A computational study of bond dissociation enthalpies in haloethenes

Carbon–hydrogen bond dissociation enthalpies (BDEs) were computed for all haloethenes, C₂H_4−nX_n (n=0–3, X=F, Cl, Br, I), at the B3LYP/6-311+G(3df,2p) level using isodesmic reactions. It was found that C–H bond strengths in the monohaloethenes varied substantially, by as much as 18 kJ mol⁻¹, dependent upon the bond's stereochemical position relative to the halogen. BDEs in the dihaloethanes varied in the order CX₂CH–H>(E)-CHXCX–H>(Z)-CHXCX–H. Trends in the computed bond enthalpies were discussed and explained on the basis of relative steric repulsions and hyperconjugative delocalization interactions, as determined from Natural Bond Orbital analysis.     

Competition STD NMR for the detection of high-affinity ligands and NMR-based screening

The reported competition STD NMR method combines saturation transfer difference (STD) NMR with competition binding experiments to allow the detection of high-affinity ligands that undergo slow chemical exchange on the NMR time-scale. With this technique, the presence of a competing high-affinity ligand in the compound mixture can be detected by the disappearance or reduction of the STD signals of a low-affinity indicator ligand. This is demonstrated on a BACE1 (beta-site amyloid precursor protein cleaving enzyme 1) protein-inhibitor system. This method can also be used to derive an approximate value, or a lower limit, for the dissociation constant of the potential ligand based on the reduction of the signal intensity of the STD indicator, which is illustrated on an HSA (human serum albumin) model system. This leads to important applications of the competition STD NMR method for lead discovery: it can be used (i) for compound library screening against a broad range of drug targets to identify both high- and low-affinity ligands and (ii) to rank order analogs rapidly and derive structure-activity relationships, which are used to optimize these NMR hits into viable drug leads.     

Synthesis, Characterization and Two-Photon Absorption Properties of a Novel Pyridinium Salt

A novel pyridinium salt, 2,4-bis[p-(N,N-dimethylamino)styryll-N-metlayl pyridinium iodide (BMSPI) was synthesized and characterized by TG, ^1H NMR spectroscopy and elemental analysis, and the reaction process was studied by using ES-MS. When BMSPI was pumped by a pulsed 1064 nm, 50 ps laser beam, it manifests highly efficient TPA (Two-Photon Absorption) and up-conversion superradiance. The up-conversion efficiency was 6.0% at the pump energy of 4-6 mJ and the lifetime of two-photon fluorescence was measured as 59 ps.     

Analysis of phosphorus herbicides by ion-pairing reversed-phase liquid chromatography coupled to inductively coupled plasma mass spectrometry with octapole reaction cell

A reversed phase ion-pairing high performance liquid chromatographic (RPIP-HPLC) method is developed for the separation of two phosphorus herbicides, Glufosinate and Glyphosate as well as Aminomethylphosphonic acid (AMPA), the major metabolite of Glyphosate. Tetrabutylammonium hydroxide is used as the ion-pairing reagent in conjunction with an ammonium acetate/acetic acid buffering system at pH 4.7. An inductively coupled plasma mass spectrometer (ICP-MS) is coupled to the chromatographic system to detect the herbicides at m/z = 31P. Historically, phosphorus has been recognized as one of the elements difficult to analyze in argon plasma. This is due to its relatively high ionization potential (10.5 eV) as well as the inherent presence of the polyatomic interferences 14N16O1H+ and 15N16O+ overlapping its only isotope at m/z = 31. An octapole reaction cell is utilized to minimize the isobaric polyatomic interferences and to obtain the highest signal-to-background ratio. Detection limits were found to be in the low ppt range (25-32 ng/l). The developed method is successfully applied to the analysis of water samples collected from the Ohio River and spiked with a standard compounds at a level of 20 microg/l.     

Dissociation quotients of succinic acid in aqueous sodium chloride media to 225°C

The first and second molal dissociation quotients of succinic acid were measured potentiometrically with a hydrogen-electrode, concentration cell. These measurements were carried out from 0 to 225°C over 25° intervals at five ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The dissociation quotients from this and two other studies were combined and treated with empirical equations to yield the following thermodynamic quantities for the first acid dissociation equilibrium at 25°C: log K_1a=–4.210±0.003; H _1a ⁰ =2.9±0.2 kJ-mol^–1; S _1a ⁰ =–71±1 J-mol^–1-K^–1; and C _p1a ⁰ =–98±3 J-mol^–1-K^–1; and for the second acid dissociation equilibrium at 25°C: log K_2a=–5.638±0.001; H _2a ⁰ = –0.5±0.1 kJ-mol^–1; S _2a ⁰ =–109.7±0.4 J-mol^–1-K^–1; and C _p2a ⁰ = –215±8 J-mol^–1-K^–1.     

Laser-induced fluorescence detection schemes for the analysis of proteins and peptides using capillary electrophoresis

Over the past few years, a large number of studies have been prepared that describe the analysis of peptides and proteins using capillary electrophoresis (CE) and laser-induced fluorescence (LIF). These studies have focused on two general goals: (i) development of automatic, selective and quick separation and detection of mixtures of peptides or proteins; (ii) generation of new methods of quantitation for very low concentrations (nm and subnanomolar) of peptides. These two goals are attained with the use of covalent labelling reactions using a variety of dyes that can be readily excited by the radiation from a commonly available laser or via the use of noncovalent labelling (immunoassay using a labelled antibody or antigen or noncovalent dye interactions). In this review article, we summarize the works which were performed for protein and peptide analysis via CE-LIF.     

A theoretical study of five nitrates: Electronic structure and bond dissociation energies

Three density-functional methods (B3P86, B3PW91, and B3LYP) are employed to investigate the O–NO₂ bond lengths, frontier orbital energies, and O–NO₂ bond dissociation energies (BDEs) of n-propyl nitrate (NPN), isopropyl nitrate (IPN), 2-ethylhexyl nitrate (EHN), triethylene glycol dinitrate (Tri-EGDN), and tetraethylene glycol dinitrate (Tetra-EGDN). It is found that the O–NO₂ bond lengthens (destabilizes) in the order of IPN, NPN, EHN, Tetra-EGDN, and Tri-EGDN. From the data of frontier orbital energies (E_HOMO, E_LUMO), and energy gaps (ΔE), we estimate the relative thermal stability ordering of five nitrates and their corresponding radicals. The predicted BDEs of O–NO₂ bond in NPN, IPN, EHN, Tri-EGDN, and Tetra-EGDN, are 176.6, 174.5, 168.1, 156.1, and 159.3 kJ mol⁻¹, respectively. Based on the finding that the present results of BDEs are well coincident with the experimental results of apparent activation energies from the literature, we can draw a conclusion that the experimental thermolysis of five nitrates is only unimolecular homolytical cleavage of the O–NO₂ bonds.     

Rapid and sensitive determination of ephedrine and pseudoephedrine by micellar electrokinetic chromatography with on-line regenerating covalent coating

A rapid, sensitive and reproducible micellar electrokinetic chromatographic method using hexamethyldisilazane as on-line regenerating covalent coating was developed for the quantification of ephedrine (E) and pseudoephedrine (PE). E and PE were derivatized with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazol for laser-induced fluorescence detection. The on-line regenerating covalent coating formed a combinative double coating with the subsequently produced dynamic SDS coating. The total coating can be easily removed and conveniently regenerated on-line. The simple coating procedure was described. By a series of optimization, a running buffer of 20 mm Na(2)B(4)O(7) + 16 mm SDS was applied for the separation of the derivatives. Linear relationships for E and PE were obtained in the range of 0.044-6.60 microg mL(-1) (correlation coefficients: 0.9975 for E, 0.9981 for PE), and the detection limits for E and PE were 1.71 and 0.67 ng mL(-1), respectively. The separation speed, the reproducibility and the sensitivity were much improved over those of other capillary electrophoresis methods more recently reported. The method was applied to the analysis of the two alkaloids in traditional herbal preparations with recoveries in the range 92.8-104.8%.     

3-苯基-1-丁炔-3-醇质谱碎裂中[C8H7]^+的结构及分子内质子迁移反应

报道了3-苯基-1-丁炔-3-醇的常规电子轰击质谱(EIMS)。利用碰撞诱导解离(CID)技术研究了质谱碎裂过程中产生的[C8H7]^+的气相离子结构。同时, 氘代同位素交换、亚稳(MI)和CID实验进一步证实了m/z 103离子的形成并不是分子离子的质谱碎裂中顺次失去甲基自由基和中性CO分子的直接氢迁移的协同反应, 而是在失去CO分子前后发生了二次质子迁移反应的逐步过程。在此基础上提出了一种独特的双分子质子键合复合物中间体的碎裂机理。     

三芳基咪唑类化合物的合成及其双光子吸收性能研究

双光子吸收是指物质分子或原子在强激光激发下同时吸收两个光子,从基态跃迁到两倍光子能量激发态的过程。目前,双光子吸收材料已广泛地应用到双光子激发显微[1]、频率上转换激射[2-3]、光学限幅、三维光信息存储[4]以及光生物学等领域。但在已报道的双光子诱导上转换荧光的有机