Theoretical study on the ring-opening isomerization reaction mechanism of the ring isomers of N8H8

Ring-opening isomerization from ring-shaped isomers to chain-shaped isomers of N(8)H(8) has been studied by a density function B3LYP method at 6-311+ +G** level. 20 ring-shaped isomers have been found to be able to transform into chain-shaped isomers, with 20 possible transition states got by ring-opening structure optimization. Furthermore, the ring-openings have been found in the longer N-N single bond by analyzing the length change of N-N bond of ring-shaped isomers in ring-opening processes. In addition, with the activation energies in ring-opening processes, the differences of the activation energies in isomerization between the isomers have been found according to the classification of rings. The activation energies in ring-opening isomerization of six-membered ring-shaped isomers are higher than that of the four-membered ring-shaped isomers. It indicates that six-membered ring-shaped isomers difficult in ring-opening in the isomerization are the steadiest ring-shaped isomers of N(8)H(8) while four-membered ring-shaped isomers easy in ring-opening are the most unstable.     

Separation of isomers of dienoic acids by electromigration techniques

Different capillary electromigration techniques were employed to resolve geometrical isomers of sorbic acid, decadienoic acid, and ethyl sorbate. Since these substances differ in their polarity, shape, and size, various electromigration approaches were investigated to separate the four geometrical isomers of each compound. With capillary electrophoresis (CE) modified with a cyclodextrin (β‐CD) the four isomers of sorbic acid were separated using a buffer that consists of 60 mM tetraborate and 8 mg/mL β‐CD. The separation of decadienoic acid geometrical isomers was not possible, even at elevated tetraborate and cyclodextrin concentrations. The four isomers of decadienoic acid were successfully separated using micellar electrokinetic chromatography (MEKC) with a buffer consisting of 30 mM tetraborate and 100 mM SDS and microemulsion electrokinetic chromatography (MEEKC). Ethyl sorbate is the least polar of all the studied substances and its isomers could not be separated by MEKC or MEEKC. The resolution was improved and isomers were fully separated using capillary electrochromatography (CEC) with ODS stationary phase and a mobile phase consisting of 10 mM boric acid in 50% acetonitrile. Minor differences in the polarity and the shape of isomers and high resolving power of the applied techniques were sufficient for separation of very similar compounds. We have shown that versatile electromigration techniques can be applied for separation of geometrical isomers of dienoic acids and their esters.     

用短毛细管柱分离一、二、三硝基甲苯的异构体

一、二、三硝基甲苯(简称MNT、DNT和TNT)共有15个异构体.分离这些异构体文献中虽有一些报道^[1-9],但均不太完善.我们曾用液晶填充柱分离和测定MNT的三个异构体[10];用OV-225填充柱分离DNT的六个异构体^[11];用OV-25和OV-210混合固定液填充柱分离TNT六个异构体^[12].     

Computational study on the structures of the [H, Si, N, C, O] isomers: possible species of interstellar interest

Ab initio methods have been used to study the lowest lying [H, Si, N, C, O] isomers, which are of astrochemical interest. Over 20 [H, Si, N, C, O] isomers in the 1A' electronic state have been investigated at the MP2/aug-cc-pVTZ level of theory. Of these, the seven lowest isomers have been further investigated using different levels of theory, including B3LYP and QCISD(T). It has been found that the relative energies of the isomers in their ground electronic state (1A') are very dependent on the level of theory used with either the cis-HOSiCN or cis-HOSiNC isomers being the lowest in energy. Overall, the four lowest isomers are within 6 kcal/mol of each other, and a further three isomers are less than 15 kcal/mol higher in energy than the lowest lying isomer, including HSiNCO, which has recently been detected spectroscopically. Natural bond analysis has been carried out on the ground electronic states of the seven lowest lying isomers to examine their electronic structure. The enthalpies of formation of the seven lowest isomers have also been evaluated using the G3MP2 and G3B3 multilevel methods and show that the isomers are relatively thermodynamically stable. The structures and energies of lowest lying 1A' and 3A' electronic states of these isomers have also been investigated and show that for most of the isomers the optimized structures in these excited electronic states correspond to a transition state structure.     

Theoretical study on the structures and stabilities of the isomers of N6H6

Forty-eight possible isomers of N₆H₆ have been investigated at the B3LYP/6-311++G^** and G3B3 levels. The results show that heats of formation of all isomers are positive and A2-3 is the lowest and C2 is the highest. In addition, the energies and the heats of formation of the ring-shaped isomers are higher than those of the chain-shaped isomers. Among the ring-shaped isomers, the four-membered ring isomers have the highest heat of formation and the five-membered ring isomers have the lowest. For six-membered ring isomers, only the chair-shaped isomers are stable. Among all those chain-shaped isomers, the ones which have N–N bond and its length is in the range of NN double bond can improve the stability of the compounds.     

核桃仁中多酚类物质的液相/电喷雾质谱分析

利用HPLC-ESI-MSn方法对核桃(Juglans regia L.)仁中多酚类物质组成进行了分析,发现的多酚类物质有鞣花酰基葡萄糖4种异构体、橡椀酰基葡萄糖2种异构体、二鞣花酰基葡萄糖2种异构体、二没食子酰基葡萄糖2种异构体、鞣花酰基和橡椀酰基葡萄糖3种异构体.结果表明,采用HPLC-ESI-MSn方法可以对核桃仁中多酚类物质进行有效的分析.     

Improved HRGC separation of cis, trans CLA isomers as Diels-Alder adducts of alkyl esters

This paper reports the separation of four isomers of conjugated linoleic acid (CLA), c,t/t,c-8,10; c,t/t,c-9,11; c,t/t,c-10,12; c,t/t,c-11,13, after reaction of esterification with aliphatic alcohols of different chain length and adduct formation with 4-methyl-1,2,4-triazoline-3,5-dione (MTAD). The high resolution gas chromatographic analyses were carried out using a simple 50-m cyanopropyl polysiloxane capillary column both with a flame ionization detector and a mass spectrometer. The resolution between the two pair of isomers: c,t/t,c-9,11 and c,t/t,c-10,12 and between c,t/t,c-10,12 and c,t/t,c-11,13 isomers were good for all the investigated alkyl esters and increased with the chain length of alcohol esterified to carboxylic moiety of CLA isomers. The most interesting result was relative to the c,t/t,c-8,10 and c,t/t,c-9,11 isomers, critical pair of isomers also when analyzed with a 120-m cyanopropyl polysiloxane capillary column; their resolution also increased from methyl to hexyl esters of CLA isomers and reached an acceptable value (0.8) in the case of hexyl esters. The best resolutions of the four considered CLA isomers were obtained with the hexyl esters of MTAD adducts of the isomers, without excessive analysis time. This method was useful and simple to evaluate the profile of the four main c,t isomers in commercial CLA samples.     

Fluorescence study of toluic and anisic acid isomers

The fluorescence of benzoic acid and some of its derivatives (including toluic and anisic acids isomers) in 10% glacial acetic acid-chloroform as a solvent is presented. The percentage of glacial acetic acid that maximizes the fluorescence of these isomers is between 5 and 10%. The excitation and the emission wavelengths of all isomers are presented in addition to limits of detection, relative standard deviations, and fluorescence band half-widths. The stabilities of these isomers are studied and the determination of one isomer in the presence of other isomers is reported.     

Determination of ortho-cresyl phosphate isomers of tricresyl phosphate used in aircraft turbine engine oils by gas chromatography and mass spectrometry

Tricresyl phosphate (TCP) is used as an anti-wear additive in aircraft turbine engine oil. Concerns about its toxicity are largely based on the tri-o-cresyl phosphate isomer content. However, the presence of other and more toxic isomers has been previously suggested. In this work, the structural isomers of TCP have been determined by two methods (experimental and semi-theoretical). First, the TCP isomers were separated by gas chromatography (GC) and identified by mass spectrometry (MS). Second, after base cleavage of TCP, GC was used to quantify the cresol precursors. These results were used to calculate the TCP isomer distribution based on the assumption of a statistical distribution of the TCP isomers. The results from the two determinations showed reasonable agreement for three of the four oils studied. The o-cresyl isomers were found to be present almost exclusively as the more toxic mono-o-cresyl isomers in the concentration range 13-150 mg/L. The ability to analyse for the mono-o-cresyl isomers allows the toxicity of TCP to be based on the latter isomers rather than on the less toxic tri-o-cresyl phosphate isomer.     

对环芳烷二氯化产物组成的GC—MS表征

对环芳烷([2,2]Paracyclophane,简称PCP）是指苯环对位桥联的化合物,PCP及其氯代PCP的重要用途是通过升华、裂解、聚合等步骤沉积于物体表面形成聚合膜,该聚合膜具有透湿和透气小、抗化学腐蚀和抗辐射等优良性能[1,2].     

Rapid and sensitive differentiation of anomers,linkage, and position isomers of disaccharides using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS)

A challenging aspect of structural elucidation of carbohydrates is gaining unambiguous information for anomers, linkage, and position isomers. Such isomers with identical mass can't be easily distinguished in mass spectrometry and a separation step is required prior to mass spectrometry identification. In our laboratory, gas-phase separation and differentiation of anomers, linkage, and position isomers of disaccharides was achieved using High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS). The FAIMS method responds to changes in ion mobility at high field rather than absolute values of ion mobility, and was shown to provide efficient separation and identification of disaccharide isomers at high sensitivity. Separation of analyzed disaccharide isomers can be accomplished at low nM level in a matter of seconds without sample purification or fractionation. Capability for examining a large population of ionic species of disaccharides by this method allowed for correlating structural details of disaccharide isomers with their separation properties in FAIMS. Results for disaccharide isomers indicate that this method could be applied to a larger group of carbohydrates.     

Prediction of low-energy isomers of large fullerenes from C132 to C160

To predict energetically favored isomers, we used a topological scheme as a prescreening tool to select candidate isomers for each fullerene from C(106) to C(160). Comparison with the PM3 and tight-binding (TB) potential calculated results and few published data for the low-energy isomers of C(106) to C(130) indicates that the prescreening approach is feasible. For each fullerene from C(132) up to C(160), the selected 1000 candidate isomers were further optimized by PM3 and TB potential. The analysis of the semiempirical PM3 and TB results of C(106) to C(160) provides some qualitative features of the large fullerenes. Furthermore, calculations at the B3LYP/6-31G*//B3LYP/3-21G level of theory were carried out on the top ten PM3 and TB low-energy isomers of C(132) to C(160) to accurately predict the stable isomers, and the HOMO-LUMO gap, the ionization energy, and electron affinity of the lowest-energy isomers were also investigated at the same level.     

Characterisation of sorbate geometrical isomers

trans,trans Isomers of sorbic acid, its potassium salt and ethyl sorbate isomerise under UV irradiation. All four geometrical isomers of the acid, salt and ester were separated using high-performance liquid chromatography on a nonpolar reversed-phase column (C18) and the ester also by gas chromatography on a VOCOL capillary column. The limit of detection and the interval of linearity were determined for all chromatographic methods. Individual isomers were identified with NMR analysis. Resolved chemical shifts of protons adjacent to the double bonds enabled qualitative and quantitative determination of isomers in the mixture. Antimicrobial activity of potassium sorbate isomers was tested on yeast Saccharomyces cerevisiae. Results show that the pure trans,trans isomer has a higher antimicrobial activity than the mixture of isomers.     

Analysis of triacylglycerol positional isomers by silver ion high performance liquid chromatography

Silver ion chromatography, utilizing two commercially available HPLC columns connected in series, was used to separate a variety of triacylglycerol positional isomers. The isomers had a fatty acid composition of ABA and AAB, where A and B were fatty acids containing zero to three double bonds. Isocratic solvent systems of 0.5% to 1.0% acetonitrile in hexane were used to separate the triacylglycerol isomers. This methodology can be used to rapidly determine the isomeric purity of synthesized or isolated triacylglycerols and to isolate specific positional isomers.     

Analysis for the Isomers of Naphthylamine by Reversed-Phase HPLC with Temperature Controlled

Generally, the analysis of isomers by high performance liquid chromatography (HPLC) requires the liquid-solid adsorption chromatography (LSC) since few satisfactory separations of isomers can be achieved by regular reversed-phase chromatography. Because the moisture absorption from air by the mobile phase will change markedly the separable property of isomers on LSC system, the analysis of isomers by LSC is still a troublesome problem.     

Enumeration of Isomers of Acyclic Saturated Hydroxyl Ethers

In this paper, we present generating functions for counting constitutional isomers and stereo isomers of acyclic saturated compounds consisting of O, C and H for the first time, and obtain the numbers of constitutional isomers and stereo isomers of acyclic saturated hydroxyl ethers. The numerical results are tabulated.     

Series analysis methods in enumeration of chemical isomers

Summary Methods of series analysis are used to interpret the results of a computer enumeration of chemical isomers. It is demonstrated how a typical asymptotic behavior that controls the number of isomers arises from singular points of the generating function. The approach is tested on the examples of the isomers of alkanes, alkenes, alkyl radicals and polyenes.     

三重态SiCP2异构体的结构和稳定性的理论研究

采用DFT,QCISD和CCSD(T)等理论计算方法对三重态SiCP2异构体的结构和稳定性进行了理论研究.在B3LYP/6-311G(d)水平下,共计算得到由17个过渡态相连接的15个异构体.在CCSD(T)/6-311 +G(2df)//QCISD/6-311G(d)水平下,考虑重点振动能相对能量最低的三元环状异构体P-cCSiP 8(0.0 kJ/mol)及四元环状结构的cPCSiP 4具有相当大的动力学稳定性,在一定的实验室和星际条件下可能被检测到.另外,对它们的成键性质也进行了分析.     

Synthesis of the major isomers of Aprepitant and Fosaprepitant

The synthesis of two isomers of Aprepitant （APT） and three isomers of Fosaprepitant （FPT）, crucial components for quality control in manufacturing, is described. Herein, three chiral centers in the isomers of Aprepitant are established in high yield by induced crystallization and chiral reduction. Additionally, the isomers of Aprepitant are utilized to synthesize the isomers of Fosaprepitant with the same stereochemistry. All the target compounds were confirmed by elemental analyses, 1R, NMR and MS data analysis.     

Improved identification of conjugated linoleic acid isomers using silver-ion HPLC separations

Silver-ion high-performance liquid chromatography (Ag+-HPLC) has been shown to be effective in the resolution of most of the isomers of conjugated octadecadienoic acids (18:2), also known as conjugated linoleic acid (CLA). The CLA isomers identified in natural fats from ruminants are a mixture of numerous positional and geometric isomers from 7,9- to 12,14-18:2. Ag+-HPLC separates both geometric (trans,trans < cis/trans < cis,cis) and positional CLA isomers using the mobile phase hexane/acetonitrile (99.9:0.1). The elution volumes for the CLA isomers were not only affected by the concentration of acetonitrile (in the prepared mobile phase) but also with successive runs during the day using a prepared mobile phase batch, due to the partial solubility of acetonitrile in hexane. However, this drift does not affect the relative resolution of the CLA isomers. The addition of diethyl ether to the mobile phase partly stabilizes the solvent mixture. In order to facilitate the interpretation of Ag-+HPLC chromatograms, the relative retention volumes (RRV) were calculated for each CLA isomer. Toluene was added to all the test portions and served as an estimator of dead volume, whereas the elution of the ubiquitous 9c,11t-CLA isomer was chosen as unity (1.00). Expressing the elution of all the CLA isomers as their RRV greatly helped to standardize each CLA isomer, resulting in relatively small coefficients of variation (% CV) for the trans,trans (<1.5%) and cis/trans (<0.5%) CLA isomers. The identification of the CLA isomers was further facilitated by synthesis of authentic CLA isomers. All the geometric CLA fatty acid methyl esters (FAME) from positions 6,8- to 13,15-CLA were commercially available or synthesized by a combination of partial hydrazine reduction of known polyunsaturated fatty acids followed by alkali isomerization, isolation of products, and further iodine-catalyzed geometric isomerization. Based on expressing the elution volume as RRV and the availability of the synthetic CLA isomers, a unique reversal of the elution order of the c/t CLA isomers was found. It is also proposed that the retention times of CLA isomers by gas chromatography (GC) should be expressed as their relative retention times (RRT) relative to methyl gamma-linoleneate. The availability of CLA reference materials and the application of RRV and RRT to Ag+-HPLC and GC separations, respectively, will greatly improve in the identifications of CLA isomers.