Unimolecular dissociation of protonated trans-1,4-diphenyl-2-butene-1,4-dione in the gas phase: rearrangement versus simple cleavage

Fragmentation mechanisms of trans-1,4-diphenyl-2-butene-1,4-dione were studied using a variety of mass spectrometric techniques. The major fragmentation pathways occur by various rearrangements by loss of H(2)O, CO, H(2)O and CO, and CO(2). The other fragmentation pathways via simple alpha cleavages were also observed but accounted for the minor dissociation channels in both a two-dimensional (2-D) linear ion trap and a quadrupole time-of-flight (Q-TOF) mass spectrometer. The elimination of CO(2) (rather than CH(3)CHO or C(3)H(8)), which was confirmed by an exact mass measurement using the Q-TOF instrument, represented a major fragmentation pathway in the 2-D linear ion trap mass spectrometer. However, the elimination of H(2)O and CO becomes more competitive in the beam-type Q-TOF instrument. The loss of CO is observed in both the MS(2) experiment of m/z 237 and the MS(3) experiment of m/z 219 but via the different transition states. The data suggest that the olefinic double bond in protonated trans-1,4-diphenyl-2-butene-1,4-dione plays a key role in stabilizing the rearrangement transition states and increasing the bond dissociation (cleavage) energy to give favorable rearrangement fragmentation pathways.     

Theoretical investigation of the Fe+-catalyzed oxidation of acetylene by N2O

The gas-phase Fe(+)-mediated oxidation of acetylene by N2O on both sextet and quartet potential energy surfaces (PESs) is theoretically investigated using density functional theory. Geometries and energies of all the stationary points involved in the catalytic reaction are located. For the catalytic cycles, the crucial step is the initial N2O reduction by Fe(+) to form FeO(+), in which a direct O-abstraction mechanism is located on the sextet PES, whereas the quartet pathway favors a N-O insertion mechanism. Spin inversion moves the energy barrier for this process downward to a position below the ground-state entrance channel. The second step of the catalytic cycles involves two mechanisms corresponding to direct hydrogen abstraction and cyclization. The former mechanism accounts for the ethynol formation with the upmost activation barrier below the entrance channel by about 5 kcal/mol. The other mechanism involves a "metallaoxacyclobutene" structure, followed by four possible pathways, i.e., direct dissociation, C-C insertion, C-to-O hydrogen shift, and/or C-to-C hydrogen shift. Among these pathways, strong exothermicities as well as energetically low location of the intermediates suggest oxidation to ketene and carbon monoxide along the C-to-C hydrogen shift pathway is the most favorable. Reduction of the CO loss partner FeCH2(+) by another N2O molecule constitutes the third step of the catalytic cycles, which contains direct abstraction of O from N2O giving OFeCH2(+), intramolecular rearrangement to form Fe(+)-OCH2, and nonreactive dissociation. This reaction is also energetically favored considering the energy acquired from the initial reactants.     

应用腙作为配体的铜催化二芳醚合成反应

一种简单易得的腙,2-吡啶醛 1-苯基腙 （1）,能够作为有效配体,极大地促进碘化铜催化的芳基溴与酚的C–O偶联反应。该偶联反应条件温和（二氧六环为溶剂、磷酸钾为碱、90–100 0C 的反应温度）,适用于各种芳基溴（电中性、贫电子和富电子）和酚（电中性和富电子）的底物,能够相容一些碱敏感官能团如酯基、醛基和羰基等。     

Synthesis of Copolymers of Methacrylate BearingAminonitro┐StilbeneDyesandTheirNonlinearOpticalProper

报道了含ａｍｉｎｏｎｉｔｒｏ－ｓｔｉｌｂｅｎｅ基团的甲基丙烯酸酯类单体的合成、该单体与ＭＭＡ的共聚反应及对所得聚合物进行的非线性光学性能测定。结果表明，缩短ｓｔｉｌｂｅｎｅ片段与甲基丙烯酰基的连接链长有助于改善单体的聚合性能，增强聚合物在有机溶剂中的溶解能力。对所得聚合物的ＳＨＧ测定显ｄ３３值可达到５４ｐｍ／ｖ。     

Initial hydrogenations of pyridine on MoP(001): a density functional study

The initial hydrogenations of pyridine on MoP(001) with various hydrogen species are studied using self-consistent periodic density functional theory (DFT). The possible surface hydrogen species are examined by studying interaction of H(2) and H(2)S with the surface, and the results suggest that the rational hydrogen source for pyridine hydrogenations should be surface hydrogen atoms, followed by adsorbed H(2)S and SH. On MoP(001), pyridine has two types of adsorption modes, i.e., side-on and end-on; and the most stable η(5)(N,C(α),C(β),C(β),C(α)) configuration of the side-on mode facilitates the hydrogenation of pyridine. The optimal hydrogenation path of pyridine with surface hydrogen atoms in the Langmuir-Hinshelwood mechanism is the formation of 3-monohydropyridine, followed by producing 3,5-dihydropyridine, in which the two-step hydrogenations take place on the C(β) atoms. When adsorbed H(2)S is considered as the source of hydrogen, slightly higher hydrogenation barriers are always involved, while the energy barriers for hydrogenations involving adsorbed SH are much lower. However, the hydrogenation of pyridine should be suppressed by the adsorption of H(2)S, and the promotion effect of adsorbed SH is limited.     

Gas-phase Meerwein reaction of epoxides with protonated acetonitrile generated by atmospheric pressure ionizations

Ethylnitrilium ion can be generated by protonation of acetonitrile (when used as the LC-MS mobile phase) under the conditions of atmospheric pressure ionizations, including electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) as well as atmospheric pressure photoionization (APPI). Ethylnitrilium ion (CH₃ - C o mathop N⁺ HCH_3 - C equiv mathop Nlimits^ + H and its canonical form CH₃ - mathop C⁺ = NHCH_3 - mathop Climits^ + = NH) is shown to efficiently undergo the gas-phase Meerwein reaction with epoxides. This reaction proceeds by the initial formation of an oxonium ion followed by three-to-five-membered ring expansion via an intramolecular nucleophilic attack to yield the Meerwein reaction products. The density functional theory (DFT) calculations at the B3LYP/6-311 + G(d,p) level show that the gas-phase Meerwein reaction is thermodynamically favorable. Collision-induced dissociation (CID) of the Meerwein reaction products yields the net oxygen-by-nitrogen replacement of epoxides with a characteristic mass shift of 1 Da, providing evidence for the cyclic nature of the gas-phase Meerwein reaction products. The gas-phase Meerwein reaction offers a novel and fast LC-MS approach for the direct analysis of epoxides that might be of genotoxic concern during drug development. Understanding and utilizing this unique gas-phase ion/molecule reaction, the sensitivity and selectivity for quantitation of epoxides can be enhanced.     

Molecularly imprinted magnetic nanoparticles for determination of the herbicide chlorotoluron by gate-controlled electro-catalytic oxidation of hydrazine

Microchimica Acta - We have synthesized magnetic nickel hexacyanoferrate (NiHCF) nanoparticles and coated them with a polymer that was molecularly imprinted for the herbicide chlorotoluron. The...     

Approaching the electromagnetic mechanism of surface-enhanced Raman scattering: from self-assembled arrays to individual gold nanoparticles

Surface-enhanced Raman scattering (SERS) has been intensively explored both in theory and applications and has been widely used in chemistry, physics and biology for decades. A variety of SERS substrates have been developed in order to investigate the mechanisms behind, which give rise to the enormous enhancement even enabling single molecule detection. The Raman enhancement, which involves an electromagnetic enhancement (EM) and a chemical enhancement (CM), reflects both the physical principle of light/metal interactions and the molecule/metal interactions. In this tutorial review, we focus on the EM enhancement of SERS active substrates made of colloidal gold nanoparticles (GNPs), varying from self-assembled arrays down to single particles, for the purpose of investigating the EM coupling effect and probing the distribution of the induced electric field of single GNPs.