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In this study, a facile colorimetric assay for 2,4,6-trinitrotoluene (TNT) in aqueous solution is developed based on the intrinsic peroxidase-like activity of cysteine protected molybdenum disulfide nanosheets (Cys-MoS2 NSs). The Cys-MoS2 NSs, obtained from sonication-assisted exfoliation of bulk MoS2 crystals, possess a great intrinsic peroxidase-like activity for the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2) to produce a blue-colored solution. Significantly, the oxidation of TMB was weakened or even prevented in the presence of TNT because of the formation of a Meisenheimer complex between TNT and TMB, which results in a marked decrease in the absorption and a color change of MoS2–TMB–H2O2 solution (TMB reaction system). The relative absorbance is proportional to the logarithm of concentration of TNT in the range of 0–50 μM with a 3σ limit of detection of 10 nM. Moreover, this assay for TNT is also applicable to real water samples.

Recent developments in experimental and computational chemistry have identified a rapidly growing class of nucleophilic aromatic substitutions that proceed by concerted (cSNAr) rather than classical, two‐step, SNAr mechanisms. Whereas traditional SNAr reactions require substantial activation of the aromatic ring by electron‐withdrawing substituents, such activating groups are not mandatory in the concerted pathways., Concerted or stepwise? A class of nucleophilic aromatic substitutions has been developed that proceeds by concerted (cSNAr) rather than classical, two‐step, SNAr mechanisms. Whereas traditional SNAr reactions require substantial activation of the aromatic ring by electron‐withdrawing substituents, such activating groups are not mandatory in the concerted pathways.

Owing to the importance of explosive detection for the security of land and the environment, the exploration of new methodologies for sensing electron-deficient nitroaromatics explosives (NAEs) is urgently imperative. In this work, we firstly reported a colorimetric sensor for visual detection of 2,4,6-trinitrotoluene (TNT) based on Cysteamine capped-CdSe quantum dots (QDs) decorated graphene-chitosan xerogel (GSXS), which is shown to have high signal-to-background ratio. Meisenheimer complex formation, which is a well-known sensing mechanism, is characterized by steady state and time resolved spectroscopy supported by Density-Functional-Theory (DFT). Upon Green Fluorescent Protein (GFP) illumination, this stable Meisenheimer complex actively suppresses the attributed fluorescence of QD-GSXS and thus providing a novel path for chemical sensing applications. Under optimized conditions, the sensor displayed a wide linear range from 0.0 to 311.4 μ M with a limit of detection 9.7 μ M. The developed chemosensor showed several advantages, including good selectivity and excellent stability. The proposed approach can be utilized for the rapid and sensitive detection of NAEs in the solution phase and also for clinical application.

Common methods of 2,4,6-trinitrotoluene (TNT) detection are based on direct detection to the concentration of TNT. If TNT can be accumulated first to increase partial concentration near the sensor and detected by sensing materials then, the detection performance can be significantly improved. As we know, fluorescence resonance energy transfer (FRET) is widely used as the sensing mechanism in TNT detection, which has an effective range of 10 nm. Based on this, we built an accumulation layer between sensing materials and TNT within effective range of FRET, where exists a large quantity of amino groups. The Amino groups can adsorb TNT and realize accumulation function. The accumulation and sensing layer played different roles in the detection of TNT based on distinct mechanism. Accumulation layer was realized by forming Meisenheimer complex between TNT and amino groups while sensing layer was realized by FRET. Via both two layers and mechanisms, the sensors achieved ultra-high sensitivity to TNT, with a limit of detection (LOD) of 0.6 nM and a linear range of 1–20 nM. Compared with pure graphene quantum dots (GQDs), the KSV of sensor in this work to TNT increased for about 14 times. The accumulation-detection path provides an effective method for the measurement of TNT with ultra-low concentration.

Nucleophilic aromatic substitution (SNAr) is widely used by organic chemists to functionalize aromatic molecules, and it is the most commonly used method to generate arenes that contain (18)F for use in positron-emission tomography (PET) imaging. A wide range of nucleophiles exhibit SNAr reactivity, and the operational simplicity of the reaction means that the transformation can be conducted reliably and on large scales. During SNAr, attack of a nucleophile at a carbon atom bearing a 'leaving group' leads to a negatively charged intermediate called a Meisenheimer complex. Only arenes with electron-withdrawing substituents can sufficiently stabilize the resulting build-up of negative charge during Meisenheimer complex formation, limiting the scope of SNAr reactions: the most common SNAr substrates contain strong π-acceptors in the ortho and/or para position(s). Here we present an unusual concerted nucleophilic aromatic substitution reaction (CSNAr) that is not limited to electron-poor arenes, because it does not proceed via a Meisenheimer intermediate. We show a phenol deoxyfluorination reaction for which CSNAr is favoured over a stepwise displacement. Mechanistic insights enabled us to develop a functional-group-tolerant (18)F-deoxyfluorination reaction of phenols, which can be used to synthesize (18)F-PET probes. Selective (18)F introduction, without the need for the common, but cumbersome, azeotropic drying of (18)F, can now be accomplished from phenols as starting materials, and provides access to (18)F-labelled compounds not accessible through conventional chemistry.

A reactivity and selectivity study of O,O-diethyl 2,4-dinitrophenyl phosphate () and O,O-diethyl 2,4-dinitrophenyl thionophosphate () with a series of thiols of low molecular weight: N-acetyl cysteine (NAC), l-cysteine (Cys), homocysteine (Hcys), glutathione (GSH), and d-penicillamine (Pen) was conducted. Results show that (i) these nucleophiles only attack at the aromatic moiety of both triester derivatives, (ii) a kinetic control product by sulfhydryl attack of thiols was observed in the reactions of both triesters with Cys and Hcys, followed by an intramolecular amine attack leading to a thermodynamic control product. The kinetic study leads to the proposal of Meisenheimer complex formation and then proton transfer to the reaction media as the mechanism of these reactions.

The total stabilization energies (TSEs) and anion stabilization energies (ASEs) of ring-substituted (X-) Meisenheimer complexes featuring two NO2 groups in the ring were determined using appropriate isodesmic reactions. The structures and energies of respective species were calculated at the B3LYP/6–311+G(2d,p) level of theory. Ten series of substituent effects were examined by varying substituent Y, which is connected to the sp3 carbon of the ring. The substituent effects were successfully analyzed using an extended Yukawa–Tsuno equation, [Formula: see text]. The r− values for the TSEs were identical to those for the ASEs, whereas the s values for the TSEs were significantly different from those for the ASEs. This shows that the effect of neutral species contributes to the s values of the TSEs. The r− and s values for the ASEs of all Meisenheimer complexes were distributed in a narrow range because substituent Y was insulated from the π-conjugation system. The r− values were large and the s values were small. This shows that the r− and s values were independent of each other and that the extended three-term Yukawa–Tsuno equation was intrinsic for substituent-effect analyses of anions. Although the variation was not substantial, the change in the r− values was clearly explained by the orbital interaction between substituent Y and the π-conjugation system. The r− values exhibited a good correlation with the bond lengths between the ring and the 4-NO2 group among all Meisenheimer complexes and benzylic anions. These facts provide a physical meaning: the r− value is a parameter that reveals the degree of the additional π interactions between the electron-withdrawing substituents and the π-conjugation systems of the ring.

The reaction between four tested nitroaromatic compounds with tetrapropyl ammonium hydroxide (TPAH) was used to produce Meisenheimer complex colour. This result was used to prepare a new fast, selective, and sensitive kit (F-MOST) composed of polyvinyl alcohol and glycerin beside the active agent (TPAH) with gel – like appearance. The gel-like appearance of F-MOST kit gave an advantage through excellent efficiency continued more than six months.

Trinitrotoluene (TNT) is a primary component in chemical explosives, making them a common focus in public safety detection. However, it is very difficult to achieve selective and sensitive detection of the TNT molecule in practical application. In the present study, a simple surface enhanced Raman scattering (SERS) sensing based on monoethanolamine (MEA) - modified gold nanoparticles (Au NPs) was expanded for high selectivity and sensitive detecting of TNT in an envelope, luggage, lake water, and clothing through a quickly sampling and detection process. The monoethanolamine molecule based on Meisenheimer complex lights up ultra-high Raman scattering of a nonresonant molecule on the superficial coat of gold nanoparticles. Using this detection sensor, a molecular bridge can be established to selectively detect trinitrotoluene with a detection limit of 21.47 pM. We were able to rapidly identification trinitrotoluene molecule with a powerful selective over the familiar interfering substances nitrophenol, picric acid, 2,4-dinitrophenol, and 2,4-dinitrotoluene. The outcome in this work supply an efficient solution to the test of trinitrotoluene and to establishing a SERS sensor analytical strategy. The studies have demonstrated that the MEA-Au NPs based SERS sensing can be potentially used in field detection the trace amount of chemical explosives for public security.

The explosive 2,4,6-trinitrotoluene (TNT) is a highly toxic pollutant. Biodegradation is inevitably one of the most cost-effective and enviromentally friendly means of removing TNT pollution. However, the aromatic derivatives from the reduction of nitro groups by several classic enzymes are still toxic. Besides the reduction of nitro groups, pentaerythritol tetranitrate reductase (PETNR) offers a potential route to ring fission and complete degradation of TNT through the pathway of the Meisenheimer complex. This work is devoted to deeply understand the essence of the Meisenheimer pathway and mainly focus on the crucial hydrogen-transfer reaction by means of molecular dynamics (MD) simulations. We obtain three valuable findings. Firstly, the parallel π-π stacking between TNT and the flavin mononucleotide (FMN) cofactor is a precondition. The key residue controlling this conformation is His181. Although His184 does not interact with TNT, the mutation from His184 to Asn184 would abolish the π-π structure. Secondly, the data of the empirical valence bond (EVB) show that the Meisenheimer pathway is predominant because its activation barrier is 6.7 kcal mol-1 far less than that of nitro reduction (26.6 kcal mol-1). Finally, based on the results of thermodynamic integration (TI), the type of transferred hydrogen is also ensured, that is, the H anion (H-) for the Meisenheimer complex and the H radical (H˙) for nitro reduction. Our findings provide an exhaustive understanding for the first hydrogen transfer reaction that has a decisive effect on two competing pathways, and help in searching for and designing new enzymes that can effectively degrade TNT.

Rapid, reliable, onsite approaches for detection trace level of trinitrotoluene (TNT) is a pressing necessity for both homeland security and environmental protection. To this end, hydrophilic amine(-NH2) protected Au@Ag nanoparticles (NPs) were developed and fabricated as colorimetric paper sensor for delicate detection of TNT. The as-developed nanoprobe selectively reacts with TNT through classic Meisenheimer complex formation by means of charge transfer process from an electron-rich NH2 group of β-cysteamine to an electron-deficient nitro group on TNT. Due to the absence of this particular interaction of other nitroaromatics, the proposed probe is highly selective for TNT detection with a better linear range (0–20 μg/mL) and limit of detection (LOD) of 0.35 μg/mL. The present work provides a novel and facile strategy to fabricate colorimetric paper sensors with rapid and selective recognition ability for label free analysis of TNT.

2,4,6-Trinitrotoluene (TNT) forms a red-colored Meisenheimer complex with 3-aminopropyltrienthoxysilane (APTES) both in solution and on solid phase. The TNT–APTES complex is unique since it forms yellow-colored complexes with 2,4,6-trinitrophenol and 4-nitrophenol, and no complex with 2,4-dinitrotoluene. The absorption spectrum of TNT–APTES has two absorption bands at 530 and 650 nm, while APTES complexes with 2,4,6-trinitrophenol and 4-nitrophenol have absorption maxima at around 420 nm, and no absorption change for 2,4-dinitrotoluene. The TNT–APTES complex facilitates the exchange of the TNT-CH3 proton/deuteron with solvent molecules. The red color of TNT–APTES is immediately visible at 1 μM of TNT. Keywords: TNT detection, Nitroaromatics, Meisenheimer complex

A kinetic study on SNAr reactions of 1-halo-2,4-dinitrobenzenes (6a–6d) with alkali-metal ethoxides (EtOM; M = Li, Na, K and 18-crown-6-ether-complexed K) is reported. The plots of pseudo-first-order rate constant (kobsd) vs. [EtOM] curve upward or downward depending on the size of M+ ions. The reactions are catalyzed or inhibited by the M+ ions, e.g., the large K+ ion complexed by 18-crown-6-ether (18C6) acts as a catalyst while the small Li+ and Na+ ions behave as an inhibitor. Reactivity of 6a–6d toward EtOM decreases linearly as the halide ion becomes less basic regardless of the size of M+ ions, indicating that expulsion of the leaving group occurs after the rate-determining step (RDS). Thus, the reactions have been proposed to proceed through a stepwise mechanism with formation of a Meisenheimer complex being the RDS. Computational studies using B3LYP density functional theory have revealed that Mulliken charge density of the electrophilic center decreases as the halogen atom becomes less electronegative. Thus, it has been concluded that the SNAr reactivity of 6a–6d toward EtOM is governed by electrophilicity of the reaction center but not by nucleofugality of the leaving group. A π-complexed transition-state structure has been proposed to account for the experimental and computational results.

We report for the first time a photothermal paper sensor for the selective and sensitive detection of 2,4,6-trinitrotoluene (TNT) down to 14 ng/cm(2). In the presence of TNT, a Meisenheimer complex was formed by means of a charge transfer process from an electron-rich group in polyaniline (PANI) to an electron-deficient nitro group in TNT, which resulted in the near-infrared absorption around 800 nm. Upon irradiation with an 808 nm diode laser, the photothermal effect of the PANI/TNT complex caused the temperature increase, and the temperature difference (ΔT) was proportional to the TNT concentration, while the temperature increase was hardly observed for other nitroaromatics including 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenol (TNP), and nitrobenzene (NB), affording high selectivity toward TNT. All the tests can be conducted both in solution and on paper. Therefore, the proposed photothermal strategy not only offers a fast and convenient protocol for selective detection of TNT but also indicates great potential in practical applications, especially for airport/railway security inspection and prevention of terrorist attacks.

A flexible fluorescent sensing strategy for the recognition and detection of bisphenol A (BPA) has been proposed based on molecularly imprinted polymers (MIPs)-coated gold nanoclusters (AuNCs), by taking advantages of the high selectivity of MIPs and the strong fluorescence property of AuNCs. SiO2@AuNCs were initially prepared by making use of the powerful amido bonds between carboxyl-terminated AuNCs and amino-functionalized SiO2 nanoparticles. Then MIPs-coated AuNCs were formed by anchoring MIP layer on the surface of SiO2@AuNCs via a solgel process. In the presence of imprinting template BPA, a Meisenheimer complex could be formed between BPA and the primary amino groups on the surface of the AuNCs, and the photoluminescent energy of AuNCs would be transferred to the complex, and thereby result in the fluorescence quenching of AuNCs. The fluorescence-quenching fractions of the sensor presented a satisfactory linearity with BPA concentrations over the range of 013.1 mu M and the detection limit could reach 0.10 mu M. Distinguished selectivity was also exhibited to BPA over other possibly competing molecules. Moreover, the sensor was successfully applied to determine BPA in seawater, and the average recoveries of BPA at three spiking levels ranged from 91.3 to 96.2% with relative standard deviations below 4.8%. This AuNCs-MIPs based sensor provided great potentials for recognition and determination of phenolic environmental estrogens in complicated samples. (C) 2015 Elsevier B.V. All rights reserved.

We report rapid and ultra-sensitive detection system for 2,4,6-trinitrotoluene (TNT) using unmodified gold nanoparticles and surface-enhanced Raman spectroscopy (SERS). First, Meisenheimer complex has been formed in aqueous solution between TNT and cysteamine in less than 15 min of mixing. The complex formation is confirmed by the development of a pink colour and a new UV-vis absorption band around 520 nm. Second, the developed Meisenheimer complex is spontaneously self-assembled onto unmodified gold nanoparticles through a stable Au-S bond between the cysteamine moiety and the gold surface. The developed mono layer of cysteamine-TNT is then screened by SERS to detect and quantify TNT. Our experimental results demonstrate that the SERS-based assay provide an ultra-sensitive approach for the detection of TNT down to 22.7 ng/L. The unambiguous fingerprint identification of TNT by SERS represents a key advantage for our proposed method. The new method provides high selectivity towards TNT over 2,4 DNT and picric acid. Therefore it satisfies the practical requirements for the rapid screening of TNT in real life samples where the interim 24-h average allowable concentration of TNT in waste water is 0.04 mg/L.

A facile and sensitive surface-enhanced Raman scattering substrate was prepared by controlled potentiostatic deposition of a closely packed single layer of gold nanostructures (AuNS) over a flat gold (pAu) platform. The nanometer scale inter-particle distance between the particles resulted in high population of ‘hot spots’ which enormously enhanced the scattered Raman photons. A renewed methodology was followed to precisely quantify the SERS substrate enhancement factor (SSEF) and it was estimated to be (2.2 ± 0.17) × 105. The reproducibility of the SERS signal acquired by the developed substrate was tested by establishing the relative standard deviation (RSD) of 150 repeated measurements from various locations on the substrate surface. A low RSD of 4.37 confirmed the homogeneity of the developed substrate. The sensitivity of pAu/AuNS was proven by determining 100 fM 2,4,6-trinitrotoluene (TNT) comfortably. As a proof of concept on the potential of the new pAu/AuNS substrate in field analysis, TNT in soil and water matrices was selectively detected after forming a Meisenheimer complex with cysteamine.

The “Addition–NGP (neighbouring group participation)–Elimination” technique has been used for the ratiometric sensing of H2S/HS−. We report herein the sensing of H2S by the nucleophilic addition of H2S/HS− to a –CHO group of R1 adjacent to naphthyl hydroxyl followed by NGP of thiol through a probable six-membered transition state that undergoes the cleavage of the dinitrophenyl ether moiety. In contrast, R2 undergoes a probable addition and elimination mechanism forming a Meisenheimer complex, which also cleaves the ether moiety. However, herein, the reaction is very slow compared to R1, which works through NGP. The probe can be highly promising for selectivity and sensitivity, to detect H2S/HS− with rapid response (

When an amino acid-capped quantum dots solution meets 2,4,6-trinitrotoluene (TNT), it changes from colorless to red and its fluorescence is quenched. This is a recently developed technique for the detection of TNT in trace amounts. However, what causes the changes in coloration and fluorescence remains controversial. Using density functional theory calculations, we studied the structures and optical properties of the products of TNT reacting with cysteine. Two compounds, namely, a Meisenheimer complex and a TNT anion, which are obtained from an addition reaction and an acid–base reaction respectively, were characterized, but neither of them could be used solely to interpret the experimental results. Our calculations proposed the possibility of their coexistence in the solution from their similar thermodynamic stability and their predicted absorption and vibrational spectra. The superposition of their calculated optical absorption spectra produces band distributions similar to those of the experiments. Moreover, the measured Raman spectra that had been used to characterize the formation of the Meisenheimer complex cannot exclude the formation of the TNT anion whose characteristic vibrations are buried by those of the former. Our calculations also revealed that in the Meisenheimer complex the electron delocalization in the phenyl ring of TNT is blocked by the attached cysteine, while for the TNT anion, the removal of the hydrogen atom enhances the electron delocalization and leads to a redshift of its first excitation in comparison with that of the Meisenheimer complex. Therefore, the key to TNT detection is to control the size of the QDs so as to adjust their emission to a wavelength around the absorption bands of either the Meisenheimer complex or the TNT anion that are formed with TNT and the amino acid.

This research presents a critical evaluation of chitin- and chitosan-based materials as innovative treatment alternatives for water contaminated with insensitive munitions (IMs) compounds. Specifically, chitin, chitosan, amine-functionalized chitin (AFC) were evaluated for adsorptive removal of these compounds. Cellulose and cellulose triacetate were evaluated for adsorptive performance for comparison. Chitosan-graphene oxide (CSGO) composite membranes were evaluated for removal via adsorption and filtration and compared against nanofiltration and reverse osmosis membranes in the current market. Insensitive munitions evaluated include nitrotriazolone (NTO), nitroguanidine (NQ), and 2,4-dinitroanisole (DNAN); 2,4,6-trinitrotoluene (TNT) was also studied as a traditional munition for comparison. AFC is an effective adsorbent for NTO, DNAN, and TNT. Cellulose triacetate was the only commercially available biopolymer adsorbent effective at removing munitions compounds from solution; only DNAN and TNT were removed. CSGO membranes effectively removed NTO, DNAN, and TNT, but removal performance degraded with time. Overall, this research shows that the materials studied are viable options for removing IM and traditional munitions from water.

Stable zwitterionic spirocyclic Meisenheimer compounds were synthesized using a one-step reaction between picric acid and diisopropyl (ZW1) or dicyclohexyl (ZW3) carbodiimide. A solution of these compounds displays intense orange fluorescence upon UV or visible light excitation, which can be quenched or "turned-off" by adding a mole equivalent amount of F⁻ or CN⁻ ions in acetonitrile.Fluorescence is not quenched in the presence of other ions such as Cl⁻, Br⁻, I⁻, NO₂⁻ , NO₃⁻ , or H₂PO₄⁻ These compounds can therefore be utilized as practical colorimetric and fluorescent probes for monitoring the presence of F⁻ or CN⁻ anions.

The recycle, cleaning, and reuse of water are highly important for environmental remediation. This issue is addressed by creating a fluorescent zwitterionic spirocyclic Meisenheimer complex with high chelating propensity for toxic metals using low-cost starting materials and a one-pot synthesis technique. The resulting material is able to detect fluoride up to 12.8 ppb level and remove 82% aqueous fluoride from 1000 mL of 100 ppm fluoride solution in a single contact. The material demonstrates rapid kinetics and is capable of dropping the toxic metal ion (Pb/Hg/Cd) concentration below 0.2 ppb within 10 min. A resin-free, precipitation-free, and reusable technique has been developed for the removal of toxic metal ions and fluoride from extremely polluted water. Moreover, utilizing its extreme hydrophobicity, polystyrene sponges have been coated with the Meisenheimer complex to mop up oil spill and organic solvents from a biphasic mixture.

Microsomal glutathione transferase 1 (MGST1) is a detoxification enzyme belonging to the Membrane Associated Proteins in Eicosanoid and Glutathione Metabolism (MAPEG) superfamily. Here we have used electron crystallography of two-dimensional crystals in order to determine an atomic model of rat MGST1 in a lipid environment. The model comprises 123 of the 155 amino acid residues, two structured phospholipid molecules, two aliphatic chains and one glutathione (GSH) molecule. The functional unit is a homotrimer centered on the crystallographic three-fold axes of the unit cell. The GSH substrate binds in an extended conformation at the interface between two subunits of the trimer supported by new in vitro mutagenesis data. Mutation of Arginine 130 to alanine resulted in complete loss of activity consistent with a role for Arginine 130 in stabilizing the strongly nucleophilic GSH thiolate required for catalysis. Based on the new model and an electron diffraction data set from crystals soaked with trinitrobenzene, that forms a dead-end Meisenheimer complex with GSH, a difference map was calculated. The map reveals side chain movements opening a cavity that defines the second substrate site.

Selective catalytic SNAr reaction of polyfluoroaryl C-F bonds with chloride is shown. Stoichiometric TMSCl makes the reaction exergonic and allows catalysis, which involves ground state elevation of chloride, aromatic donor-acceptor interactions, and stabilization of the Meisenheimer complex. Traditional cross-coupling of the products is now possible and demonstrates the utility.

Shewanella oneidensis, a Gram-negative γ-proteobacterium with an extensive redox capacity, possesses four old yellow enzyme (OYE) homologs. Of these, Shewanella yellow enzyme 4 (SYE4) is implicated in resistance to oxidative stress. Here, we present a series of high-resolution crystal structures for SYE4 in the oxidized and reduced states, and in complex with phenolic ligands and the nitro-aromatic explosive picric acid. The structures unmask new features, including the identification of a binding platform for long-chain hydrophobic molecules. Furthermore, we present the first structural observation of a hydride-Meisenheimer complex of picric acid with a flavoenzyme. Overall, our study exposes the binding promiscuity of SYE4 toward a variety of electrophilic substrates and is consistent with a general detoxification function for SYE4.

Ammonia is not only a highly important gas for civilization but also contribute significantly for climate change and human health hazard. Highly sensitive ammonia sensor has been developed from a fluorescent zwitterionic spirocyclic Meisenheimer complex. Moreover, formation of this Meisenheimer complex can also be utilized for selective as well as naked eye instant detection of nitro aromatic explosive picric acid. The presence of a quaternary nitrogen atom directly attached to the spiro carbon is the unique feature of this Meisenheimer complex. This excellent photoluminescent (PL) Meisenheimer complex has two distinct stimuli responsive sites. One is sensitive towards acid while the other one is towards the base. These two positions can be modulated by adding one equivalent acid and one equivalent base to result two new products which are non fluorescent. One of these two non fluorescent species was found very exciting because of its UV/Vis transparency. Utilizing this concept we have fabricated an on-line sensor for measuring ammonia in dry or humid and condensing sewer air. The sensor was robust against ambient temperature and humidity variation. We have also developed an invisible ink from this Meisenheimer complex, with potential application for security purpose.

bibtex: ISI:000416584000037 bibtex\location:'1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA',publisher:'AMER ASSOC ADVANCEMENT SCIENCE',type:'Article',affiliation:'Hill, MS (Reprint Author), Univ Bath, Dept Chem, Bath BA2 7AY, Avon, England. Maron, L (Reprint Author), Univ Toulouse, 135 Ave Rangueil, F-31077 Toulouse, France. Maron, L (Reprint Author), Univ Toulouse III Paul Sabatier, CNRS, Inst Natl Sci Appl, UMR 5215,LPCNO, 135 Ave Rangueil, F-31077 Toulouse, France. Wilson, Andrew S. S.; Hill, Michael S.; Mahon, Mary F., Univ Bath, Dept Chem, Bath BA2 7AY, Avon, England. Dinoi, Chiara; Maron, Laurent, Univ Toulouse, 135 Ave Rangueil, F-31077 Toulouse, France. Dinoi, Chiara; Maron, Laurent, Univ Toulouse III Paul Sabatier, CNRS, Inst Natl Sci Appl, UMR 5215,LPCNO, 135 Ave Rangueil, F-31077 Toulouse, France.','author-email':'m.s.hill@bath.ac.uk maron@irsamc.ups-tlsc.fr',da:'2018-12-05','doc-delivery-number':'FO2DN',eissn:'1095-9203','funding-acknowledgement':'University of Bath; Engineering and Physical Sciences Research Council','funding-text':'We thank the University of Bath and the Engineering and Physical Sciences Research Council for funding of a Doctoral Training Partnership Ph.D. studentship (A.S.S.W.). The crystallographic data for compounds 6, 7, 8, 9, and 6-d have been deposited with the Cambridge Crystallographic Data Centre as entries 1565865 to 1565869, respectively. All other experimental data are presented in the supplementary materials. Data file S1 contains the Cartesian coordinates for the calculated structures described in this study. Data file S2 contains the Cartesian coordinates of the transition state for the alternative sigma-bond metathesis pathway that was located +29.7 kcal mol\textlessSUP\textgreater-1\textless/SUP\textgreater higher in energy than the metathesis transition state (TSHI) illustrated in Fig. 5.','journal-iso':'Science','keywords-plus':'SOLUBLE CALCIUM HYDRIDE; HEAVY GRIGNARD-REAGENTS; ALKALINE-EARTH METAL; STYRENE POLYMERIZATION; CHEMICAL BEHAVIOR; COMPLEXES; SUBSTITUTION; HYDROGEN; CATALYSIS; STRONTIUM','number-of-cited-references':'40','orcid-numbers':'Wilson, Andrew/0000-0003-3972-9571 Hill, Michael/0000-0001-9784-9649','research-areas':'Science & Technology - Other Topics','times-cited':'15','unique-id':'ISI:000416584000037','usage-count-last-180-days':'27','usage-count-since-2013':'88','web-of-science-categories':'Multidisciplinary Sciences'\; The electrophilic aromatic substitution of a C-H bond of benzene is one of the archetypal transformations of organic chemistry. In contrast, the electron-rich p-system of benzene is highly resistant to reactions with electron-rich and negatively charged organic nucleophiles. Here, we report that this previously insurmountable electronic repulsion may be overcome through the use of sufficiently potent organocalcium nucleophiles. Calcium n-alkyl derivatives-synthesized by reaction of ethene, but-1-ene, and hex-1-ene with a dimeric calcium hydride-react with protio and deutero benzene at 60 degrees C through nucleophilic substitution of an aromatic C-D/H bond. These reactions produce the n-alkyl benzenes with regeneration of the calcium hydride. Density functional theory calculations implicate an unstabilized Meisenheimer complex in the C-H activation transition state.

Identifying solvent- and catalyst-free conditions for polymerizations of engineering thermoplastics is of increasing interest due to new polymer processing technologies such as 3-D printing. We report the selective formation of linear poly(aryl ether sulfone)s (PESs) from the polycondensation of trimethylsilyl-protected bisphenol A (TMS-BPA) with nitro-substituted diaryl fluorides without added solvent or catalyst. DFT calculations show that nitro groups strategically placed in the ortho-position to the fluoride leaving group form a stable Meisenheimer complex during polyether synthesis. This strategy represents a route to linear PESs that employs anionic conditions, destabilizing propagating phenoxide chain ends preventing backbiting while simultaneously stabilizing the Meisenheimer complex intermediate. Thermodynamic over kinetic control in the polycondensation minimizes cyclic PES formation and promotes the formation of pure linear PESs.

A kinetic study is reported for SNAr reaction of 1-fluoro-2,4-dinitrobenzene (5a) and 1-chloro-2,4-dinitrobenzene (5b) with alkali-metal ethoxides (EtOM, M = Li, Na, K and 18-crown-6-ether complexed K) in anhydrous ethanol. The second-order rate constant increases in the order kEtOLi < kEtO− < kEtONa < kEtOK < kEtOK/18C6 for the reaction of 5a and kEtOLi < kEtONa < kEtO− < kEtOK < kEtOK/18C6 for that of 5b. This indicates that M ion behaves as a catalyst or an inhibitor depending on the size of M ion and the nature of the leaving group (F vs. Cl). Substrate 5a is more reactive than 5b, although the F in 5a is ca. 10 pKa units more basic than the Cl in 5b, indicating that the reaction proceeds through a Meisenheimer complex in which expulsion of the leaving group occurs after the rate-determining step (RDS). M ion would catalyze the reaction by increasing either the nucleofugality of the leaving group through a four-membered cyclic transition state or the electrophilicity of the reaction center through a π-complex. However, the enhanced nucleofugality would be ineffective for the current reaction, since expulsion of the leaving group occurs after the RDS. Thus, it has been concluded that M ion catalyzes the reaction by increasing the electrophilicity of the reaction center through a π-complex between M ion and the π-electrons in the benzene ring.

A new fluorescent sensor based on single water-soluble β-cyclodextrin (β-CD) molecule as the support carrier of donor and acceptor complex was designed, and used for TNT detection by fluorescence resonance energy transfer (FRET). In this sensing platform, per-6-amino-β-CD (per-6-NH 2 -β-CD) was used as the detection vehicle. The probe dye of fluorescein 5(6)-isothiocyanate (FITC) was covalently linked onto the per-6-NH 2 -β-CD rim with the grafting molar ratio of 1:1, the residual amino groups of per-6-NH 2 -β-CD can adsorb TNT molecules by forming Meisenheimer complex (TNT–amine complex) with TNT. The absorbtion spectrum of this complex has a spectral overlapping with the emission of FITC in aqueous solution, so it can strongly suppress the fluorescence emission of the adjacent FITC through FRET on β-CD vehicle. This FRET-based fluorescent sensing technique provides a facile, ultrasensitive and selective detection method for TNT molecule. The observed linear fluorescence intensity change could allow the quantitative detection TNT with the detection limit of 20 nM in water.

Time-resolved optical waveguide absorption spectroscopy (OWAS) makes use of an evanescent field to detect the polarized absorption spectra of sub-monomolecular adlayers. This technique is suitable for the investigation of kinetics at the solid/liquid interface of dyes, pigments, fluorescent molecules, quantum dots, metallic nanoparticles, and proteins with chromophores. In this work, we demonstrate the application of positive matrix factorization (PMF) to analyze time-resolved OWAS for the first time. Meanwhile, PCA is researched to compare with PMF. The absorption/desorption kinetics of Rhodamine 6G (R6G) onto a hydrophilic glass surface and the dynamic process of Meisenheimer complex between Cysteine and TNT are selected as samples to verify experimental system and analytical methods. The results are shown that time-resolved OWAS can well record the absorption/desorption of R6G onto a hydrophilic glass surface and the dynamic formation process of Meisenheimer complexes. The feature of OWAS extracted by PMF is dynamic and consistent with the results analyzed by the traditional function of time/wavelength-absorbance. Moreover, PMF prevents the negative factors from occurring, avoids contradicting physical reality, and makes factors more easily interpretable. Therefore, we believe that PMF will provide a valuable analysis route to allow processing of increasingly large and complex data sets.

The reaction of picryl benzoate derivatives 1a–g with aniline in methanol proceeds through COO and ArO bond cleavage pathways. Furthermore, the reactivity of these esters toward anilinolysis is correlated to the energy gap between highest occupied molecular orbital aniline and lowest unoccupied molecular orbital of each ester. The regioselectivity of acyloxygen versus aryloxygen cleavage is also discussed. The overall rate constants ktot split into kCOO (the rate constant of acyl-oxygen cleavage) and kArO (rate constant of aryl-oxygen cleavage). The COO bond cleavage advances through a stepwise mechanism in which the formation of the tetrahedral intermediate is the rate-determining step. The ArO bond cleavage continues through a SNAr mechanism in which the departure of the leaving group from the Meisenheimer complex occurs rapidly after its formation in the rate-determining step.

8-Hydroxyquinolines (HQs) functionalized ZnS nanoparticles (NPs) with an amine-capping layer (ZnS–NH2–Q NPs) were prepared by a ligand-exchange process. FT-IR, XRD, NMR, TEM and fluorescence spectrometer were used to characterize the novel ZnS–NH2–Q NPs. Upon the addition of 2,4,6-trinitrotoluene (TNT), the amino groups on the surface of ZnS–NH2–Q NPs can bind TNT molecule from solution by forming Meisenheimer complex. This complex absorbs the green part of visible light, and strongly suppresses the fluorescence emission of the ZnS–NH2–Q NPs through Fluorescence Resonance Energy Transfer (FRET). The observed linear fluorescence intensity response for TNT in the range of 0–1.89 μM allows the quantitative detection TNT, with a detection limit down to 10 nM.

Stable crystalline σ-complexes (Meisenheimer adducts) were isolated for the first time in the reactions of superelectrophilic 4,6-dinitrobenzofuroxan with dodecyl- and hexadecylamines, and their structure was determined by X-ray analysis.

The gas-phase reactions of the reactive λ(3)-phenyl(trifluoromethyl)iodonium (PhI(+)(III)CF3, 1 at m/z 273) to the radical cation of iodobenzene (PhI(•+), 2 at m/z 204) via the loss of ·CF3 and the radical cation of trifluoromethylbenzene (PhCF3(•+), 3 at m/z 146) via the loss of ·I, were studied by electrospray ionization tandem mass spectrometry (ESI-MS/MS). Interestingly, the gas-phase intramolecular coupling reaction of CF3 with phenyl via the CF3 migration process of 1 at m/z 273 from iodine to the phenyl to give 3 at m/z 146 could only occur according to an intramolecular aromatic substitution mechanism. Density functional theory (DFT) calculations showed that the gas-phase intramolecular aryltrifluoromethylation of 1 at m/z 273 to 3 at m/z 146 occurred via a Meisenheimer complex intermediate (MC), where the triplevalent I center of 1 was reduced to monovalent I. Most importantly, the structure of 3 at m/z 146 derived from 1 at m/z 273 in ESI-MS/MS process was confirmed by comparison of its MS/MS with that of an authentic PhCF3(•+) at m/z 146 acquired from the electron ionization (EI)-MS/MS analysis of PhCF3. Thus, our studies revealed the intrinsic reactivity tendencies of λ(3)-phenyl(trifluoromethyl)iodonium under solvent-free conditions.

A highly selective fluorescent probe for the effective discrimination of N-acetylcysteine (NAC) from cysteine (Cys) is proposed. Probe 1 contains an N,N-diethylrhodol ( DER ) dye and a dinitrophenyl ether moiety. Upon mixing with NAC in aqueous cetyltrimethylammonium bromide (CTAB) micellar solution, 1 was thiolyzed by NAC to release DER , thus affording a significant increase in fluorescence emission. Whereas for Cys, it gives only a dim response at the same reaction conditions. The significant difference in reaction rates can be explained via the fact that NAC shows more hydrophobicity than Cys, therefore the Meisenheimer complex intermediate ( 2a ) of its nucleophilic aromatic substitution with 1 can embed in CTAB micelles effectively, which will facilitate the formation of 2a and hence affords an acceleration of reaction rates. The proposed method shows an excellent selectivity for NAC over Cys, homocysteine (Hcy) and other amino acids.

The reactivity of the explosive tetryl (N-methyl-N,2,4,6-tetranitroaniline; Mw = 287 u) was studied using electrospray ionization in negative mode. The main species detected in the spectrum corresponds to the ion observed at m/z 318 (previously assumed to be the odd-electron ion [tetryl + HNO](-•), C7H6O9N6). In this study, we show using D-labeling combined with high-resolution mass spectrometry that this species corresponds to an even-electron anion (i.e. C8H8O9N5), resulting from the formation of a Meisenheimer complex between tetryl and the methanol used as the solvent. Fragmentation of this complex under CID conditions revealed an unexpected fragment: the formation of a 2,4,6-trinitrophenoxide anion at m/z 228. (18)O-labeling combined with quantum chemical calculations helped us better understand the reaction pathways and mechanisms involved in the formation of this product ion. This occurs via a transition state leading to a SN2-type reaction, consequently evolving toward an ion-dipole complex. The latter finally dissociates into deprotonated picric acid.

Water soluble methionine functionalized CdTe quantum dots (QDs) and CdTe/ZnS core–shell QD samples have been prepared by a reflux condensation method and have been used to detect explosive chemicals, such as dinitrotoluene (DNT), nitrotoluene (NT) and nitrobenzene (NB) in liquid media. Meisenheimer complex formation between the QD surface attached methionine and aromatic explosive molecules has helped to detect them selectively via a fluorescent quenching process. Fluorescence quenching occurred because of the transfer of excited electrons from QD to the explosive molecules. Depending upon the number of nitro groups present on the explosive molecule, the quenching efficiency of different analytes varied. Due to surface passivation and inductive effects, the methionine capped CdTe/ZnS core–shell quantum dot sample resulted in the maximum quenching constant.

Nitrobenzothiazinones are among the most potent antituberculosis agents. Herein, we disclose an unprecedented in vivo reduction process that affords Meisenheimer complexes of the clinical candidates BTZ043 and PBTZ169. The reduction is reversible, occurs in all mammalian species investigated, has a profound influence on the in vivo ADME characteristics, and has considerable implications for the design and implementation of clinical studies. The reduction was confirmed by chemical studies that enabled the complete characterization of the Meisenheimer complex and its subsequent chemistry. Combination of the in vivo and chemical studies with LC-MS characterization and assay development also provides a basis for rational lead optimization of this very promising class of antituberculosis agents.