The (14)N quadrupole coupling in hexamethylene triperoxide diamine (HMTD)

Using high-field NMR, we have determined the magnitude of the nuclear quadrupole interaction in hexamethylene triperoxide diamine (HMTD), the explosive allegedly used in the London bombings of July 2005. The experimental quadrupolar coupling constant, 5.334 MHz, is in good agreement with quantum chemical calculations. The predicted single zero-field transition frequency should lie in a relatively empty part of the (14)N nuclear quadrupole resonance (NQR) spectrum; the spin relaxation rate is reasonably fast.     

Surface amine‐functionalization of UHMWPE fiber by bio‐inspired polydopamine and grafted hexamethylene diamine

Ultrahigh molecular weight polyethylene (UHMWPE) fibers exhibit excellent mechanical property, but their low surface activity limits the application in many fields. In this work, an efficient method was used to improve the surface activity and adhesion property of UHMWPE fibers. The amine functionalized UHMWPE fibers were prepared by the combination of bio‐inspired polydopamine (PDA) and grafted hexamethylene diamine (HMDA). The chemical structure of UHMWPE fibers was characterized by X‐ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy. The surface morphologies and mechanical property of the fibers were investigated by scanning electron microscopy and tensile testing respectively. In addition, a single‐fiber pull‐out test was carried out to investigate the adhesion property of the fibers with epoxy resin matrix. The results showed that PDA was coated on the surface of UHMWPE fibers and then HMDA was successfully grafted on the PDA layers. The excellent mechanical property of UHMWPE fibers had no obvious change. Compared with the pristine UHMWPE fibers, the interfacial shear strength of the PDA coated UHMWPE fibers with the epoxy resin matrix improved by 28.3%, while the IFSS of the HMDA grafted UHMWPE fibers had an increase of 82.7%. Copyright © 2017 John Wiley & Sons, Ltd.     

Determination of the helical screw sense and side-group chirality of a synthetic chiral polymer from Raman optical activity

Splitting it up: Excellent agreement between the experimental and the quantum-chemically simulated Raman optical activity (ROA) spectrum of (+)-poly(trityl methacrylate) shows that the polymer backbone adopts a left-handed helical conformation while the trityl side groups display a left-handed propeller conformation. Thus ROA can be used to determine the complete structure of synthetic chiral polymers in solution.     

Theoretical study of helical structure caused by chirality of cysteine dimer

In this work we report a theoretical study of the helix structure and chiral discrimination on the interactions between the chiral cysteine–cysteine. Two reasonable geometries on the potential energy hypersurface of the cysteine–cysteine system are considered with the global minimum. Accurate geometric structures, relative stabilities, harmonic vibrational frequencies, and infrared (IR) intensities were investigated. To take into account the water solvation effect, the Onsager model within the self‐consistent reaction field (SCRF) method and the polarized continuum (PCM) method were used to evaluate the interaction energy, ΔG_solv at the same level employed in the gas phase. The results indicate that the polarity of the solvent plays an important role in the structures and relative stabilities of different isomers. Computational results indicate that the global minimum should be conformer I regardless of whether in the gas phase or in aqueous solution, which differs from previous theoretical reports. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Enantiomeric differentiation of oxygenated p-menthane derivatives by 13C NMR using Yb(hfc)3

The (13)C NMR behaviour of 21 p-menthanic terpene bearing an oxygenated function (alcohol, ketone, acetate) was examined in the presence of a chiral lanthanide shift reagent (Yb(hfc)(3)). For each monocyclic compound, we measured the lanthanide-induced shift (LIS) on the signals of the carbons and the splitting of signals allowing the enantiomeric differentiation. Some general features were found about their LIS behaviour: experimental data establishing distinct patterns for carvomenthone-like compounds and menthone-like compounds. The enantiomeric splitting was observed for the majority of signals in the spectrum of each compound. In the case of alcohols and acetates, the influence of the relative stereochemistry (cis vs trans) of isopropyl(ene) and the binding function was discussed.     

Theoretical study on the polymerization mechanism of substituted maleimides by using a chiral catalyst with Zn

It is possible to synthesize poly(N-substituted maleimide) by using a chiral complex consisting of a zinc and N-diphenylmethyl-1-benzyl-2-pyrrolidinoethanamine (DPhBP). The optical specific rotations [α]₄₃₅²⁵ in obtained polymers depend on the chirality of ligands in the catalysts. In the present study, density functional theory (DFT) calculations were adopted to investigate the polymerization mechanism in detail. The bulky diphenylmethyl group in the chiral ligand is effective to enhance the formation of the product in the initiation reaction. The geometry related to the pyrrolidine ring of the chiral ligand in the Zn catalyst is responsible for determining the configuration of polymers. It was also confirmed that the bulky substituent on the N atom of the N-substituted maleimide is another factor for obtaining polymers with high [α]₄₃₅²⁵.     

Hydrogen bonding in two benzene‐1,2‐diaminium pyridine‐2‐carboxylate salts and a cocrystal of benzene‐1,2‐diamine and benzoic acid

The isostructural salts benzene‐1,2‐diaminium bis(pyridine‐2‐carboxylate), 0.5C₆H₁₀N₂²⁺·C₆H₄NO₂^?, (1), and 4,5‐dimethylbenzene‐1,2‐diaminium bis(pyridine‐2‐carboxylate), 0.5C₈H₁₄N₂²⁺·C₆H₄NO₂^?, (2), and the 1:2 benzene‐1,2‐diamine–benzoic acid cocrystal, 0.5C₆H₈N₂·C₇H₆O₂, (3), are reported. All of the compounds exhibit extensive N—H…O hydrogen bonding that results in interconnected rings. O—H…N hydrogen bonding is observed in (3). Additional π–π and C—H…π interactions are found in each compound. Hirshfeld and fingerprint plot analyses reveal the primary intermolecular interactions and density functional theory was used to calculate their strengths. Salt formation by (1) and (2), and cocrystallization by (3) are rationalized by examining pK_a differences. The R₂²(9) hydrogen‐bonding motif is common to each of these structures.     

Density functional calculations of 19F and 235U NMR chemical shifts in uranium (VI) chloride fluorides UF6−nCln: Influence of the relativistic approximation and role of the exchange‐correlation functional

Relativistic density functional theory (DFT) has been applied to the calculation of the ¹⁹F nuclear magnetic resonance (NMR) chemical shifts of the title compounds. It is shown that, while large‐core effective core potentials (ECP) fail completely for the calculation of ligand NMR chemical shifts in uranium compounds, small‐core ECPs are a valid relativistic method for this purpose. In an earlier study of the same systems, certain differences between theory and experiment had been observed, for instance, in the relative chemical shift of the A₄ and X sites in UF₅Cl. The reason for these deviations has been investigated further in the current paper. By comparing different relativistic methods, it is shown that the relativistic approximation is not responsible for these deviations. The role of the approximation to the exchange‐correlation (XC) functional of DFT has been probed, and generalized gradient approximations (GGA) as well as hybrid DFT methods have been investigated. None of these methods corrects the mentioned errors. It is argued that the neglect of environmental factors (solvent effects) remains as a possible error source, although the approximate XC functional appears to be the more likely cause of the problem. ²³⁵U NMR shieldings and chemical shifts have been calculated, and the trends predicted earlier have been confirmed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

The role of salt on cellulose dissolution in ethylene diamine/salt solvent systems

Ethylene diamine (EDA)/salt solvent systems can dissolve cellulose without any pretreatment. A comparison of the electrical conductivity of different salts in EDA was made at 25 °C, and conductivity decreased in the order of KSCN>KI>NaSCN at the same molar concentration. Among the salts tested, potassium thiocyanate (KSCN) was capable of dissolving both high molecular weight (DP>1000) and low molecular weight (DP = 210) cellulose, and this was confirmed by polarized light microscopy. ³⁹K and ¹⁴N NMR experiments were conducted at 70 °C as a function of cellobiose concentration with EDA/KSCN as the solvent. The results showed that the K⁺ ion interacts with cellobiose more than the SCN⁻ ion does. Recovered cellulose was studied by infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). Changes in the FTIR absorption bands at 1,430 and 1,317 cm⁻¹ were associated with a change in the conformation of the C-6CH₂OH group. The changes in positions and/or intensities of absorption bands at 2,900, 1,163, and 8,97cm⁻¹ were related to the breaking of hydrogen bonds in cellulose. X-ray diffraction studies revealed that cellulose, recovered by precipitating cellulose solutions with water, underwent a polymorphic transformation from cellulose I to cellulose II.     

New Chiral Kemp's Acid Diamides for Chiral Amine Recognition by 1 H NMR

Two Kemp's acid diamides were synthesized and applied to chiral amine recognition using ¹H NMR analysis. One derivative based on 1-(1-naphthyl)ethylamine had good chiral recognition of six amines and was useful to determine the optical purity for three amines, i.e., methylbenzylamine, 1-(1-naphthyl)ethylamine and 1- henylpropylamine,however, the cyclohexylethylamine derivative showed little discrimination for the amines studied. Together with the results for alkylamines, it was shown that aromatic structure was important for aromatic shielding anisotropy and –– interactions between host and guest. The structure of the 1-(1-naphthyl)ethylamine derivative in solution was also considered based on ¹H NMR data and computer simulation.     

Degrees of Chirality in Helical Structures

Long helical structures occur in both natural and synthetic polymers. Their “degree of chirality” is quantified through the calculation of an overlap‐based chirality index and an infinite hierarchy of pseudoscalar parameters. It is shown that these quantities are related, since they may be constructed from a common set of cylindrical Fourier coefficients. The formal analysis is illustrated by the application to helical ribbons. It is found that the chirality of helical ribbons is a monotonically increasing function of the ratio h/a and a decreasing function of the ratio τ/a, where h and a are the pitch and the radius of the helix, whereas τ is the height of the ribbon. Chirality achieves a maximum asymptotic value as h → ∞.     

Parameterization of peptide 13C carbonyl chemical shielding anisotropy in molecular dynamics simulations.

NMR chemical shielding anisotropy (CSA) relaxation is an important tool in the study of dynamical processes in proteins and nucleic acids in solution. Herein, we investigate how dynamical variations in local geometry affect the chemical shielding anisotropy relaxation of the carbonyl carbon nucleus, using the following protocol: 1) Using density functional theory, the carbonyl (13)C' CSA is computed for 103 conformations of the model peptide group N-methylacetamide (NMA). 2) The variations in computed (13)C' CSA parameters are fitted against quadratic hypersurfaces containing cross terms between the variables. 3) The predictive quality of the CSA hypersurfaces is validated by comparing the predicted and de novo calculated (13)C' CSAs for 20 molecular dynamics snapshots. 4) The CSA fluctuations and their autocorrelation and cross correlation functions due to bond-length and bond-angle distortions are predicted for a chemistry Harvard molecular mechanics (CHARMM) molecular dynamics trajectory of Ca(2+)-saturated calmodulin and GB3 from the hypersurfaces, as well as for a molecular dynamics (MD) simulation of an NMA trimer using a quantum mechanically correct forcefield. We find that the fluctuations can be represented by a 0.93 scaling factor of the CSA tensor for both R(1) and R(2) relaxations for residues in helix, coil, and sheet alike. This result is important, as it establishes that (13)C' relaxation is a valid tool for measurement of interesting dynamical events in proteins.     

Benchmark approach to search of cost-effective and accurate density functional for homolytic cleavage of C─Mg bond of Grignard reagent

Grignard reactions are of importance in organic chemistry for the synthesis β-keto esters and diethyl malonate, alcohols, aldehydes or ketones, monocarboxylic acids, and other organometallic compounds. Generally, the heterolytic dissociation of C─Mg bond in Grignard reagent is the key step in these reactions. Recently, homolytic cleavage of the C─Mg bond in Grignard reagents has been reported in the preparation of stable radicals. These reactive species react with other compounds, which result in the formation of hydrocarbons and their derivatives. Therefore, the study of homolytic cleavage of C─Mg bonds is quite vital to better understand the kinetics and thermodynamics of these reactions. In the current study, a benchmark approach is adopted to find a cost-effective and accurate density functional (DF) for bond dissociation energies measurement of the C─Mg bond of Grignard reagents. Twenty-nine DFs from 13 density functional theory (DFT) classes with three types of basis sets (Pople' 6-31G(d) and 6-311G(d), Dunning's aug-cc-pVDZ, and Karlsruhe' def2-SVP basis sets) are implemented for the measurement of dissociation energies of the C─Mg bond. Theoretical dissociation energy values are compared with experimental reported values of the C─Mg bond of selected Grignard reagents. TPSSTPSS of the meta-GGA class with 6-31G (d) basis set gave accurate results, and its Pearson's correlation is 0.95. SD, root mean square deviation, and mean unsigned error of this method are 2.36 kcal mol⁻¹, 2.33 kcal mol⁻¹, and −0.46 kcal mol⁻¹, respectively. TPSSTPSS of the meta-GGA class is a one-electron, self-interaction, error-free Tao-Perdew-Staroverov-Scuseria functional that performed better with the 6-31G(d) basis set.     

Mycoepoxydiene核磁共振谱的理论研究

用密度泛涵理论(Density Functional Theory,DFT)的B3PW 91方法在6-311G**基组下优化了标题化合物mycoepoxyd iene的几何构型,同时计算了红外光谱.并用规范不变原子轨道(gauge independent atom ic orb ital)G IAO/B3PW 91以及G IAO/HF方法分别在6-311G**,6-311++G**等基组进行了核磁共振谱研究,计算结果与实验结果吻合很好.     

1,5-萘二胺衍生物的密度泛函理论研究

用量子化学的密度泛函理论方法,在B3LYP/6 31G水平上,对N,N′ 二苯基 N,N′ 二(1 萘基) 1,5 萘二胺(NPN)进行了理论计算.结果发现:NPN有两个平衡构型(trans NPN,cis NPN),trans NPN比cis NPN稳定,谐振动频率分析表明它们都是稳定构型.在PM3/CIS水平上计算了它们的电子光谱,得到了由基态到各激发态的垂直跃迁能和相应的振子强度,计算结果与实验符合得很好.     

Chiral recognition in solution and the gas phase. Experimental and theoretical studies of aromatic D- and L-amino acid-Cu(II)-chiragen complexes

This study was focused on distinguishing L- and D-enantiomers of amino acids using electrospray ionization mass spectrometry (ESI-MS) of ternary complexes with Cu(II) and chiral derivatives of bipyridine. A pinene-annulated derivative of 2,2'-bipyridine, (5R,7S,8S)-(--)-5,6,7,8-tetrahydro-6,6,8-trimethyl-2-(pyridin-2-yl)-5,7-methanoquinoline, called a chiragen, was used as the auxiliary ligand bound to Cu(II) to study the complexation of D- and L-phenylalanine and D- and L-tryptophan and their detection by MS. NMR studies showed that the D- and L-amino acid complexes can be distinguished in solution by the difference in the amount of band broadening of the alpha-carbon proton, with the D-complex showing greater broadening from a more intense interaction with the paramagnetic copper center. In ESI-MS studies, the ion abundances for the analyte complexes were compared with those of internal standards to investigate competition in binding to Cu(II)chiragen between the internal standard and D- and L-amino acids. The D-Phe complex showed stronger binding than the L-Phe complex, although differences in response related to solvent effects were also apparent. In studies involving two separate internal standards, the Trp enantiomers were practically indistinguishable in all but one solvent environment. In 100% methanol, the L-Phe and D-Phe complexes were readily distinguished and the L-Phe complex out-competed the L-Phe complex by a factor of two. Density functional theory calculations were performed on D- and L-complexes of Phe and Trp to determine the optimized geometries and the most energetically favorable structures. Calculations agreed with experiments, where the D-Phe complex was the most stable structure, while the L-Trp and D-Trp complexes were comparably stable in the gas phase.     

Hydrogen bonding in 4‐nitrobenzene‐1,2‐diamine and two hydrohalide salts

The structures of 4‐nitrobenzene‐1,2‐diamine [C₆H₇N₃O₂, (I)], 2‐amino‐5‐nitroanilinium chloride [C₆H₈N₃O₂⁺·Cl⁻, (II)] and 2‐amino‐5‐nitroanilinium bromide monohydrate [C₆H₈N₃O₂⁺·Br⁻·H₂O, (III)] are reported and their hydrogen‐bonded structures described. The amine group para to the nitro group in (I) adopts an approximately planar geometry, whereas the meta amine group is decidedly pyramidal. In the hydrogen halide salts (II) and (III), the amine group meta to the nitro group is protonated. Compound (I) displays a pleated‐sheet hydrogen‐bonded two‐dimensional structure with R₂²(14) and R₄⁴(20) rings. The sheets are joined by additional hydrogen bonds, resulting in a three‐dimensional extended structure. Hydrohalide salt (II) has two formula units in the asymmetric unit that are related by a pseudo‐inversion center. The dominant hydrogen‐bonding interactions involve the chloride ion and result in R₄²(8) rings linked to form a ladder‐chain structure. The chains are joined by N—H...Cl and N—H...O hydrogen bonds to form sheets parallel to (010). In hydrated hydrohalide salt (III), bromide ions are hydrogen bonded to amine and ammonium groups to form R₄²(8) rings. The water behaves as a double donor/single acceptor and, along with the bromide anions, forms hydrogen bonds involving the nitro, amine, and ammonium groups. The result is sheets parallel to (001) composed of alternating R₅⁵(15) and R₆⁴(24) rings. Ammonium N—H...Br interactions join the sheets to form a three‐dimensional extended structure. Energy‐minimized structures obtained using DFT and MP2 calculations are consistent with the solid‐state structures. Consistent with (II) and (III), calculations show that protonation of the amine group meta to the nitro group results in a structure that is about 1.5 kJ mol⁻¹ more stable than that obtained by protonation of the para‐amine group. DFT calculations on single molecules and hydrogen‐bonded pairs of molecules based on structural results obtained for (I) and for 3‐nitrobenzene‐1,2‐diamine, (IV) [Betz & Gerber (2011). Acta Cryst. E 67 , o1359] were used to estimate the strength of the N—H...O(nitro) interactions for three observed motifs. The hydrogen‐bonding interaction between the pairs of molecules examined was found to correspond to 20–30 kJ mol⁻¹.     

Theoretical Studies on the Infrared Vibrational Spectra ,Thermodynamic Properties and Nuclear Magnetic Resonance Spectra for Polynitro-1,3-bishomopentaprismanes

Based on the optimized molecular geometries at the DFT‐B3LYP/6‐31G?? level, IR spectra, thermodynamic functions, as well as ¹³C and ¹H NMR chemical shifts, were obtained and discussed for polynitro‐1,3‐bishomo‐pentaprismanes (PNBPP). The comparison of the calculated IR frequencies and NMR chemical shifts showed considerable agreements with the available experimental results. IR regions, ¹³C and ¹H NMR chemical shifts of PNBPP were assigned. The relationships of the thermodynamic functions with temperature and the number of nitro groups were discussed, and it was found that the latter showed a good group additivity rule. These calculated data and discussions would be helpful for the further study of PNBPP.     

Theoretical Study on a 42 T Model of Beta Zeolite by Density Functional Theory Simulation

Density functional theory was applied to study the structure of Beta zeolite. A model cluster containing 41Si atoms, 1 Al atom, 70 O atoms and 29 H atoms was constructed. The model structures were optimized using the Becke＇s three-parameter hybrid method with the Lee-Yang-Parr correlation functional （B3LYP） and the 6-31G basis set applying the Gaussian03 program package. The NMR parameters were calculated to validate the rationality of the model. It was found that in the optimization models, all O-H bond lengths were in range of 0.984-0.985A^°, among which the model with O-H bond length of 0.98478A^° was more stable than the others. The ^1H and ^27Al chemical shifts of the most stable model were 4.03434 and 55.74 ppm, which were pretty consistent with Larry＇ s experimental data of 4.1 and 54 ppm. The relationship between other structure parameters and total relative electric energy has also been found. All the results exhibit that the 42 T （the total number of Si and Al atoms is 42） model has common properties of the standard of zeolite Beta.