共查询到20条相似文献,搜索用时 15 毫秒
1.
Joseph J. Brophy Chang-Ming Sun Berhane Tecle Robert F. Toia 《Journal of mass spectrometry : JMS》1989,24(8):609-614
The detailed mass spectral fragmentation pathways of a series of both naturally occurring and synthetic 5-methyl-3-alkyl-2-aIkenylpyrazines have been elucidated with the aid of deuterium labels placed specifically in the alkenyl side-chain. The influence on the fragmentation pattern of the stereochemistry of the alkenyl group as well as the relative placement of the groups on the pyrazine nucleus have also been evaluated. This provides information of some diagnostic value in the assignment of structures by mass spectrometry to new compounds within this class. 相似文献
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Henri Edouard Audier Georges Sozzi Arielle Milliet Steen Hammerum 《Journal of mass spectrometry : JMS》1990,25(7):368-374
The reactions of propyl ether radical cations close to threshold are initiated by (reversible) formation of γ-disitonic isomers, R$ \mathop {\rm O}\limits^ + $ (H)CH2CH2CH2·. The three methylene groups in these ions lose their positional identity by ring closure/ring opening via [cyclopropane + alcohol]+· intermediates. Extensive hydrogen exchange occurs within the C3-chain. When R is not methyl the γ-distonic isomer undergoes further intramolecular hydrogen atom transfer reactions that lead to formation of α- and β-distonic ions. The α-distonic isomers expel ethyl and propyl radicals by C? O bond cleavage. 相似文献
4.
The fixed wavelength photoelectron—photoion coincidence technique has been employed to study the fragmentation behaviour of excited acetaldehyde molecular cations with internal energies up to 7 eV. The recorded breakdown curves of the parent ion as well as the C2H3O+, CHO+ and CH3+ fragment ions enable to reject state specific fragmentation behaviour of the title compound into the CHO+ and CH3+ fragment ion channels. The present data give evidence of a fast isomerization of the CH3CHO+ cation from its first electronically excited state ā(2A″) to the oxirane cation in its electronical ground state X?(2B2). 相似文献
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E. T. Denisov 《Russian Chemical Bulletin》2013,62(7):1533-1539
A comparison of experimental data and results of the rate constant calculations by the method of intersecting parabolas (MIP) showed that abstraction of H atom by a peroxyl radical from the Cα-H bond in hydroperoxide is accompanied by concerted fragmentation of the molecule. The complicated character of the elementary event is due to high exothermicity of the reaction. The kinetic parameters of isomerization with fragmentation of peroxyalkyl, peroxyalkoxyl, and peroxyperoxyl radicals were calculated within the framework of the MIP method. The enthalpies, activation energies, and rate constants for a series of isomerization reactions of peroxyl radicals with concerted fragmentation were also obtained from the MIP calculations. Factors influencing these reactions are analyzed. 相似文献
8.
Nian-hua Luo Xiang Sun Wen-tao Wei Xue-jing Zhang Ming Yan 《Tetrahedron: Asymmetry》2013,24(7):402-408
An organocatalytic cascade Michael/Michael reaction between curcumins and 2-arylidene-1,3-indandiones has been studied. Prolinol, chiral thiourea-tertiary amines, and cinchona alkaloids were evaluated as catalysts. Quinine was identified as the best catalyst for the transformation. Multicyclic spiro-1,3-indandiones were prepared in moderate to excellent yields, diastereoselectivities, and enantioselectivities. 相似文献
9.
Janovský I Knolle W Naumov S Williams F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(21):5524-5534
The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed. 相似文献
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The fragmentation patterns of n-paraffins in the C10–C30 range are accounted for using a statistical, maximal entropy, formalism. The theory can be used in a predictive manner. Particular attention is given here to a quantitative test of the theory which is independent of the uncertainties in the structure of the fragments. 相似文献
11.
Anife Ahmedova Petja Marinova Gordana Pavlovi? Maya Guncheva Neyko Stoyanov Mariana Mitewa 《Journal of the Iranian Chemical Society》2012,9(3):297-306
A series of seven 2-cinnamoyl-1,3-indandiones and their metal(II) complexes were synthesized and characterized by means of spectroscopic (IR, NMR, electron absorption and emission spectroscopy) and/or single-crystal X-ray diffraction methods. The optical spectra of the organic compounds show very strong absorption in the visible region and weak fluorescence with moderate to strong Stokes shift. The effect of concentration, water addition and metal ion complexation on the optical properties was also studied. In search of potential practical application, the complexation of 2-cinnamoyl-1,3-indandiones with metal(II) ions was investigated. A series of non-charged complexes with Cu(II), Cd(II), Zn(II), Co(II) and Ni(II) was isolated and analyzed by elemental analyses and IR. Most of the complexes show presence of water molecules, most probably coordinated to the metal ion, thus forming octahedral geometry. For the paramagnetic Cu(II) complexes a distorted, flattened tetrahedral structure is proposed, basing on the EPR data. The optical properties of the metal complexes, however, do not differ appreciably from those of the free ligands. 相似文献
12.
Z. Wang Katty X. Wan R. Ramanathan John S. Taylor Michael L. Gross 《Journal of the American Society for Mass Spectrometry》1998,9(7):683-691
Understanding the product-ion spectra of T-rich tetradeoxynucleotides is a starting point in the development of a mass spectrometric scheme to determine the mutagenicity of individual types of DNA damage. We obtained product-ion spectra for electrospray-produced ions that were activated in the ion source (electrospray ionization-source collision-activated-dissociation) and by high-energy collisions in the MS/MS mode of a four-sector instrument. We also activated singly and doubly charged ions by low-energy collisions in an ion-trap mass spectrometer and investigated post source decompositions of matrix-assisted laser desorbed ions in a time-of-flight mass spectrometer. The various methods of extracting structural information give remarkably consistent results. The difference in the relative abundances of w n and d n ions of the singly charged oligonucleotides and the formation of [a3 ? B3] ions, where B3 is the base on the third position, are effective for identification and distinction of pairs of isomeric tetranucleotides. A sufficient number of tetramers and pentamers were studied to enable us to propose a charge-remote mechanism for the formation of site-specific [a n ? B n ] ion. 相似文献
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Extending our previous study on the title species (J. Phys. Chem. A2010, 114, 6787), we investigated the dimer cations that are formed on oxidation of the glucobrassin derivatives indole-3-carbinol (I3C) and diindolylmethane (DIM) and of parent indole (I). Radiolysis in ionic liquid and Ar matrices shows that, at sufficiently high concentrations and/or on annealing the solid glasses, intense intermolecular charge-resonance (CR) absorption bands in the NIR herald the formation of sandwich-type dimer cations. The molecular and electronic structure of these species is modeled by calculations with the double-hybrid B2-PLYP-D density functional method which yields predictions in good accord with experiment. The radical cation of DIM also shows a CR band, but unlike in the case of I and I3C, its occurrence is not dependent on the concentration but instead on the solvent: in ionic liquid the CR band is initially absent and arises only on annealing, whereas in Ar matrices it is present from the outset and undergoes blue shifting and sharpening on annealing. These puzzling findings are rationalized on the basis of B2-PLYP-D calculations which predict that neutral DIM exists in the form of two conformers, present in different relative amounts in the two experiments, which on vertical ionization form distinct radical cations, a nonsymmetric one where the odd electron is largely localized on one of the two indole moieties and one with C(2) symmetry where charge and spin are completely delocalized over both halves of the molecule, thus giving rise to an intramolecular CR transition. On annealing, the nonsymmetric cation relaxes to a similarly delocalized structure with C(s) symmetry, thus explaining the observed increase and the shift of the CR band. We believe that DIM(?+) represents the first example of a radical cation which can exist under the same conditions as a localized and a delocalized complex cation. 相似文献
14.
Anjaiah Aitha Satyanarayana Yennam Manoranjan Behera Jaya Shree Anireddy 《Tetrahedron letters》2017,58(6):578-581
The reaction of 1,3,4-oxathiazol-2-one derivative with 2-arylidene-1,3-indandione to furnish novel spiroindene-1,3-dione isothiazoline derivatives by Michael/1,3-dipolar [3+2]-cycloaddition reaction was investigated. The key 1,3-dipolar cycloaddition reaction step was examined in toluene solvent at reflux temperature to obtain mixture of two regioisomers (6a and 6b – 14a and 14b) and single isomers (15–20). The scope of this new reaction was demonstrated with many examples with high reactivity and yields. 相似文献
15.
Paul H. Chen William F. Kuhn Donald C. Kleinfelter James M. Miller 《Journal of mass spectrometry : JMS》1972,6(7):785-796
The mass spectra of isomeric 3- and 7-cyclohexyl-2-norbornanols have been studied. The 3-substituted norbornanols give very similar mass spectra, whereas large differences were observed between the mass spectra of 7-syn and 7-anti isomers. While the mass spectra of these bicyclic alcohols show extensive rearrangements to give rise to many odd-electron ions, the Wagner-Meerwein rearrangement of the molecular ion prior to fragmentation or during the process of dehydration is not important. The origins and the mechanisms for the formation of some major ions are discussed in terms of low voltage spectra, defocused metastables and their relative abundances, and ionization or appearance potentials of the ions of interest. 相似文献
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The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N-dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the O-O bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be -0.55+/-0.15 V and 20+/-3 kcal mol(-1), respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the O-O bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene-cis-1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane-cis-1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a beta-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for beta-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. 相似文献
17.
Hachizo Muto Kazumi Toriyama Keichi Nunome Machio Iwasaki 《Chemical physics letters》1984,105(6):592-597
A 4 K matrix ESR study shows that the molecular radical cations of isopropyl formate and acetate, produced radiolytically in halocarbon matrices at 4.2 K, undergo spontaneous rearrangement due to a selective intramolecular hydrogen shift from the tertiary CH bond in the isopropyl group to the carbonyl oxygen atom giving RC+(OH)OC(CH3)2, where R = H or CH3. The radical cation of tert-butyl acetate undergoes further fragmentation at the ester CO bond following a similar rearrangement to give an isobutene radical cation in CFCl3. 相似文献
18.
The mass spectral fragmentations of 2-methylsparteine (1), 2, 17-dimethylsparteine (2), 2-methyl-17-isopropylsparteine (3), 2-methyl-17-oxosparteine (4), 2-oxo-17-methylsparteine (17-methyllupanine) (5) and 2-oxo-17-isopropylsparteine (17-isopropyllupanine) (6) were investigated. Fragmentation pathways, whose identification was assisted by accurate mass measurements and a correlation between the abundances of the M(+.) and selected fragment ions of the investigated compounds, are discussed. The data obtained create the basis for distinguishing the structural isomers and metamers. 相似文献
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I. Yu Shchapin V. I. Feldman V. N. Belevskii A. V. Khoroshutin A. A. Bobylyova 《Radiation Physics and Chemistry》1999,55(5-6):559-563
A four-step mechanism of isomerization of tricyclo[4.3.0.03,7]nona-4,8-diene radical cations to tricyclo[4.2.1.04,9]nona-2,7-diene radical cations in γ-irradiated frozen Freon-113 (CFCl2CF2Cl) matrix was suggested on the basis of ESR data. The rearrangement was found to occur via distonic form of the radical cations with spin and charge separation. Furthermore, it was shown that the primary radical cations abstracts hydrogen atom from methylene group of the parent molecule, whereas distonic radical cations reacts via attachment to the C=C bond at 110–119 K. 相似文献
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After identification of the impurities, separated from synthesized orthoterphenyl (I) labelled in position 2 with 13C, i.e. o-xenylcyclohexanol (II), o-xenylcyclohexene (III), hexahydrotriphenylene (IV) and o-xenylcyclohexane (V), the fragmentation processes under electron-impact of molecules I, II and IV have been studied, advantage being taken of their 13C labelling. Mass spectrometric studies were carried out on an AEI MS-9 for determination of the atomic composition of the main ions and on a MAT CH-4 instrument for quantitative determination of 12C and 13C species, at low energy, assuming an equal sensitivity. The 13C labelling of position 2 in I, has not yet given clear evidence of a formerly assumed rearrangement into phenanthrenic structure after ring opening. The fragmentation of molecule II occurs in two ways, i.e. into a series of hydrocarbon fragments, produced from the molecular ion by loss of H2O or into fragments still containing the oxygen atom. The difference between the fragmentation processes of the two series depends on the fact that the loss of H2O from the molecule stabilizes the two carbon atoms adjacent to the C carrying the hydroxyl group. This is shown by the relatively higher 13C content of the hydrocarbon fragment in C15 compared with that of the equivalent oxygenated fragment. The labelling in IV shows how the rupture of the saturated ring occurs under electron-impact, the first C removed being the one nearest to the central ring. 相似文献