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.     

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.     

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.     

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.     

Esters of 2,5-multisubstituted-1,3-dioxane-2-carboxylic acid: their conformational analysis and selective hydrolysis

The carbomethoxy group at the C2 position of the 2,5-multisubstituted 1,3-dioxanes prefers the axial conformation rather than the equatorial one due to an anomeric effect. The trans isomers of the 5-monosubstituted compounds are more selectively hydrolyzed than the cis isomers. Based on the calculated results, hydrolysis to the trans isomers is attributed to the larger carbonyl charges of the trans than those of the cis isomers. The anomeric and homoanomeric effects will explain the axial preference of the carbomethoxy group and selective hydrolysis to the trans isomers. Furthermore, the calculated stability between the cis and trans isomers is in good agreement with the experimental results in the equilibrium state.     

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.     

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].     

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.     

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.     

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.     

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.     

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.     

Molecular structure and retention behaviour of some polycyclic aromatic and perhydroaromatic hydrocarbons on graphitized carbon black

Summary On efficient glass capillary columns packed with graphitized thermal carbon black (GTCB) the stereoisomers of aromatic and saturated tricyclic hydrocarbons are completely separated and identified. The Henry's adsorption constants of the individual isomers were determined from mixtures at different temperatures. Five isomers of perhydroanthracene, five isomers of perhydrofluorene, two isomers of perhydrophenalene, four isomers of perhydroacenaphthene, as well as phenalene, dihydrophenalene, acenaphthene and acenaphthylene were investigated. The increase of the hydrogenization degree of unsaturated tricyclic hydrocarbons reduces the retention. Among the saturated tricyclic isomers the retention becomes shorter with the larger bending of the molecules, i.e. from the molecule having more equatorial bonds to the molecules having more axial connections. The experimentally determined Henry's constants were qualitatively compared with the structure of the separated molecules.     

Chromatographic analysis of cis/trans carotenoid isomers.

Cis/trans configurations of carotenoids are known to effect the biochemistry of carotenoids in certain situations. Methodology for separating carotenoid cis/trans isomers is of importance to nutritionists and food scientists because cis isomers of provitamin A carotenoids have lower provitamin A activities than the all-trans form. Traditional food processing and preservation methods, especially thermal treatments, induce the formation of cis isomeric forms. However, many challenges, are apparent for identifying and analyzing cis/trans isomers present in foods and other biological tissues. The development of current chromatographic methods for the separation of carotenoid cis/trans isomers is reviewed. For the separation of beta-carotene isomers, most procedures employ either Ca(OH)2 or Vydac C18 columns. In general, polymeric C18 columns allow for the detection of cis carotenes, while monomeric C18 columns provide for some separation of certain xanthophylls. The main cis isomers detected in foods are the 13-cis and 9-cis forms, although other forms have also been found (mainly 15-cis and various di-cis isomers). More studies involving the metabolism and physiological consequences of cis/trans isomers in the diet are needed. However, due to limitations in current techniques, further method development in the area of separation, detection and quantitation of cis/trans carotenoid isomers will be required.     

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.     

High-pressure liquid chromatography combined with fluorescence detection and solvent extraction for simultaneous determination of coproporphyrins I and III in human urine

A precise and reproducible high-pressure liquid chromatography (HPLC) method for the determination of coproporphyrin I and III isomers (cp isomers) in urine was investigated. The cp isomers were extracted into diethyl ether, the solution was evaporated and the residue dissolved prior to HPLC. The recoveries of both cp isomers were 87.8 +/- 3.2 and 90.0 +/- 2.1% and detection limits (S/N = 3) were 0.07 ng, respectively.     

Stereochemistry of heterocycles

2-Methyl-and 2,5-dialkyl-5-phenoxy-1,3-dioxanes were synthesized and separated into their geometrical isomers by precision rectification. The configuration and conformations of these isomers were determined by PMR spectroscopy. It is shown that the low-boiling trans isomers exist primarily in the chair conformation with an equatorial phenoxy group, whereas the high-boiling cis isomers exist primarily in the chair conformation with an axial phenoxy group.     

Enumeration of isomers of alkylcyclopropanes by means of alkyl 1,1‐biradicals

Henze and Blair [2] have successfully derived algorithms for enumerating the number of constitutional isomers of aliphatic compounds using alkyl radicals; however, the algorithms cannot be extended to enumerate constitutional isomers of cyclic compounds. Similarly, Read [4] has advocated the use of alkyl biradicals to enumerate constitutional isomers of aliphatic compounds, but not cyclic compounds. Apparently, the use of alkyl biradicals in the enumeration of constitutional isomers of cyclic compounds has been neglected. In this communication, an algorithm using alkyl biradicals to enumerate the number of constitutional isomers of cyclic compounds, namely alkylcyclopropanes, is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Analysis of the optical and geometrical isomer distributions in selected propylene glycol acetals

For the first time, relative distributions of optical and geometrical isomers in selected propylene glycol acetals are determined. Resolution of the four acetal isomers possible through the reaction of racemic propylene glycol (PG) with selected aldehydes is demonstrated. The four isomers are ascribed to the presence of syn and anti geometrical isomers for each optically active PG acetal enantiomer. Thus, the (+) and well as (-) enantiomer are found to have a pair of syn and anti geometrical isomers. The ratio of the (+) and (-) isomers in the product remains at an approximate 50:50 ratio, as expected. However, somewhat unexpectedly, the syn/anti geometrical isomer ratio systematically varies with the nature of the substituent comprising the side chain of the aldehyde. Mechanisms involving electronic and minimal steric effects are advanced as possible reasons for the change in the syn/anti PG acetal ratios.