Solvent effects in nuclear magnetic resonance spectra of some pyrazines,pyrimidines and their N-oxides

The benzene-induced solvent effects upon the proton chemical shifts of various pyrazines, pyrimidines and their N-oxides are described. Larger chemical shift effects, implying closer benzeneheterocycle association, are noted in the N-oxides as compared to the non-oxidized heterocycles. The solvent-induced chemical shift changes can be used to establish the site of N-oxidation in those instances where different isomers can be formed.     

Natural abundance 17O NMR spectroscopy of heterocyclic N-oxides and Di N-oxides. Structural effects

The ¹⁷O chemical shift data for a series of azine N-oxides, diazine N-oxides and di-N-oxides at natural abundance are reported. Isomeric methyl substituted quinoline N-oxides exhibited chemical shifts which are interpreted in terms of electronic and compressional effects. The ¹⁷O chemical shift for 8-methylquinoline N-oxide (370 ppm) is deshielded by 25 ppm more than predicted, based upon electronic considerations. The ¹⁷O chemical shift for the N-oxide of 8-hydroxyquinoline (289 ppm) is substantially shielded as a result of intramolecular hydrogen bonding. The relative ¹⁷O chemical shifts for diazine N-oxides of pyrazine, pyridazine and pyrimidine follow predictions based on back donation considerations. Because of solubility limitations, spectra of only two N,N′-dioxides were obtained. The chemical shift of benzopyrazine di N-oxide in acetonitrile was shielded by 18 ppm compared to that of its mono N-oxide.     

Chemical interaction between Paeonia lactiflora and Glycyrrhiza uralensis,the components of Jakyakgamcho‐tang,using a validated high‐performance liquid chromatography method: Herbal combination and chemical interaction in a decoction

The herbal combination is the basic unit of a herbal formula that affects the chemical characteristics of individual herbs. In the present study, a method of simultaneous determination of the 11 marker compounds in Jakyakgamcho‐tang was developed using high‐performance liquid chromatography with photodiode array detection. The validated analytical method was successfully applied to approach the chemical interaction between Paeonia lactiflora and Glycyrrhiza uralensis in co‐decoction. In P. lactiflora, the contents of gallic acid, oxypaeoniflorin, (+)‐catechin, paeoniflorin, and benzoylpaeoniflorin were decreased, while those of albiflorin and benzoic acid were increased; in G. uralensis, the contents of liquiritin, isoliquiritin, ononin, and glycyrrhizin were decreased, when decocting two herbs together. Moreover, as the ratio between P. lactiflora and G. uralensis was increased, the contents of chemical contents from each herb were proportionally increased. However, each content of marker compound per the gram of herbal medicine was decreased as the ratio of combinative herbs increased. The results showed that P. lactiflora and G. uralensis affect the extraction efficiency of chemical compounds in a Jakyakgamcho‐tang decoction. Overall, the method established in this study was simple, rapid, and accurate, and would be useful for the determination of marker compounds and for the investigation of the chemical interaction between herbal medicines.     

Effect of the character of homo- and heteronuclear chemical bond on the intermolecular interaction energy and properties of halogens and hydrogen halides

The effect of the chemical bond character (the degrees of covalence C _c, metallicity C _m, and ionicity C _i) on the bond rigidity and the components of the van der Waals intermolecular interaction have been shown for halogens and hydrogen halides as an example. The force constant is determined by the chemical bond character. The intermolecular interaction energy of these compounds can be quantified using the C _m and C _i values of the constituent bonds. Thus, the known treatment of intermolecular interaction as residual or secondary to the chemical interaction has been proved to be correct.     

Identifying the chemical markers in raw and wine-processed Scutellaria baicalensis by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry coupled with multiple statistical strategies

Herb processing is a typical pharmaceutical preparation process for traditional Chinese medicine. After processing, its clinical applications and pharmacological effects vary greatly, which is most commonly attributed to the changing chemical properties between raw herb and processed products. In this work, a total of 53 chemical compounds were detected, among which 17 compounds were identified as discriminatory chemicals between raw and wine-processed Scutellaria baicalensis, and 10 components were identified as chemical markers with a cumulative content contribution of 88.75%. In addition, this work revealed that the best wine-processed time was 18 min by investigating the changes of chemical markers in S. baicalensis during processing. This work demonstrated that ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry coupled with multiple statistical strategies is an effective approach for screening and identifying discriminatory chemical markers in complex traditional Chinese medicine.     

一种温和有效的合成手性α-氨基膦酸的方法

An improved procedure for the asymmetric synthesis of α-aminoalkylphosphonic acids using S-2-anilinomethylpyrrolidine as the chiral auxiliary was described. The chemical transformations involved in this protocol could proceed under mild reaction condition to provide good chemical and enantiomeric yields.     

Assessment of the extended Koopmans' theorem for the chemical reactivity: Accurate computations of chemical potentials,chemical hardnesses,and electrophilicity indices

The extended Koopmans' theorem (EKT) provides a straightforward way to compute ionization potentials and electron affinities from any level of theory. Although it is widely applied to ionization potentials, the EKT approach has not been applied to evaluation of the chemical reactivity. We present the first benchmarking study to investigate the performance of the EKT methods for predictions of chemical potentials (μ) (hence electronegativities), chemical hardnesses (η), and electrophilicity indices (ω). We assess the performance of the EKT approaches for post‐Hartree–Fock methods, such as Møller–Plesset perturbation theory, the coupled‐electron pair theory, and their orbital‐optimized counterparts for the evaluation of the chemical reactivity. Especially, results of the orbital‐optimized coupled‐electron pair theory method (with the aug‐cc‐pVQZ basis set) for predictions of the chemical reactivity are very promising; the corresponding mean absolute errors are 0.16, 0.28, and 0.09 eV for μ, η, and ω, respectively. © 2015 Wiley Periodicals, Inc.     

Synthesis and cyclization reactions with pyrazolopyrimidinyl keto esters part I. Reactivity of pyrazolopyrimidinyl β-keto ester and pyrazolopyrimidinyl α,β-unsaturated ketones

5-Acetyl-4,5-dihydro-1-phenylpyrazolo[3,4-d]pyrimidin-4-one was prepared and subjected to various chemical transformations to give novel 5-heterocyclic pyrazolopyrimidinone derivatives of expected important biological activity. Then, the latter compounds were used to obtain β-keto ester and α,β-unsaturated carbonylpyrazolopyrimidinones which were used as alternate precursors to produce new pyrazolopyrimidinones substituted with five-membered heterocycles such as pyrazole and isoxazole. The structure of these compounds was identified on the basis of their chemical behaviour as well as elemental and spectral analysis.     

Infrared and nuclear magnetic resonance properties of benzoyl derivatives of five‐membered monoheterocycles and determination of aromaticity indices

Benzophenones, 2‐benzoylthiophenes, 2‐benzoylpyrroles, and 2‐benzoylfurans, which have substituents at m‐ and p‐positions of the benzoyl ring were prepared and their ir and nmr spectra were obtained in 0.1 M chloroform‐d solution. The chemical shift values of each series were plotted against the Hammett substituent parameters to give good correlation, with the exception of the ortho‐Hs and ‐Cs. The slopes as well as the differences in chemical shift gave sets of meaningful values for the indices of aromaticy.     

Selenate—An internal chemical shift standard for aqueous 77Se NMR spectroscopy

The ⁷⁷Se NMR spectra of selenate were studied under various circumstances, such as concentration, pH, temperature, ionic strength, and D₂O:H₂O ratio, in order to examine its potential as a water-soluble internal chemical shift standard. The performance of selenate as a chemical shift reference and that of other attempted ones from the literature (dimethyl selenide, tetramethylsilane/TMS, and 3-(trimethylsilyl)propane-1-sulfonate/DSS) was also explored. The uncertainty in the resulting chemical shift relative to the effective spectral width is comparable to that of DSS. Compared to the currently prevalent water-soluble external chemical shift reference, selenic acid solution, the properties of internal selenate are much more favorable in terms of ease of use. We have also demonstrated that selenate can be used in reducing media, which is inevitable for the analysis of selenol compounds. Thus, it can be stated that sodium selenate is a robust internal chemical shift reference in aqueous media for ⁷⁷Se NMR measurements; the chemical shift of this reference in a solution containing 5 V/V% D₂O at 25°C and 0.15 mol·dm⁻³ ionic strength is 1048.65 ppm relative to 60 V/V% dimethyl selenide in CDCl₃ and 1046.40 ppm relative to the ¹H signal of 0.03 V/V% TMS in CDCl₃. In summary, a water-soluble, selenium-containing internal chemical shift reference compound was introduced for ⁷⁷Se NMR measurements for the first time in the literature, and with the aforementioned results all previous ⁷⁷Se measurements can be converted to a unified scale defined by the International Union of Pure and Applied Chemistry.     

Essential oil composition of Rydingia michauxii (Briq.) Scheen & V.A. Albert endemic of Iran

The chemical composition of the essential oil (EO) obtained from the aerial parts of Rydingia michauxii (Briq.) Scheen & V.A. Albert was analysed by gas chromatography and gas chromatography/mass spectrometry (GC/MS). Eighty components were identified in the oil with caryophyllene oxide (20.1%), trans-verbenol (10.2%), linalool (5.3%) and humulene epoxide II (4.6%) as main constituents. We reported here the chemical compositions of R. michauxii EOs from southern Zagros of Iran with a distinct chemical profile for the first time.     

Identifying chemical markers in wine-processed Salvia miltiorrhiza using ultrahigh-performance liquid chromatography-quadrupole-time-of-flight-tandem mass spectrometry

To find the chemical markers of wine-processed Salvia miltiorrhiza (WSM), 76 constituents, including diterpenoid quinones and phenolic acids in Salvia miltiorrhiza (SM) and WSM, were profiled using ultrahigh-performance liquid chromatography-quadrupole-time-of-flight-tandem mass spectrometry (UPLC-Q-TOF-MS/MS) in positive- and the negative-ion modes. Thirty compounds were screened out as candidate differential components using chemometrics analysis, and the concentration of most compounds increased after processing with wine. Seven compounds, namely tanshinone IIA, magnesium lithospermate B, salvianolic acid G, cryptotanshinone, isocryptotanshinone, salvianolic acid B, and rosmarinic acid, were selected as chemical markers of WSM using variable importance of the project. This study revealed the chemical markers of WSM and confirmed that WSM can improve the extraction and solubility effect of chemical constituents.     

15N-, 1H-, and 13C-NMR chemical shifts and electronic properties of aromatic diamines and dianhydrides

We measured the ¹⁵N-, ¹H-, and ¹³C-NMR chemical shifts for a series of aromatic diamines and aromatic tetracarboxylic dianhydrides dissolved in DMSO-d₆, and discuss the relationships between these chemical shifts and the rate constants of acylation (k) as well as such electronic-property-related parameters such as ionization potential (IP), electronic affinity (EA), and the energy ε of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The ¹⁵N chemical shifts of the amino group of diamines (δ_N) depend monotonically on the logarithm of k (log k) and on IP. We inferred the reactivities of diamines whose acylation rates have not been measured from their δ_N, and we propose an arrangement of diamines in the order of their reactivity. The ¹H chemical shift of amino hydrogens (δ_H) and the ¹³C chemical shift of carbons bonded to nitrogen (δ_C) are roughly proportional to δ_N, but these shifts are not as closely correlated with log k and IP. Although the ¹³C chemical shifts of the carbonyl carbon of dianhydrides (δ_C,) varies much less than the δ_C and δ_N of diamines, δ_C, can be an index of acylation reactivity for dianhydrides because it is closely correlated with ε_LUMO. These facts indicate that the chemical shifts of diamines and dianhydrides are displaced according to their electron-donor and electron-acceptor properties, and that these chemical shifts can be used as indices of the electronic properties of monomers. Changes in reactivity caused by the introduction of trifluoromethyl groups into diamines and dianhydrides are inferred from the displacements of δ_N and δ_C © 1992 John Wiley & Sons, Inc.     

High Resolution of Racemic Mandelic Acid through a Method of Bubble Fractionation

A method of bubble fractionation was developed for the resolution of racemic mandelic acid (MA), using 2‐hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) as chiral collector. The influences of concentration of HP‐β‐CD, reflux equilibriation time, gas flow rate, packing height of column and pH of buffer on resolution performance were investigated, respectively. According to the similar physical behavior of bubble fractionation and chemical reaction processes, the equivalent chemical reaction constant was introduced. The resolution process was preliminarily analyzed by means of kinetics. The results show that the enantiomeric excess of 60.7% can be obtained under the optimal conditions. The process could be regarded as a first order chemical reaction, where the equivalent speed constant was k_l=0.00376. This method is helpful for realizing high resolution and linear amplification of device.     

The 19F chemical shift in oxygen containing carbon fluorine products

We have examined the ¹⁹F NMR spectra of a number of oxygen-containing fluorocarbon products and obtained a comprehensive set of ¹⁹F chemical shift values, which enabled us to determine the influence of an oxygen atom bonded to a fluorocarbon group on the ¹⁹F chemical shift. The influence of neighbouring fluorocarbon groups, either directly connected or separated by an oxygen atom, was also considered. Our results may be summarized as follows. An oxygen atom bonded by a single bond (ether type bond) to a fluorine substituted carbon atom decreases the ¹⁹F chemical shift, as does the introduction of a further fluorine atom. Considering two adjacent fluorocarbon groups, a variation of x ppm in the ¹⁹F chemical shift of one of the two groups gives a variation of 0·12 x ppm in the opposite sense on the ¹⁹F chemical shift of the other group. If the two groups are connected by an ether oxygen atom, the effect is only about 0·06 x ppm.     

The topology and the aromaticity of coumarins

2H-l-benzopyran-2-one (coumarin), its isomers and derivatives are considered as aromatic or potentially aromatic chemical species and analyzed as such by use of standard and parameter-varying graph-theoretical procedures. Theoretical results are interpreted within the bounds set by existing chemical evidence.     

An ESCA study of chemical reactions on the surfaces of cellulose fibers

Cellulose fibers in the form of paper sheets were chemically modified with different functional groups using trichloro-s-triazine as coupling moiety. The treatments made the paper surfaces hydrophobic, as was indicated by an increase in contact angle against water. ESCA was used for the chemical characterization of the paper surfaces. The shape of the carbon 1s peak depended on the chemical functionality of the triazine derivatives. As a reference, ESCA spectra were also recorded for the triazine derivatives precipitated on aluminum plates. The chemical composition of modified cellulose surfaces could then be determined using a computer program for the peak separation and peak area measurement.     

Chemical and electrochemical grafting of polyaniline onto poly(vinyl chloride): synthesis,characterization, and materials properties

We have designed and developed a new strategy for the chemical and electrochemical graft copolymerization of aniline onto poly(vinyl chloride). For this purpose, first phenylamine groups were incorporated into poly(vinyl chloride) via a nucleophilic substitution reaction in the presence of a solvent composed of 4‐aminophenol, potassium carbonate, and dry N,N‐dimethylformamide at room temperature, in order to avoid cross‐linking. The macromonomer obtained was used in chemical and electrochemical oxidation copolymerization with aniline monomer to yield a poly(vinyl chloride)‐g‐polyaniline (PVC‐g‐PANI) graft copolymer. The chemical structures of samples as representatives were characterized by means of Fourier transform infrared and ¹H nuclear magnetic resonance spectroscopies. The electroactivity behaviors of the synthesized samples were verified under cyclic voltammetric conditions. The electrical conductivity and electroactivity measurements showed that the PVC‐g‐PANI graft copolymer has lower electrical conductivity as well as electroactivity than those of the pure PANI. However, the lower electrical conductivity and electroactivity levels in this material can be improved at the price of solubility and processability. Moreover, the thermal behavior and chemical composition of the synthesized graft copolymer were investigated. Copyright © 2016 John Wiley & Sons, Ltd.