乙丙共聚物的13C NMR研究

研究用化学反应法制备的镁钛系载体高效催化剂合成的乙丙共聚物的¹³C NMR谱图,提出了无需进行分峰处理,通过将谱图直接分为八个独立的区域,并选取其中受仪器测试条件等影响较小的五个谱区的强度,直接计算此类共聚物序列分布的方法,并分别采用伯努利模型和马尔科夫模型对共聚物的三单元组序列分布进行处理,指出在化学反应法制备的载体催化剂催化的乙丙共聚合扫次末端基效应机理进行,且催化剂活性中心有利于乙丙各自的均聚合.     

13C PGSE NMR experiment with heteronuclear dipolar decoupling to measure diffusion in liquid crystals and solids

A new PGSE NMR experiment, designed to measure molecular diffusion coefficients in systems with nonvanishing static dipolar coupling, is described. The fast static dipolar dephasing of the single-quantum (13)C coherences is removed by multiple-pulse heteronuclear decoupling. The resulting slow dephasing of the (13)C coherences allows for inserting appropriate gradient pulses into the pulse sequence. The presence of the large magnetic field gradient reduces the efficiency of the decoupling sequences which is compensated for by introducing a scheme of sequential slice selection across the sample. The method is demonstrated by (19)F-decoupled (13)C PGSE NMR experiments in a lyotropic nematic and lamellar liquid crystal.     

取代基效应（SCS）与乙烯—α—烯烃共聚物的序列结构：Ⅰ,乙烯—丙烯共聚物

本文应用取代基效应方法处理乙丙共聚物的~(13)C NMR谱,第一次得到这个共聚物(CH_3)的取代基参数,在此基础上归属了乙丙共聚物的~(13)C谱,讨论了共聚物序列结构的几种表示方法,进而用一套普适的公式计算了几个共聚物的序列分布,并作了比较。     

Suppression of artifacts induced by homonuclear decoupling in amino-acid-type edited methyl 1H-13C correlation experiments

A detailed theoretical and experimental analysis of the artifacts induced by homonuclear band-selective decoupling during CT frequency labeling is presented. The effects are discussed in the context of an amino-acid-type editing filter implemented in (1)H-(13)C CT-HSQC experiments of methyl groups in proteins. It is shown that both Bloch-Siegert shifts and modulation sidebands are efficiently suppressed by using additional off-resonance decoupling as proposed by Zhang and Gorenstein [J. Magn. Reson. 132 (1998) 81], and appropriate adjustment of a set of pulse sequence parameters. The theoretical predictions are confirmed by experiments performed on (13)C-labeled protein samples, yielding artifact-free amino-acid-type edited methyl spectra.     

The effectiveness of 1H decoupling in the 13C MAS NMR of paramagnetic solids: an experimental case study incorporating copper(II) amino acid complexes

The use of continuous-wave (CW) 1H decoupling has generally provided little improvement in the 13C MAS NMR spectroscopy of paramagnetic organic solids. Recent solid-state 13C NMR studies have demonstrated that at rapid magic-angle spinning rates CW decoupling can result in reductions in signal-to-noise and that 1H decoupling should be omitted when acquiring 13C MAS NMR spectra of paramagnetic solids. However, studies of the effectiveness of modern 1H decoupling sequences are lacking, and the performance of such sequences over a variety of experimental conditions must be investigated before 1H decoupling is discounted altogether. We have studied the performance of several commonly used advanced decoupling pulse sequences, namely the TPPM, SPINAL-64, XiX, and eDROOPY sequences, in 13C MAS NMR experiments performed under four combinations of the magnetic field strength (7.05 or 11.75T), rotor frequency (15 or 30kHz), and 1H rf-field strength (71, 100, or 140kHz). The effectiveness of these sequences has been evaluated by comparing the 13C signal intensity, linewidth at half-height, LWHH, and coherence lifetimes, T2('), of the methine carbon of copper(II) bis(dl-alanine) monohydrate, Cu(ala)(2).H2O, and methylene carbon of copper(II) bis(dl-2-aminobutyrate), Cu(ambut)(2), obtained with the advanced sequences to those obtained without 1H decoupling, with CW decoupling, and for fully deuterium labelled samples. The latter have been used as model compounds with perfect 1H decoupling and provide a measure of the efficiency of the 1H decoupling sequence. Overall, the effectiveness of 1H decoupling depends strongly on the decoupling sequence utilized, the experimental conditions and the sample studied. Of the decoupling sequences studied, the XiX sequence consistently yielded the best results, although any of the advanced decoupling sequences strongly outperformed the CW sequence and provided improvements over no 1H decoupling. Experiments performed at 7.05T demonstrate that the XiX decoupling sequence is the least sensitive to changes in the 1H transmitter frequency and may explain the superior performance of this decoupling sequence. Overall, the most important factor in the effectiveness of 1H decoupling was the carbon type studied, with the methylene carbon of Cu(ambut)(2) being substantially more sensitive to 1H decoupling than the methine carbon of Cu(ala)(2).H2O. An analysis of the various broadening mechanisms contributing to 13C linewidths has been performed in order to rationalize the different sensitivities of the two carbon sites under the four experimental conditions.     

Synchronized adiabatic decoupling

A new decoupling scheme termed "synchronized adiabatic decoupling" is developed for use in the indirectly detected dimension. After each increment, the decoupling sequence is replaced by another one with different period T or different initial period T(ini) so that sampling always occurs at the end of a complete decoupling period. The effects of J coupling are therefore completely averaged out for all data points. As a result, all decoupling sidebands disappear and the center band increases correspondingly. Since the synchronized adiabatic decoupling does not require conventional editing techniques to cancel the sidebands, it is useful in high-field gradient-enhanced multidimensional experiments with only a single scan per increment.     

高分子共聚物组成的13C-NMR测定

用DEPT编辑谱方法确定了丙烯酸丁酯-醋酸乙烯酯-丙烯酸共聚物的¹³C-NMR谱线归属,采用门控去偶,增加延迟时间,减小脉冲翻转角的方法,用¹³C-NMR定量地测定了其主要组成。     

Improvement of the inverse-gated-decoupling sequence for a faster quantitative analysis of various samples by 13C NMR spectroscopy

The inverse-gated-decoupling sequence enables quantitative (1)H decoupled (13)C spectra to be obtained. We modified this sequence so as to obtain the same result in less time for molecules containing carbons with various relaxation properties. For that, we determined the optimal (13)C longitudinal-magnetization initial value for a faster relaxation while (1)H decoupler is stopped. This value can be calculated precisely via the nuclear Overhauser effects, the longitudinal relaxation times, together with the determination of the relaxation rate constants of carbons while (1)H are out of equilibrium. A supplementary delay of (1)H decoupling and/or a series of selective pulses applied at the beginning of the recovery delay allow an acceleration of (13)C longitudinal relaxation. We applied this method to the molecule of vanillin. The simultaneous quantification of all carbons was carried out with a recovery delay divided by two compared to the usual sequence.     

Suppression of artifacts induced by homonuclear decoupling in amino-acid-type edited methyl H–C correlation experiments

A detailed theoretical and experimental analysis of the artifacts induced by homonuclear band-selective decoupling during CT frequency labeling is presented. The effects are discussed in the context of an amino-acid-type editing filter implemented in ¹H–¹³C CT-HSQC experiments of methyl groups in proteins. It is shown that both Bloch–Siegert shifts and modulation sidebands are efficiently suppressed by using additional off-resonance decoupling as proposed by Zhang and Gorenstein [J. Magn. Reson. 132 (1998) 81], and appropriate adjustment of a set of pulse sequence parameters. The theoretical predictions are confirmed by experiments performed on ¹³C-labeled protein samples, yielding artifact-free amino-acid-type edited methyl spectra.     

Complete Dipolar Decoupling ofC and Its Use in Two-Dimensional Double-Quantum Solid-State NMR for Determining Polymer Conformations

A multiple-pulse technique for complete dipolar decoupling of directly bonded¹³C-labeled sites is described. It achieves significant spectral simplifications in a recently introduced two-dimensional double-quantum solid-state NMR experiment for determining torsion angles. Both homonuclear and heteronuclear dipolar couplings are removed by combining a¹³C multiple-pulse sequence with continuous-wave irradiation on the protons. The¹³C sequence has a fundamental 10-pulse cycle which is a significantly modified magic-sandwich-echo sequence. The crucial heteronuclear decoupling is achieved by breaking the 360° “inner” pulses in the magic sandwich into 90° pulses and spacing them by¹H 360° pulse lengths. Spectral artifacts typical of multiple-pulse sequences are eliminated by phase shifts between cycles. In contrast to many other multiple-pulse decoupling sequences, the long window in the cycle is the dwell time and can be longer than the inverse dipolar coupling, which makes the sequence practical for direct detection even with long pulse ring-down times. A modification of the sequence to scale the chemical shift and increase the effective spectral width is also presented. The 1D and double-quantum 2D experiments are demonstrated on polyethylene with 4%¹³C–¹³C spin pairs. The potential of this approach for distinguishing segmental conformations is illustrated by spectral simulations of the two-dimensional ridge patterns that correlate double-quantum and single-quantum chemical-shift anisotropies.     

Adiabatic decoupling sidebands

An analytical solution is given for amplitudes and phases of adiabatic decoupling sidebands as a function of spin inversion time tau. Since all the adiabatic decoupling phases theta(t, tau) refocus at two periods (2T) of the decoupling pulse, the sidebands are located at n/2T rather than at n/T as observed in other decoupling schemes. The real (R(n)(tau)) and imaginary (I(n)(tau)) amplitudes of the sidebands have symmetry R(n)(tau) = R(-n)(tau) and I(n)(tau) = -I(-n)(tau), forming a mirror image between the counterparts of the sidebands. When frequency sweep changes direction all I(n)(tau) are inverted while all R(n)(tau) remain unchanged, leading to pure absorption sidebands with two accumulations as demonstrated by Kupce and Freeman, and to an exchange of sidebands between counterparts. The sum of the real parts for sidebands n = 1 and 2 is almost a constant near on-resonance decoupling, and it increases substantially for large decoupling offsets. The phase defocusing can be minimized for all decoupling offsets by inserting an initial decoupling period with T(ini) = T/2, eliminating all sidebands located at n/2T (n = +/-1, +/-3, +/-5, ...).     

取代基效应(SCS)与乙烯-α-烯烃共聚物的序列结构——7.氢化丁苯共聚物(HBS)

测定了两个氢化丁苯共聚物样品的¹H、¹³C NMR谱。通过处理文献中的¹³C NMR谱数据得到了-C₂H₅和-C₆H₅的SCS参数。利用乙烯-α-烯烃共聚物的SCS方法结合DEPT NMR实验技术重新归属了该共聚物的¹³C NMR谱。明确指出,氢化丁苯共聚物分子链中主链任何一个乙烯单元的两个CH₂在序列分布上分属于二元组和三元组。CH则用三元组来表征。最后,对HBS共聚物的序列分布进行了计算。     

甲基丙烯酸甲酯-萘乙烯共聚物的核磁共振研究──1.甲基丙烯酸甲酯-萘乙烯共聚物的2D NMR谱研究

用500MHz超导核磁共振谱仪测定了甲基丙烯酸甲酯-萘乙烯共聚物的二维异核多量子化学位移相关谱和二维相敏NOESY谱，由此归属了共聚物的碳谱和氢谱，结果表明，聚合物是以无规共聚物为主，其中有一部分是以头一头（或尾一尾）相接的．文中还以反门控去偶测得其共聚含量．     

"BEST" homonuclear adiabatic decoupling for 13C- and 15N-double-labeled proteins.

The cyclic irradiation sidebands appearing in homonuclear adiabatic decoupling are calculated in detail, which reveals the origin of the antisymmetric sidebands. The sidebands can be inverted by inserting an initial decoupling with a different period, but the same f1rms as the main decoupling that is required for Bloch-Siegert shift compensation. The sidebands can be eliminated in a broad decoupling range by adding spectra of opposite sidebands. Based on this scheme, an offset-independent double-adiabatic decoupling, named Bloch-Siegert Shift Eliminated and Cyclic Sideband Trimmed Double-Adiabatic Decoupling, or "BEST" decoupling for short, is constructed, which not only compensates the Bloch-Siegert shift as shown earlier by Zhang and Gorenstein (1998) but also eliminates residual sidebands effectively.     

A two-dimensional artifact from asynchronous decoupling

Many heteronuclear NMR experiments employ decoupling to collapse the heteronuclear multiplet, using decoupling schemes with a periodic phase modulation like WALTZ, MLEV, or GARP. Because of the periodic nature of these schemes, cycling sidebands are generated, whose intensity can be strongly reduced by decoupling asynchronously. We show that the most common implementation of asynchronous decoupling on modern spectrometers is such that the cycling sidebands are subjected to a periodic modulation. For multidimensional experiments, this results in ridges that can seriously compromise the quality of the spectrum. Based on our model, the artifact in a 2D [(1)H]-(15)N NOE equilibrium experiment is simulated and it is shown that the artifact can be prevented by using synchronous decoupling.     

FX-100核磁共振谱仪上的MLEV-16宽带去偶实验

本文叙述了在FX-100核磁共振谱仪上进行的MLEV-16宽带去偶实验。实验表明:其去偶范围大大增加,效率明显提高,对常规质子去偶的¹³C谱实验所需要的去偶场B₂比谱仪原有的噪声去偶系统的去偶场B₂降低约一个数量级。并且,用谱仪原有的去偶功率实现了¹³C{¹⁹F}氟宽带去偶的¹³C谱的观测,去偶范围达14KHz,可以满足大部分氟化物的¹³C核磁共振研究的要求。     

新型茂金属聚烯烃弹性体Vistamaxx的溶液13C NMR 研究

Vistamaxx特种弹性体是一种新型的茂金属催化丙烯-乙烯共聚物,通过对其在溶液中¹³C NMR谱的归属和计算得到3种不同牌号的Vistamaxx弹性体的共聚物组成、单体和序列分布、亚甲基数均序列链长及分布等信息,并分别采用Bernoullian和一级Markovian统计模型对共聚物的三单元组序列分布进行处理,依据组成分布函数条件概率公式得到乙烯单元和丙烯单元的组成分布曲线.      

“BEST” Homonuclear Adiabatic Decoupling for C- and N-Double-Labeled Proteins

The cyclic irradiation sidebands appearing in homonuclear adiabatic decoupling are calculated in detail, which reveals the origin of the antisymmetric sidebands. The sidebands can be inverted by inserting an initial decoupling with a different period, but the same f_1rms as the main decoupling that is required for Bloch–Siegert shift compensation. The sidebands can be eliminated in a broad decoupling range by adding spectra of opposite sidebands. Based on this scheme, an offset-independent double-adiabatic decoupling, named Bloch–Siegert Shift Eliminated and Cyclic Sideband Trimmed Double-Adiabatic Decoupling, or “BEST” decoupling for short, is constructed, which not only compensates the Bloch–Siegert shift as shown earlier by Zhang and Gorenstein (1998) but also eliminates residual sidebands effectively.     

Effect of magnetic particles on NMR spectra of Murchison meteorite organic matter and a polymer-based model system

Organic matter from the Murchison meteorite shows pronounced spinning sidebands of the (1)H MAS NMR spectrum and exhibits a large bulk magnetization of 0.75emicro/g extrapolated to 94kOe at 300K. By comparison with data of diamagnetic polystyrene and laponite clay mixed with ferrimagnetic gamma-Fe(2)O(3) nano-particles, we show that the spinning sidebands arise from a combination of dipolar couplings of a given (1)H to magnetic particles, seen in a backscattered-electron image, and to other protons. Signal loss and significant broadening of protonated-carbon peaks in (13)C MAS NMR spectra of polystyrene with Fe(2)O(3) nano-particles is demonstrated, and implications for (13)C NMR spectroscopy of Murchison meteorite are discussed.     

Supercycled SW(f)-TPPM sequence for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance

The performance of a supercycled SW(f)-TPPM sequence for heteronuclear dipolar decoupling in solid-state NMR is analyzed here. The decoupling performance of this sequence with respect to experimental parameters, such as, the phase angle, proton offset and MAS frequency is studied. A comparison is made with two other commonly used decoupling schemes in solid-state NMR namely, SPINAL-64 and SW(f)-TPPM, on a sample of U-13C-labeled tyrosine. Our results show that supercycled SW(f)-TPPM performs better than the former sequences. Also, numerical spin dynamics studies are presented which support the experimentally observed efficiency in the decoupling.