Identification and quantitative analysis of the intermediate phase in a linear high-density polyethylene

The observation of chain conformation and mobility in polyethylene by solid-state ¹³C magic angle spinning (MAS) nuclear magnetic resonance (NMR) permits unambiguous identification and quantitative analysis of an intermediate phase. The carbon-carbon bonds in the intermediate phase adopt, on the average, an all-trans conformation and are more mobile than in the crystalline state (room temperature rate of reorientation ≈ 10⁷ Hz). Comparisons of crystallinities by differential scanning calorimetry, wide-angle x-ray diffraction, and NMR support the high orientation of the intermediate phase and suggest a lower heat of fusion than for the crystals. Results from ¹³C spin-lattice relaxation and ¹H spin diffusion show that the mass fractions are ≈ 20% and the domain sizes ≈ 36 Å. Both change with crystallization and annealing conditions. © 1994 John Wiley & Sons, Inc.     

Quantitative determination of aliphatic hydrocarbon compounds by 2D NMR

Quantitative analysis of complex mixtures by NMR is often hampered by heavily overlapping signals in 1D ¹H or ¹³C spectra. To resolve the overlap problem, we have been looking at the possibilities of using heteronuclear correlated 2D NMR methods for quantification. In this work, we applied 2D INEPT to analyze mixtures of tetradecane and squalane, which represent typical substructures of lube oil fractions. The factors affecting correlation peak volumes, namely the polarization transfer delays within pulse sequence, multiplicity of CH_n group and the magnitude of ¹J_{(C, H)} couplings were taken into account by product operator formalism calculations. The results indicate that if absolute precision in quantification is not essential, the current approach can be used for the quantitative analysis of the molecular composition of complex mixtures when conventional 1D NMR methods fail. Copyright © 2002 John Wiley & Sons, Ltd.     

The morphology of gel-spun polyethylene fibers,investigated by solid-state 13C NMR

Gel-spun polyethylene fibers were analyzed at room temperature with ¹³C NMR, using both, CP-MAS and BILEV (Bloch-decay with two-level decoupling). The analysis shows the existence of three different components in the fiber sample—a crystal component, an amorphous component and a third component, named the oriented, mobile component. This latter component has a ¹³ C chemical shift that is similar to the crystalline chemical shift, but with a mobility, expressed by T₁, that is closer to the amorphous component. The chemical shift and T₁ are as follows: 34.06 ppm and 28.1 s for the crystalline part; 31.70 ppm and 0.3 s for the amorphous part; and 34.06 ppm and 1.8 s for the oriented mobile component. The percentages are 63.2% crystalline; 34.0% oriented mobile, 2.8% amorphous component. Using proton spin-diffusion measurements, it was possible to estimate the domain size of the crystalline and oriented mobile components to be 62.8 and 13.2 nm, respectively, in agreement with the results of a full-pattern x-ray study on the same sample. After melting of the fiber at 450 K and recrystallinzation on cooling, the oriented, mobile component is dramatically reduced. © 1994 John Wiley & Sons, Inc.     

油菜花粉十二肽的二维核磁共振研究

用核磁共振方法对油菜花粉十二肽溶液构象进行了研究。利用２Ｄ ＣＯＳＹ和ＮＯＥＳＹ归属了＾１Ｈ ＮＭＲ谱的全部共振峰。用Ｋａｒｐｌｕｓ方程从肽键Ｎ－Ｈ质子与α－Ｈ质子的偶合常数计算出了所有肽键的二面角θ及Φ，并对其进行不同温度及不同溶剂条件下的观测。结果表明，在水溶液中该肽链为无规卷曲结构，个别肽段存在γ－ｔｕｒｎ构象。     

Progress in Hyperpolarized Ultrafast 2D NMR Spectroscopy

An important development in the field of NMR spectroscopy has been the advent of hyperpolarization approaches, capable of yielding nuclear spin states whose value exceeds by orders‐of‐magnitude what even the highest‐field spectrometers can afford under Boltzmann equilibrium. Included among these methods is an ex situ dynamic nuclear polarization (DNP) approach, which yields liquid‐phase samples possessing spin polarizations of up to 50 %. Although capable of providing an NMR sensitivity equivalent to the averaging of about 1 000 000 scans, this methodology is constrained to extract its “superspectrum” within a single—or at most a few—transients. This makes it a poor starting point for conventional 2D NMR acquisition experiments, which require a large number of scans that are identical to one another except for the increment of a certain t₁ delay. It has been recently suggested that by merging this ex situ DNP approach with spatially encoded “ultrafast” methods, a suitable starting point could arise for the acquisition of 2D spectra on hyperpolarized liquids. Herein, we describe the experimental principles, potential features, and current limitations of such integration between the two methodologies. For a variety of small molecules, these new hyperpolarized ultrafast experiments can, for equivalent overall durations, provide heteronuclear correlation spectra at significantly lower concentrations than those currently achievable by conventional 2D NMR acquisitions. A variety of challenges still remain to be solved before bringing the full potential of this new integrated 2D NMR approach to fruition; these outstanding issues are discussed.     

矢车菊素结构的2D NMR确定

从菊科药用植物蕴苞麻花头（Serratula stranglata lljin)中分离得到一含氯的愈创木内酯型的倍半萜——矢车菊素（centaurepensin),利用^1H-^1H COSY、HMQC、HMBC及^1H-^1H NOESY等二维波谱技术鉴定了该化合物的结构,并对其^1H和^13C NMR的信号进行了全归属,最终确定了其绝对构型。     

Structure elucidation of a novel group of dithiocarbamate derivatives using 1D and 2D NMR spectroscopy

Compounds 1-7 form a novel group of dithiocarbamates, first synthesized from the reaction of a series of primary amines with carbon disulfide and 3-bromo ethyl pyruvate in the presence of anhydrous potassium phosphate. Structure elucidation of this group of compounds was accomplished using extensive 1D and 2D NMR spectroscopic studies, including (1)H, (13)C, COSY, NOESY, HSQC, and gHMBC experiments. The distinction between the linear structures I, II and the cyclic structure III was made mainly on the basis of the analysis of the cross peak between H-2 and H-4a in the COSY spectra, in combination with the long-range correlation between H-2 and C-4, 6 in the gHMBC spectra.     

Automated Protein Structure Elucidation From 2D NMR

A general method for automated de novo deduction of protein structure from 2D NMR has been developed. The algorithms which extract simple spin coupling topologies from MQF-COSY, construct more complicated spin coupling topologies based upon MQ spectrum including all possible pathways, and extract spin coupling topological fragments for amino acids of a protein, have been implemented in C~(++) language and run on a SUN 4/280 work station. Compared with the manual assignments for melittin, the total identity of the automated de novo method is 86.3%.     

用2D NMR确定金色酰胺醇酯的结构

从中国南海红藻鹧鸪菜（Caloglossa leprieurii）中首次分离得到二肽金色酰胺醇酯(aurantiamide acetate)。利用^1H－^1H COSY、^13C－^1H COSY、HMBC等二维谱手段鉴定了该化合物的结构,对其^1H和^13C NMR的信号进行了归属,并且确定了其绝对构型。     

Quantitative estimation of the degree of derivatization: an alternative methodology using 1-D and 2-D NMR experiments

The precise estimation of the degree of derivatization of functional groups in polymers is important for determining their macroscopic properties. In this work, the quantitative estimation of the extent of esterification of novolac copolymers with di-tert-butyl dicarbonate was studied. Although the extent of esterification has been calculated previously by quantifying the signals from FT-IR and UV-Vis spectroscopy, these were restricted to monitoring the progress of the derivatization process. The 13C NMR signal intensities from the inverse-gated 1H-decoupled NMR spectrum have been used recently for the quantitative estimation of the degree of esterification of polymers. An alternative methodology has been suggested by us based on the fully relaxed 1H chemical shift intensities. However, since the proton signals of novolac resins are generally broad and overlapping, the proton decoupled 13C NMR spectrum was used to identify the 1H NMR signals using the 2-D HSQC technique. A TOCSY experiment was also performed to confirm further the 1H NMR signal assignments and, finally, the deconvoluted 1H NMR spectrum was used for the calculation of the extent of derivatization.     

Using 2D NMR to determine the degree of branching of complicated hyperbranched polymers

Degree of branching (DB) is a crucial structure parameter of hyperbranched polymers, which can be determined by 1H NMR, quantitative 13C NMR, degradative method, etc. However, for complicated hy-perbranched polymers, intricate structure and severe overlap of spectral signals hinder the determina-tion of DB using traditional methods. In this work, the architecture of complicated hyperbranched polymers has been elucidated with the help of 2D NMR techniques. Using such a method, overlapped NMR signals can be well separated into a two-dimensional space, and additional structural information is also available. Correspondingly, quantitative analysis for complicated systems can be realized. De-termination of DBs for three types of complicated hyperbranched polymers synthesized from step-polymerization, self-condensation vinyl polymerization and self-condensation ring-opening po-lymerization is shown as examples.     

Towards the automatic analysis of (1)H NMR spectra: Part 5. Confirmation of chemical structure with flow-NMR

A shift comparison procedure, which allows the use of flow spectra in automatic structure confirmation, is described. The effect of imperfect proton scaling at the analysis stage, loss of solute resonances under large solvent signals and the intermittent detection of labile protons are considered. The derivation of a suitable threshold acceptance criterion in the absence of explicit knowledge of spectral prediction reliability is also discussed.The methodology yielded approximately 10-15% incorrect structure confirmation, but the exact number depends on the reliability of spectral prediction and the number of incorrect but closely related structures postulated.     

Sequence Determination of a Tetrapeptide Using Long-Rang Heteronuclear Shift Correlation 2D NMR Spectroscopy

A tetrapeptide has been studied by means of NMR of ¹³C in natural abundance. By the combined application of various two-dimensional ¹H/¹H and ¹³C/¹H correlation techniques, the primary structure of the peptides could be unambiguously determined.     

New validation of molecular mass measurements by means of 2D DOSY 1H NMR experiments: Application to surfactants

This study proposes a new correlation equation between surfactant diffusion coefficients and molecular mass using 2D DOSY ¹H NMR experiment. Indeed, DOSY is a suitable NMR technique to discriminate the different components in a mixture. Using Inverse Laplace Transform (ILT), all chemical shifts in the direct dimension are dispersed along the second dimension in relation to the diffusion coefficient. This approach does not make any assumption about the number of exponential components. The empiric equation was determined for anionic and non-ionic surfactants dissolved in dimethylsulfoxide (DMSO). The parameters, K = C_r = 5.18 ± 0.70 and a shape factor α = 0.45 ± 0.02 that is related to the molecular family fractal dimension constant δ_F = 2.21 ± 0.11, have been extracted from the data. The equation validity was checked for simple commercial raw material and a simple mixture composed of three surfactants classically used in detergent formulations. This equation, which describes quite satisfactorily the diffusion phenomenon of surfactants, could be used for deformulation purpose to check commercial products in case of quality control or adulteration assessment.     

Structural elucidation by 1D and 2D NMR of three isomers of a carotenoid lysophosphocholine and its synthetic precursors

A carotenoic acid was used to obtain a long-chain unsaturated lysophosphocholine. The carotenoid lysophosphocholine was synthesized by two methods. The first method resulted in mixtures of regioisomers for each step in the synthetic route. Homo- and heteronuclear 1D and 2D NMR methods were employed to elucidate the structures of the individual isomers and their intermediates. The pure regioisomer [1-(beta-apo-8'-carotenoyl)-2-lyso-glycero-3-phosphocholine] was obtained by a second method, but in low yield. The 1D 1H NMR subtraction spectrum of the mixture and the pure regioisomer was used to interpret the 1H shifts of the unsaturated acyl moieties. The 1H and 13C signals of the acyl chain show characteristic shifts depending on the positions of the choline and the acyl group attached to the glycerol backbone. Therefore, the unsaturated acyl chain signals have diagnostic values for the identification of isomers of unsaturated (lyso)phosphocholines. Chemical shifts and indirect coupling constants are reported for each of the major components of the mixtures. The methods used were 1D (1H, 13C and 31P) and 2D (H,H-COSY, HMBC, HSQC and HETCOR) NMR.     

N‐Alkenyl amide rotational barriers by 2D EXSY NMR

The rotational barriers of some N‐alkenylamides were measured by 2D exchange spectroscopy (2D EXSY) NMR techniques. It was found that the conjugated double bond lowers E,Z barriers by 2.6 kJ mol^?1. Copyright © 2003 John Wiley & Sons, Ltd.     

Rheo-optical near-infrared (NIR) spectroscopy study of low-density polyethylene (LDPE) in conjunction with projection two-dimensional (2D) correlation analysis

A rheo-optical characterization technique based on near-infrared (NIR) spectroscopy is developed specifically to probe the submolecular-level deformation caused during a mechanical test. An illustrative example of the mechanical deformation of low-density polyethylene (LDPE) is provided to show how it can be utilized. A set of NIR spectra of the polymer sample were collected by using an acousto-optic tunable filter (AOTF) NIR spectrometer coupled with a tensile testing machine as an excitation device. While the substantial level of variation of spectral intensity was readily captured during the mechanical deformation of the LDPE, main feature of the NIR spectra was overwhelmed by the contribution from the baseline change. Projection 2D correlation analysis was then applied to selectively extract the signal contribution from the baseline fluctuation. The 2D correlation spectra revealed the predominant extension of amorphous tie chains followed by the rotation of crystalline lamellae, which induce elastic and plastic deformation of the LDPE, respectively.     

2D NMR analysis for unambiguous structural elucidation of phenolic compounds formed through reaction between (+)‐catechin and glyoxylic acid

Various phenolic compounds were synthesized in an aqueous–alcoholic solution containing (+)‐catechin and glyoxylic acid which was used as a model of fruit‐derived food browning that usually occurs during aging. After purification by semi‐preparative HPLC, the isolated compounds were subjected to homo‐ and heteronuclear proton and carbon NMR analysis including COSY, TOCSY, ROESY, HSQC and HMBC techniques. These experiments allowed the structural elucidation and complete ¹H and ¹³C NMR assignment of the isolated compounds. The strategies followed for the assignment of all proton and carbon resonances in addition to the linkage site determination are discussed. Copyright © 2002 John Wiley & Sons, Ltd.     

Dynamic NMR: A new procedure for the estimation of mixing times in the 2D EXSY experiments. A four-site exchange system studied by 1D and 2D EXSY spectroscopy

A procedure for the estimation of the mixing time between the last two 90° pulses in the classic three-pulse sequence NOESY/EXSY is proposed and tested and some considerations for the treatment of the two-dimensional (2D) ¹H NMR exchange spectra are given. The rate constants are thus obtained with reasonable precision. This procedure was followed to obtain the 2D spectra of the model compound α-[bis(dimethylamino)methylene]-4-nitrophenylacetonitrile, which represents a four-site exchange system. The barriers to restricted rotations found in this compound were also determined from one-dimensional (1D) ¹H NMR spectra, which were processed with the iterative complete lineshape analysis (CLSA) method. The double-fit approach was incorporated in the CLSA method. It is shown that the results from the 2D dynamic NMR spectral studies corroborate those obtained by the CLSA double-fit method.