic DEPT experiment is proposed. Its behavior has been studied theoretically and experimentally and has been compared critically with alternative methods dedicated for the same purpose. Its marked insensitivity to experimental parameters and its potential for complete and efficient spectral editing makes DEPTQ the ideal experimental platform for a semi- or fully automated analysis of 1D13C spectra.  相似文献   

16.
Metabolic imaging of hyperpolarized [1-13C] pyruvate co-polarized with [13C]urea by dynamic nuclear polarization with rapid dissolution is a promising new method for assessing tumor metabolism and perfusion simultaneously in vivo. Novel pulse sequences are required to enable dynamic imaging of multiple 13C spectral lines with high spatiotemporal resolution. The goal of this study was to investigate a new frequency-specific approach for rapid metabolic imaging of multiple 13C resonances using the spectral selectivity of steady-state free precession pulse (SSFP) trains. Methods developed in simulations were implemented in a dynamic frequency-cycled balanced SSFP pulse sequence on a 14.1-T animal magnetic resonance imaging scanner. This acquisition was tested in thermal and hyperpolarized phantom imaging studies and in a transgenic mouse with prostate cancer.  相似文献   

17.
With an improved version of the 13C DEPTQ NMR experiment all carbon multiplicities (Cq, CH, CH2 und CH3) can be identified unequivocally and most conveniently in two experiments and at best with only one scan each. This simplifies the analysis of 13C-NMR data on a routine level in general. With higher sample amounts and/or exploiting the high sensitivity of cryogenically cooled 13C probeheads the efficiency of such investigations may be improved. This makes this method attractive for fast 13C analysis of small-to-medium sized molecules in high-throughput laboratories.  相似文献   

18.
A suite of 3D NMR experiments for measuring15N–{1H} NOE,15NT1, and15NTvalues in large proteins, uniformly labeled with15N and13C, is presented. These experiments are designed for proteins that exhibit extensive spectral overlap in the 2D1H–15N HSQC spectrum. The pulse sequences are readily applicable to perdeuterated samples, which increases the spectral resolution and signal-to-noise ratio, thereby permitting the characterization of protein dynamics to be extended to larger protein systems. Application of the pulse sequences is demonstrated on a perdeuterated13C/15N-labeled sample of the 44 kDa ectodomain of SIV gp41.  相似文献   

19.
Localized 1H NMR spectroscopy using the 90°−t1−180°−t1+t2−180°−t2−Acq. PRESS sequence can lead to a signal loss for the lactate doublet compared with signals from uncoupled nuclei which is dependent on the choice of t1 and t2. The most striking signal loss of up to 78% of the total signal occurs with the symmetrical PRESS sequence (t1=t2) at an echo time of 2/J (290 ms). Calculations have shown that this signal loss is related to the pulse angle distributions produced by the two refocusing pulses which leads to the creation of single quantum polarization transfer (PT) as well as to not directly observable states (NDOS) of the lactate AX3 spin system: zero- and multiple-quantum coherences, and longitudinal spin orders. In addition, the chemical shift dependent voxel displacement (VOD) leads to further signal loss. By calculating the density operator for various of the echo times TE=n/J, n=1, 2, 3, …, we calculated quantitatively the contributions of these effects to the signal loss as well as their spatial distribution. A maximum signal loss of 75% can be expected from theory for the symmetrical PRESS sequence and TE=2/J for Hamming filtered sinc pulses, whereby 47% are due to the creation of NDOS and up to 28% arise from PT. Taking also the VOD effect into account (2 mT/m slice selection gradients, 20-mm slices) leads to 54% signal loss from NDOS and up to 24% from PT, leading to a maximum signal loss of 78%. Using RE-BURP pulses with their more rectangular pulse angle distributions reduces the maximum signal loss to 44%. Experiments at 1.5 T using a lactate solution demonstrated a maximum lactate signal loss for sinc pulses of 82% (52% NDOS, 30% PT) at TE=290 ms using the symmetrical PRESS sequence. The great signal loss and its spatial distribution is of importance for investigations using a symmetrical PRESS sequence at TE=2/J.  相似文献   

20.
Electron impact energy loss spectra at an impact energy of 100 eV and a scattering angle of 2 degrees are presented for propenal (CH2 = CHCHO) and methyl vinyl ketone (CH2 = CHCOCH3). The spectra are tentatively assigned using quantum defects and term values derived from ionization potentials measured by photoelectron spectroscopy. The duality of Rydberg and valence transition assignments in propenal is discussed. It is shown that a transition earlier assigned by Walsh to be π → π* is most probably a Rydberg transition.  相似文献   

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