Selective removal of metallic single-walled carbon nanotubes with small diameters by using nitric and sulfuric acids

Coexistence of metallic and semiconducting carbon nanotubes has often been a bottleneck in many applications and much fundamental research. Single-walled carbon nanotubes (SWCNTs) were dissolved in HNO3/H2SO4 mixture to confirm differing reactivity between metallic (m) and semiconducting (s) SWCNTs. With HNO3/H2SO4 treatment, s-SWCNTs remained intact, while m-SWCNTs were completely removed for SWCNTs with small diameters less than 1.1 nm, which was confirmed by resonant Raman and optical absorption spectra. We also showed that nitronium ions (NO2+) dissolved in the HNO3/H2SO4 solution could preferably attack the m-SWCNTs, which was supported by our theoretical calculation. This clear selectivity can be explained by the preferential adsorption of positively charged NO2+ on m-SWCNTs due to more available electron densities at the Fermi level in the m-SWCNTs. We report for the first time a selective removal of small-diameter m-SWCNTs by using HNO3/H2SO4 solution, which presented a striking contrast to the diameter-selective removal of SWCNTs by oxidative etching reported previously.     

琼脂糖性质对凝胶电泳法分离金属性和半导体性单壁碳纳米管的影响

利用琼脂糖凝胶电泳分离单壁碳纳米管(SWCNTs)技术, 考察了MB, Agarose, Agarose B和LRU 4种琼脂糖对SWCNTs分离效率的影响. 紫外-可见-近红外(UV-Vis-NIR)吸收光谱研究结果表明, 不同的琼脂糖对SWCNTs中s-SWCNTs的分离效率影响较小, 而对m-SWCNTs的分离效率影响较大. 分析4种琼脂糖凝胶的凝胶强度和凝胶网孔尺寸等发现, 影响SWCNTs中m-SWCNTs分离效率的主要因素是琼脂糖的凝胶强度和琼脂糖凝胶形成的网孔尺寸, 小的凝胶网孔尺寸有利于m-SWCNTs富集, 高凝胶强度则不利于其富集.     

Interaction of narrow carbon nanotubes with nitronium tetrafluoroborate salts

Density functional calculations have been performed to investigate the destruction of narrow carbon nanotubes (CNTs) under the attack of nitronium tetrafluoroborate salts. The dissociation of these salts in a solvent produces nitronium and tetrafluoroborate ions which coadsorb on the external surface of the tubes. It is shown that the ions bind strongly to both metallic and semiconducting narrow nanotubes, although stronger to the metallic ones. The nitronium cations bind to the CNTs through a charge transfer mechanism, whereas the tetrafluoroborate anions remain negatively charged upon adsorption on the nanotubes. The surface of the nanotubes gets substantially deformed around the adsorption site of the nitronium ion, but it is hardly changed around the adsorption site of the tetrafluoroborate ion. These results are the theoretical basis to explain the destruction of the narrow CNTs found in the experiments and also to unravel, in agreement with the experimental interpretation, the distinct role played by the nitronium and the tetrafluoroborate ions. The tetrafluoroborate ions contribute to separate the CNTs from the bundles into individual tubes, without affecting the tubes. The nitronium ions, in contrast, modify the electronic and geometrical structures of the narrow tubes leading eventually to their destruction. The implications for the selective removal of intermediate diameter metallic CNTs found in the experiments are also discussed. The adsorption of the neutral nitrogen dioxide molecule is also studied, and the results show that the weak interactions of this molecule with both metallic and semiconducting tubes cannot be used as a model for the strong attack of the nitronium ions to the narrow tubes. The sensor effect of the nanotubes toward adsorption of nitrogen dioxide is also discussed.     

Fragmentation of negative ions from carbohydrates: Part 2. Fragmentation of high-mannose N-linked glycans

[M + NO3]- And [M + (NO3)2]2- ions were produced by electrospray from neutral high-mannose ([Man](5-9)[GlcNAc]2, [Glc](1-3)[Man](4-9)[GlcNAc]2) N-linked glycans and their 2-aminobenzamide derivatives sprayed from methanol:water containing ammonium nitrate. Low energy collision-induced decomposition (CID) spectra of both types of ions were almost identical and dominated by cross-ring and C-type fragments, unlike the corresponding spectra of the positive ions that contained mainly B- and Y-type glycosidic fragments. This behavior could be rationalized by an initial proton abstraction from various hydroxy groups by the initially-formed anionic adduct. These negative ion spectra were more informative than the corresponding positive ion spectra and contained prominent ions that were diagnostic of structural features such as the composition of individual antennas that were not easily obtainable by other means. C-ions defined the sequence of the constituent monosaccharide residues. Detailed fragmentation mechanisms are proposed to account for many of the diagnostic ions.     

Electrophilic aromatic nitration: understanding its mechanism and substituent effects

Theoretical calculations and gas-phase mass spectrometric studies were performed for the reaction of the naked (NO2+) and monosolvated (CH3NO2.NO2+) nitronium ion with several monosubstituted aromatic compounds. From these studies, we propose a general model for regioselectivity based on the single-electron transfer (SET) mechanism and an alternative mechanistic scheme for electrophilic aromatic nitration. This scheme considers the SET and the polar (Ingold-Hughes) mechanisms as extremes in a continuum pathway, the occurrence and extents of both mechanisms being governed mainly by the ability, or lack of ability, of the aromatic compound to transfer an electron to NO2+.     

Unified mechanistic concept of electrophilic aromatic nitration: convergence of computational results and experimental data

The mechanism of electrophilic aromatic nitration was revisited. Based on the available experimental data and new high-level quantum chemical calculations, a modification of the previous reaction mechanism is proposed involving three separate intermediates on the potential energy diagram of the reaction. The first, originally considered an unoriented pi-complex or electron donor acceptor complex (EDA), involves high electrostatic and charge-transfer interactions between the nitronium ion and the pi-aromatics. It explains the observed low substrate selectivity in nitration with nitronium salts while maintaining high positional selectivity, as well as observed oxygen transfer reactions in the gas phase. The subsequent second intermediate originally considered an oriented "pi-complex" is now best represented by an intimate radical cation-molecule pair, C(6)H(6)(+)(*)()/NO(2), that is, a SET complex, indicative of single-electron transfer from the aromatic pi-system to NO(2)(+). Subsequently, it collapses to afford the final sigma-complex intermediate, that is, an arenium ion. The proposed three discrete intermediates in electrophilic aromatic nitration unify previous mechanistic proposals and also contribute to a better understanding of this fundamentally important reaction. The previously obtained ICR data of oxygen transfer from NO(2)(+) to the aromatic ring are also accommodated by the proposed mechanism. The most stable intermediate of this reaction on its potential energy surface is a complex between phenol and NO(+). The phenol.NO(+) complex decomposes affording C(6)H(6)O(+)(*)/PhOH(+) and NO, in agreement with the ICR results.     

Gas-phase reactions of nitronium ions with acetylene and ethylene: an experimental and theoretical study

A comparative study of the gas-phase reactions of NO2+ with acetylene and ethylene was performed by using FT-ICR, MIKE, CAD, and NfR/ CA mass spectrometric techniques, in conjunction with ab initio calculations at the MP2/6-31+G* level of theory. Both reactions proceed according to the same mechanism, that is, 1,3-dipolar cycloaddition, but yield products of different stability. The C2H2NO2+ adduct from acetylene has an aromatic character and hence is highly stabilized with respect to the C2H4NO2+ adduct from ethylene. Both cycloadducts tend to isomerize into O-nitroso derivatives, that is, nitrosated ketene and nitrosated acetaldehyde, which represent the thermodynamically most stable products from the addition of NO2+ to acetylene and ethylene, respectively. As prototypal examples of the reactivity of free nitronium ions with most simple pi systems, the reactions investigated are useful starting points to model the mechanism of aromatic nitration.     

2-(2-羟基-5-氯苯基)-1H-苯并咪唑-铜(Ⅱ)配合物作为NO荧光探针的初探

常温下合成了2-(2-羟基-5-氯苯基)-1H-苯并咪唑荧光化合物和其二价铜离子的配合物,并对配合物的结构和稳定性进行了表征,用荧光光谱、质谱、红外光谱等研究了配合物与NO反应的机理.结果表明,该配合物与NO的反应具有高度的选择性,不受其他常见干扰分子的影响.配合物应用于脂多糖(LPS)激活的小鼠巨噬细胞中NO的测定,能够得到具有较好分辨率的荧光成像结果.     

A theoretical study on nitration mechanism of benzene and solvent effects

Aromatic nitration is an important and canonical example of electrophilic substitution in organic chemistry. The research on nitration mechanism is also very important for synthesis of explosives since benzene molecule is a basic unit to build up into the energetic material. Besides the electrophilic substitution mechanism, there is an electron transfer mechanism[1,2]. The nitronium ion (NO+ 2), however, is a generally accepted active nitrating agent in the aromatic nitration. Therefore, the …     

Generation of Singlet Molecular Oxygen by Lipid Hydroperoxides and Nitronium Ion†

Singlet molecular oxygen is a reactive species involved in biological oxidative processes. The major cellular targets of singlet molecular oxygen are unsaturated fatty acids in the membrane, as well as nucleic acids and proteins. The aim of this study was to investigate whether lipids and commercial hydroperoxides generate singlet molecular oxygen, in presence of nitronium and activated nitronium ion. For this purpose, monomol light emitted in the near-infrared region (λ = 1270 nm) was used to monitor singlet molecular oxygen decay in different solvents, with different hydroperoxides and in the presence of azide. Direct measurements of the singlet molecular oxygen spectrum at 1270 nm recorded during the reaction between lipids and commercial hydroperoxides and nitronium ions unequivocally demonstrated the formation of this excited species.     

Catalytic reduction of NO to N2O by a designed heme copper center in myoglobin: implications for the role of metal ions

The effects of metal ions on the reduction of nitric oxide (NO) with a designed heme copper center in myoglobin (F43H/L29H sperm whale Mb, CuBMb) were investigated under reducing anaerobic conditions using UV-vis and EPR spectroscopic techniques as well as GC/MS. In the presence of Cu(I), catalytic reduction of NO to N2O by CuBMb was observed with turnover number of 2 mol NO.mol CuBMb-1.min-1, close to 3 mol NO.mol enzyme-1.min-1 reported for the ba3 oxidases from T. thermophilus. Formation of a His-heme-NO species was detected by UV-vis and EPR spectroscopy. In comparison to the EPR spectra of ferrous-CuBMb-NO in the absence of metal ions, the EPR spectra of ferrous-CuBMb-NO in the presence of Cu(I) showed less-resolved hyperfine splitting from the proximal histidine, probably due to weakening of the proximal His-heme bond. In the presence of Zn(II), formation of a five-coordinate ferrous-CuBMb-NO species, resulting from cleavage of the proximal heme Fe-His bond, was shown by UV-vis and EPR spectroscopic studies. The reduction of NO to N2O was not observed in the presence of Zn(II). Control experiments using wild-type myoglobin indicated no reduction of NO in the presence of either Cu(I) or Zn(II). These results suggest that both the identity and the oxidation state of the metal ion in the CuB center are important for NO reduction. A redox-active metal ion is required to deliver electrons, and a higher oxidation state is preferred to weaken the heme iron-proximal histidine toward a five-coordinate key intermediate in NO reduction.     

Synthesis and characterization of sulfonated single-walled carbon nanotubes and their performance as solid acid catalyst

Single-walled carbon nanotubes (SWCNTs) were treated with sulfuric acid at 300 °C to synthesize sulfonated SWCNTs (s-SWCNTs), which were characterized by electron microscopy, infrared, Raman and X-ray photoelectron spectroscopy, and thermo analysis. Compared with activated carbon, more sulfonic acid groups can be introduced onto the surfaces of SWCNTs. The high degree (∼20 wt%) of surface sulfonation led to hydrophilic sidewalls that allows the SWCNTs to be uniformly dispersed in water and organic solvents. The high surface acidity of s-SWCNTs was demonstrated by NH₃ temperature-programmed desorption technique and tested by an acetic acid esterification reaction catalyzed by s-SWCNTs. The results show that the water-dispersive s-SWCNTs are an excellent solid acid catalyst and demonstrate the potential of SWCNTs in catalysis applications.     

Bulk synthesis of large diameter semiconducting single-walled carbon nanotubes by oxygen-assisted floating catalyst chemical vapor deposition

Semiconducting single-walled carbon nanotubes (s-SWCNTs) with a mean diameter of 1.6 nm were synthesized on a large scale by using oxygen-assisted floating catalyst chemical vapor deposition. The oxygen introduced can selectively etch metallic SWCNTs in situ, while the sulfur growth promoter functions in promoting the growth of SWCNTs with a large diameter. The electronic properties of the SWCNTs were characterized by laser Raman spectroscopy, absorption spectroscopy, and field effect transistor measurements. It was found that the content of s-SWCNTs in the samples was highly sensitive to the amount of oxygen introduced. Under optimum synthesis conditions, enriched s-SWCNTs can be obtained in milligram quantities per batch.     

On the nitration of 2-pyrone

A 1:1 adducl of 2-pyrone and nitronium fluoroborate has been delected by nmr and is a possible precursor of 5-nitro-2-pyrone, a subsequently formed product. From comparison of the nmr spectra of the 1:1 adduct and 2-methoxypyrylium fluoroborate, the adduct is believed to be the pyrylium fluoroborate derived from nitration of the carbonyl oxygen of 2-pyrone.     

Mass spectrometry of 4-amino-4′-nitroazobenzene compounds

The mass spectra of 32 substituted 4-amino-4′-nitroazobenzene compounds have been recorded and the most intense peaks have been used to characterize these spectra. It was found that the spectra of 4-amino-4′-nitroazobenzene compounds are characterized by peaks due to: (1) molecular ions, (2) fragment ions formed by cleavage of one of the carbon-nitrogen bonds adjacent to the azo linkage with the positive charge remaining with the amine fragment, (3) ions formed by cleavage alpha to the amine nitrogen with the charge remaining with the amine substituent, (4) ions formed by cleavage beta to the amine nitrogen with the loss of the amine substituent fragment, (5) secondary ions formed by cleavage beta to the amine nitrogen with the loss of the amine substituent fragment from the primary amine fragment (2), and (6) ions formed by loss of NO from the molecular ion. This work shows that 4-amino-4′-nitroazobenzene compounds exhibit fragmentation which is dependent in a consistent manner on the types of substituents. This work provides a basis for a systematic approach to the identification of 4-amino-4′-nitroazobenzene compounds.     

Raman spectra of the double-anion salts M3ZnCl4NO3 (M+ = K+, Rb+, NH4)

Recently characterized K3ZnCl4NO3 and (NH4)3ZnCl4NO3, and newly prepared Rb3ZnCl4NO3 constitute a limited series of isomorphous double-anion salts (space group Pnma, Z = 4). Room-temperature (295 K) Raman spectra from polycrystalline samples of the compounds are reported and interpreted on the basis of the Cs site symmetry of the ZnCl4(2-) and NO3- ions with reference to the D2h factor group of the unit cell. The spectra are compared with Raman spectra of the corresponding M2ZnCl4 and MNO3 single-anion salts. Relative positions and frequencies of the ZnCl4(2-) modes vary considerably among the M3ZnCl4NO3 compounds, despite the isomorphism. The NO3- modes are more similar in all three compounds. The NO3- doubly degenerate v3 and V4 modes are split into two distinct bands as a result of the decent in symmetry from D3h for the free ion to Cs at the crystallographic site. The unequal intensities of the v3 bands observed for K3ZnCl4NO3 and Rb3ZnCl4NO3 and the equal intensities of the v4 bands observed for all three compounds suggest the same factor-group assignments as the high-temperature phase NH4NO3(III). The free-ion Raman-inactive planar deformation mode, v2, is evident in all three compounds, but with lesser intensity than its overtone 2v2. In K3ZnCl4NO3 and Rb3ZnCl4NO3, the symmetric stretching band, in addition to the very strong component for v1, shows a weak, low-frequency band found in many ionic nitrates, which has been attributed to thermally disordered nitrate ions or hot bands. This feature is not found in the spectrum of (NH4)3ZnCl4NO3. The 12 NH4+ ions in the unit cell of (NH4)3ZnCl4NO3, which occupy C1 and Cs sites in a 2:1 ratio, give rise to extremely broad bands that show no evidence of the individual symmetry distinctions of the cations. The broad band from NH4+ v4 obscures the region in which NO3- v3 bands are expected, but the NO3- overtone 2v2 is evident as a sharp peak above a similarly broad band from NH4+ v2.     

Fragmentation of negative ions from carbohydrates: Part 3. Fragmentation of hybrid and complex N-linked glycans

Hybrid and complex N-linked glycans were ionized by electrospray in the presence of ammonium nitrate to give [M + NO3]- and [M + (NO3)2]2- ions. Low energy collision-induced decomposition (CID) spectra of both types of ions were almost identical and were dominated by C-type glycosidic and cross-ring fragments, unlike the corresponding spectra of the positive ions that contained mainly B- and Y-type glycosidic fragments. Also, in contrast to fragments in the positive ion spectra, many of these ions appeared to be produced by single pathways following proton abstraction from specific hydroxy groups. Consequently, many ions were diagnostic for specific structural features. Such features included the composition of each of the two antennas, the presence or absence of a bisecting GlcNAc residue, and the location of fucose residues on the core GlcNAc residues and on the antennas. C-ions defined the sequence of the constituent monosaccharide residues. Detailed fragmentation mechanisms are proposed to account for several of the diagnostic ions.     

A highly selective and sensitive BODIPY-based colourimetric and turn-on fluorescent sensor for Hg2+ ions

A series of monostyryl boron dipyrromethenes appended with an NO(4), NO(2)S(2), N(3)O(4), or N(3)O(2)S(2)-type ligand have been prepared and characterised. While the UV-Vis spectra of the former three compounds in CH(3)CN/H(2)O (2?:?3 v/v) do not respond towards a wide range of metal ions, the derivative with an N(3)O(2)S(2)-ligand exhibits a highly selective and sensitive spectral response towards Hg(2+) ions. The absorption band is blue-shifted by 40 nm due to inhibition of the intramolecular charge transfer process upon metal complexation. The fluorescence is also turned on giving a strong emission band at 572 nm. The colour changes can be easily detected by the naked eye. The results suggest that this compound serves as a promising colourimetric and fluorescent sensor for Hg(2+) ions in this mixed aqueous medium.     

Cu2+ ions as a paramagnetic probe to study the surface chemical modification process of layered double hydroxides and hydroxide salts with nitrate and carboxylate anions

A layered zinc hydroxide nitrate (Zn5(OH)8(NO3)2.2H2O) and a layered double hydroxide (Zn/Al-NO3) were synthesized by coprecipitation and doped with different amounts of Cu2+ (0.2, 1, and 10 mol%), as paramagnetic probe. Although the literature reports that the nitrate ion is free (with D3h symmetry) between the layers of these two structures, the FTIR spectra of two zinc hydroxide nitrate samples show the C2v symmetry for the nitrate ion, whereas the g ||/A || value in the EPR spectra of Cu2+ is high. This fact suggests bonding of some nitrate ions to the layers of the zinc hydroxide nitrate. The zinc hydroxide nitrate was used as matrix in the intercalation reaction with benzoate, o-chlorobenzoate, and o-iodobenzoate ions. FTIR spectra confirm the ionic exchange reaction and the EPR spectroscopy reveals bonding of the organic ions to the inorganic layers of the zinc hydroxide nitrate, while the layered double hydroxides show only exchange reactions.     

Pi-complex structure of gaseous benzene-NO cations assayed by IR multiple photon dissociation spectroscopy

[C(6)H(6)NO](+) ions, in two isomeric forms involved as key intermediates in the aromatic nitrosation reaction, have been produced in the gas phase and analyzed by IR multiple photon dissociation (IRMPD) spectroscopy in the 800-2200 cm(-)(1) fingerprint wavenumber range, exploiting the high fluence and wide tunability of a free electron laser (FEL) source. The IRMPD spectra were compared with the IR absorption spectra calculated for the optimized structures of potential isomers, thus allowing structural information on the absorbing species. [C(6)H(6)NO](+) ions were obtained by two routes, taking advantage of the FEL coupling to two different ion traps. In the first one, an FT-ICR mass spectrometer, a sequence of ion-molecule reactions was allowed to occur, ultimately leading to an NO(+) transfer process to benzene. The so-formed ions displayed IRMPD features characteristic of a [benzene,NO](+) pi-complex structure, including a prominent band at 1963 cm(-)(1), within the range for the N-O bond stretching vibration of NO (1876 cm(-)(1)) and NO(+) (2344 cm(-)(1)). A quite distinct species is formed by electrospray ionization (ESI) of a methanol solution of nitrosobenzene. The ions transferred and stored in a Paul ion trap showed the IRMPD features of substituent protonated nitrosobenzene, the most stable among conceivable [C(6)H(6)NO](+) isomers according to computations. It is noteworthy that IRMPD is successful in allowing a discrimination between isomeric [C(6)H(6)NO](+) species, whereas high-energy collision-induced dissociation fails in this task. The [benzene,NO](+) pi-complex is characterized by IRMPD spectroscopy as an exemplary noncovalent ionic adduct between two important biomolecular moieties.