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821.
A high level of ab initio and several density functional theory studies were performed with the target being to elucidate structural, spectroscopic, and energetic properties of triafulvene, cyclobutadiene, and tetrahedrane as three C4H4 isomers. For triafulvene all of the experimental values are available, but for cyclobutadiene only the IR spectra is known, while for tetrahedrane none of these physical values have been estimated experimentally. An excellent agreement between the computed and the experimental values were obtained where data are available, which makes it confident that the computed data should be very accurate. New structural data, IR spectra, and enthalpies of formation for both cyclobutadiene and tetrahedrane are offered. 相似文献
822.
823.
建立了正相高效液相色谱测定食物中8种维生素E异构体及维生素A的方法。样品中的维生素E异构体和维生素A经皂化和液液萃取,Waters ACQUITYTM UPLC BEH Amide色谱柱(150 mm×3.0 mm, 1.7 μm)分离,体积比为90 : 10的正己烷与叔丁基甲基醚-四氢呋喃-甲醇(20:10:1, v/v/v)为流动相,荧光检测器和紫外检测器串联检测。4种生育酚在5.0~60.0 mg/L(r2≥0.9999)、维生素A和4种三烯生育酚在0.5~6.0 mg/L(r2≥0.9996)范围内具有良好的线性,各基质中目标物的检出限在20~60 μg/kg之间;9个组分在各基质中的加标回收率为79.2%~114.2%,相对标准偏差(RSD)为1%~12%。该方法简便、灵敏、可靠、环保,可用于食物中8种维生素E异构体以及维生素A含量的同时测定。 相似文献
824.
Tainara Boareto Capelari Arnaldo César Pereira Leandro Luan Gonçalves de Oliveira 《Analytical letters》2019,52(9):1462-1476
A simple, low-cost and sensitive electroanalytical method was developed for the simultaneous determination of p-nitrophenol and o-nitrophenol isomers in water samples at a glassy carbon electrode (CGE) in the presence of cationic surfactant. The electrochemical behavior of p-nitrophenol and o-nitrophenol was studied by cyclic voltammetry (CV) in 0.1?mol L?1 acetate/acetic acid buffer (pH 3.70) in the presence and absence of cetylpyridinium bromide. The resolution of overlapped cathodic peaks potentials (Epc) of isomers was successfully improved in the presence of 100.0?µmol L?1 cetylpyridinium bromide, thus making this approach ideal for the simultaneous determination of isomers. Under the optimized conditions in 0.05?mol L?1 HEPES buffer at pH 7.0 using differential pulse voltammetry (DPV) at a scan rate of 45?mV s?1, pulse amplitude of 220?mV and modulation time of 10?ms, limits of detection 0.59?µmol L?1 for p-nitrophenol and 1.14?µmol L?1 for o-nitrophenol were obtained with linear ranges from 2.0 to 60.0?µmol L?1 and 3.0 to 60.0?µmol L?1, respectively. The intraday precision was assessed as relative standard deviation (%) for 20.0 and 40.0?µmol L?1 concentrations were 4.30% and 2.41% for p-nitrophenol and 4.87% and 2.20% for o-nitrophenol, respectively. The developed method was applied for the determination of the isomers in lake water samples. The accuracy was attested by comparison with high-performance liquid chromatography with diode array detection (HPLC-DAD) as a reference analytical technique. Recovery values ranging from 90.3% to 111.8% also attested to the accuracy of method for analysis of real samples. 相似文献
825.
Takaya Murakami Yoshiaki Iwamuro Reiko Ishimaru Satoshi Chinaka Noriyuki Kato Yuki Sakamoto Natsuhiko Sugimura Hiroshi Hasegawa 《Journal of mass spectrometry : JMS》2019,54(3):205-212
A reliable method for structural analysis is crucial for the forensic investigation of new psychoactive substances (NPSs). Towards this end, mass spectrometry is one of the most efficient and facile methods for the identification of NPSs. However, the differentiation among 2‐, 3‐, and 4‐fluoromethcathinones (o‐, m‐, and p‐FMCs), which are ring‐fluorinated positional isomers part of the major class of NPSs referred to as synthetic cathinones, remains a challenge. This is mostly due to their similar retention properties and nearly identical full scan mass spectra, which hinder their identification. In this study, we describe a novel and practical method for differentiating the fluorine substitution position on the phenyl ring of FMCs, based on energy‐resolved mass spectrometry (ERMS) using an electron ionization‐triple quadrupole mass spectrometer. ERMS measurements showed that the three FMC positional isomers exhibited differences in relative abundances of both the fluorophenyl cation (m/z 95) and the fluorobenzoyl cation (m/z 123). The logarithmic plots of the abundance ratio of these two cations (m/z 95 to m/z 123) as a function of the collision energy (CE) followed the order of o‐FMC < p‐FMC < m‐FMC at each CE, which allowed the three isomers to be unambiguously and reliably differentiated. The theoretical dissociation energy calculations confirmed the relationship obtained by ERMS analyses, and additional ERMS measurements of methylmethcathinone positional isomers showed that the differences in abundance among the FMCs were attributed to the differences in their collision‐induced dissociation reactivities arising from the halogen‐induced resonance effects on the phenyl ring. Moreover, the method for differentiation described herein was successfully applied to the actual samples containing seized drugs. We expect that the described methodology will also contribute significantly to the reliable and accurate structural identification of NPSs in the fields of therapeutic, clinical, and forensic toxicology. 相似文献
826.
Fentanyl has become pervasive as a drug of abuse and as adulterant in seized drugs. Positional isomers analyzed by gas chromatography with mass spectrometry can follow the same fragmentation pathway and therefore may not be differentiated. Additionally, electron ionization leads to lack of discernible molecular ion for most fentanyl related compounds. Liquid chromatography may be used as an orthogonal identification technique with diode array ultraviolet and mass spectrometric detection. Here we provide a chromatographic method for the separation of 20 different fentanyl analogues, homologues and positional isomers using ultra high performance liquid chromatography with photodiode array ultraviolet and mass spectrometry detection. Five different columns were investigated utilizing reverse phase chromatography and hydrophilic interaction chromatography. Chromatographic systems were evaluated to determine which could separate the most compounds overall, as well as the most positional isomers. We found that isocratic elution, with a methanol modifier (35%) and formic acid (0.1%) as an additive, on a C18 column at a temperature of 25°C could resolve 10/20 compounds overall and 16/20 positional isomers. Using electrospray ionization, compounds with different masses could easily be distinguished based on their pseudo molecular ions. Ultraviolet detection facilitated differentiation of positional isomers that could not be distinguished by either electron ionization or electrospray ionization mass spectrometry alone. 相似文献
827.
该文以Nafion膜为固定相,采用动态涂覆法将其涂覆在毛细管内壁,并研究了其在气相色谱分离烷烃、醇及取代芳香烃等方面的性能。结果表明,通过改变色谱分析条件,不同碳数的直链烷烃、直链醇、蒎烯类异构体、C8芳烃异构体取代芳香位置异构体均在Nafion膜固定相上得到了有效的分离,大部分异构体在该固定相上基本按照沸点从低到高的顺序依次洗脱。此外,还对基于Nafion膜的色谱柱分离蒎烯异构体和芳香位置异构体进行了热力学研究,并发现该柱分离异构体过程为放热过程,蒎烯分离受焓变和熵变共同控制,而芳香异构体分离过程主要受焓变的控制。 相似文献
828.
Wenting Song Junge Zhi Tianyang Wang Bo Li Shanshan Ni Yanchun Ye Jin‐Liang Wang 《化学:亚洲杂志》2019,14(21):3875-3882
AIE‐active positional isomers, TTE‐o‐PhCHO , TTE‐m‐PhCHO and TTE‐p‐PhCHO , tetrathienylethene ( TTE) derivates with peripherally attached ortho‐/meta‐/para‐formyl phenyl groups, were designed and synthesized. The formyl substitution position can effectively modulate their photophysical properties, mechanochromism and fluorescent response to hydrazine. TTE‐o‐PhCHO and TTE‐m‐PhCHO exhibit remarkable AIE characteristics, and TTE‐p‐PhCHO possesses aggregation‐induced emission enhancement performance. They all exhibit high contrast mechanochromism, and TTE‐m‐PhCHO shows larger red‐shift (164 nm) than TTE‐o‐PhCHO (104 nm) and TTE‐p‐PhCHO (125 nm) due to the more twisted molecular conformation and much looser molecular packing. Moreover, TTE‐o‐PhCHO with a higher contrast color change can be used as ink‐free rewritable paper. In addition, TTE‐p‐PhCHO , as a turn‐on fluorescent probe, can selectively detect hydrazine with significant color changes that are visible by the naked eye . Therefore, the position dependence of groups would be an effective method to modulate the molecular arrangement, as well as develop AIE compounds for mechano‐stimuli responsive materials, ink‐free rewritable papers and chemosensors. 相似文献
829.
利用 1H和 13C NMR, IR, UV, SEM对合成的4种C型和S型拓扑异构体进行了结构表征和分析, 发现对于不同拓扑结构的C型和S型分子可以通过 1H NMR来鉴别, 而对于相同拓扑结构的S型cis(SC)和trans(ST)分子, C型 cis(CC)和trans(CT)分子则要结合 13C NMR来鉴别. 四种拓扑异构体的IR, UV, 波谱性质以及SEM图象没有显著的差异. 相似文献
830.
De-Ming Liu 《Chromatographia》1988,25(5):393-396
Summary D,L-amino acids were derivatized with (+), (±)-2-butanol or N-trifluoroacetyl-L-prolyl chloride (L-TPC) and then chromatographed.
Four optical isomers were separated on a Chirasil-Val capillary column. By this method, the concentration of optical impurities
arising from the commercial optically active reagents can be determined. The observed abnormal elution orders of enantiomeric
amino acid esters may be caused by a selective intermolecular force. 相似文献