Isomeric cyclic [C6H10]+• ions. The energy barrier to ring opening

1979 ◽  
Vol 57 (3) ◽  
pp. 348-354 ◽  
Author(s):  
Peder Wolkoff ◽  
John L. Holmes
Keyword(s):  
Activation Energy ◽  
Kinetic Energy ◽  
Energy Barrier ◽  
Ring Opening ◽  
Reaction Pathway ◽  
Molecular Ions ◽  
Metastable Peak ◽  
Molecular Ion

Appearance energies and metastable peak shapes for methyl loss from the molecular ions of cyclohexene, methylcyclopentenes, methylenecyclopentane, bicyclo[3.1.0]hexane, and 2-methyl-1,4-pentadiene indicate that they have a common reaction pathway which produces [cyclopentenium]+ as the daughter ion at threshold.From measurements of appearance energies and from relative peak abundances and kinetic energy releases for the metastable losses of CH3• and labelled methyl from the deuterium labelled cyclic [C6H10]+• molecular ions it was concluded that: (a) H/D mixing in [cyclohexene]+• occurs in the intact ring prior to a concerted methyl extrusion; (b) [methylcyclopentenes]+• loses methyl without any H/D mixing at or near to threshold by a straight C—C cleavage, possibly via the 3-isomer. Methyl losses involving H/D mixing are preceded by ring opening which has an activation energy of ca. 1 eV. However, [cyclopentenium]+• is again the daughter ion; (c) methylenecyclopentane and bicyclo[3.1.0]hexane molecular ions isomerize to methylcyclopentenes prior to methyl loss.H/D mixing (prior to methyl loss) in the molecular ion 1,1-2H2-2-methyl-1,4-pentadiene takes place before cyclisation to methylcyclopentene.

scholarly journals Ringöffnung CO+- und COOCH3+·-substituierter Cyclopropane und CO-Eliminierung aus isomeren C3H5CO+-Ionen / Ring Opening of CO+ and COOCH3+· Substituted Cyclopropanes and CO Loss from Isomeric C3H5CO+ Ions

1979 ◽  
Vol 34 (3) ◽  
pp. 488-494 ◽  
Author(s):  
Helmut Schwarz ◽  
Chrysostomos Wesdemiotis
Keyword(s):  
Kinetic Energy ◽  
Energy Release ◽  
Ring Opening ◽  
Collisional Activation ◽  
Kinetic Energy Release ◽  
Ester Group ◽  
Molecular Ions ◽  
Peak Shape ◽  
Metastable Peak ◽  
Substituted Cyclopropanes

Abstract The non-decomposing molecular ions of methyl cyclopropanecarboxylate (14) are found to rearrange to ionised methyl but-3-enoate (15). For ions with sufficient internal energy to decompose, this isomerization is in competition with · OCH3 loss, via direct cleavage of the ester group. Collisional activation spectroscopy may be used to distinguish between the C3H5CO+ ions formed by · OCH3 loss from the molecular ions of 14, 15 and other isomeric precursors. Four distinct C3H5CO+ species (18-21) can be identified in this way; these C3H5CO+ ions may themselves decompose, via CO elimination. Consideration of the metastable peak shape for CO loss, in conjunction with collisional activation spectroscopy on the resulting C3H5+ -ions, leads to two main conclusions, (i) Two C3H5+ ions (22 and 27) exist in potential energy wells. The very narrow metastable peaks for CO loss from 19 and 21 (leading to 22 and 27, respectively) show that these processes are continuously endothermic. In contrast, CO loss from either 18 or 20 gives rise to much broader metastable peaks. This suggests that rate-determining rearrangement of the incipient C3H5+ cations, to a more stable isomer, occurs prior to decomposition, (ii) Elimination of CO from the [M- · OCH3]+ fragment of 14 gives rise to a composite metastable peak, thus indicating the occurrence of two competing channels for dissociation. These channels are assigned to CO loss from 18 (larger kinetic energy release) and CO loss from 19 (smaller kinetic energy release).

Rearrangements in the Molecular Ions of Some ortho-Substituted Schiff Bases

1993 ◽  
Vol 46 (6) ◽  
pp. 895 ◽  
Author(s):  
T Blumenthal ◽  
M Dosen ◽  
RG Gillis ◽  
QN Porter
Keyword(s):  
Kinetic Energy ◽  
Schiff Bases ◽  
Molecular Ions ◽  
Energy Spectra ◽  
Supporting Evidence ◽  
Deuterium Labelling ◽  
Methyl Group ◽  
Molecular Ion ◽  
Fragment Ions ◽  
Ion Kinetic Energy

Under electron ionization conditions, the ortho-substituted Schiff bases N-benzylidene-o-toluidine (1a), N-(o-methylbenzylidene)aniline (1b), N-salicylideneaniline (1c) and N-(o-methoxybenzylidene)aniline (1d) give fragment ions which have been shown by collision-activated mass-analysed ion kinetic energy spectra to have the structure of the protonated molecular ions of indole (2), benzofuran (3), and 1,2-benzisoxazole (4). The molecular ion of N-(o-methylbenzylidene)-o-toluidine (1f) gives as fragment ions not only the protonated molecular ion (2) of indole and the tropylium ion but also the molecular ion of anthracene. Attempts to find supporting evidence for a mechanism for this rearrangement by deuterium labelling of a methyl group in (1b), such as (1g), have been unsuccessful.

scholarly journals Mechanistic Insights into Water-Catalyzed Formation of Levoglucosenone from Anhydrosugar Intermediates by Means of High-Level Theoretical Procedures

2016 ◽  
Vol 69 (9) ◽  
pp. 943 ◽  
Author(s):  
Wenchao Wan ◽  
Li-Juan Yu ◽  
Amir Karton
Keyword(s):  
Activation Energy ◽  
Reaction Mechanism ◽  
Ring Opening ◽  
Reaction Pathway ◽  
Fast Pyrolysis ◽  
Rate Determining Step ◽  
Dehydration Reactions ◽  
High Level ◽  
Multistep Reaction ◽  
Hydration And Dehydration

Levoglucosenone (LGO) is an important anhydrosugar product of fast pyrolysis of cellulose and biomass. We use the high-level G4(MP2) thermochemical protocol to study the reaction mechanism for the formation of LGO from the 1,4:3,6-dianhydro-α-d-glucopyranose (DGP) pyrolysis intermediate. We find that the DGP-to-LGO conversion proceeds via a multistep reaction mechanism, which involves ring-opening, ring-closing, enol-to-keto tautomerization, hydration, and dehydration reactions. The rate-determining step for the uncatalyzed process is the enol-to-keto tautomerization (ΔG‡298 = 68.6 kcal mol–1). We find that a water molecule can catalyze five of the seven steps in the reaction pathway. In the water-catalyzed process, the barrier for the enol-to-keto tautomerization is reduced by as much as 15.1 kcal mol–1, and the hydration step becomes the rate-determining step with an activation energy of ΔG‡298 = 58.1 kcal mol–1.

scholarly journals The Activating Effect of Strong Acid for Pd-Catalyzed Directed C–H Activation by Concerted Metalation-Deprotonation Mechanism

Molecules ◽  
2021 ◽  
Vol 26 (13) ◽  
pp. 4083
Author(s):  
Heming Jiang ◽  
Tian-Yu Sun
Keyword(s):  
Activation Energy ◽  
Dft Calculations ◽  
Energy Barrier ◽  
Computational Study ◽  
Strong Acid ◽  
Activation Energy Barrier ◽  
Pd Catalysts ◽  
Acid Ligand ◽  
Strong Acids

A computational study on the origin of the activating effect for Pd-catalyzed directed C–H activation by the concerted metalation-deprotonation (CMD) mechanism is conducted. DFT calculations indicate that strong acids can make Pd catalysts coordinate with directing groups (DGs) of the substrates more strongly and lower the C–H activation energy barrier. For the CMD mechanism, the electrophilicity of the Pd center and the basicity of the corresponding acid ligand for deprotonating the C–H bond are vital to the overall C–H activation energy barrier. Furthermore, this rule might disclose the role of some additives for C–H activation.

scholarly journals Theoretical Study on the Photo-Oxidation and Photoreduction of an Azetidine Derivative as a Model of DNA Repair

Molecules ◽  
2021 ◽  
Vol 26 (10) ◽  
pp. 2911
Author(s):  
Miriam Navarrete-Miguel ◽  
Antonio Francés-Monerris ◽  
Miguel A. Miranda ◽  
Virginie Lhiaubet-Vallet ◽  
Daniel Roca-Sanjuán
Keyword(s):  
Electron Transfer ◽  
Energy Barrier ◽  
Ring Opening ◽  
Dna Lesions ◽  
Barrier Heights ◽  
Dna Lesion ◽  
Electron Reduction ◽  
The Stability ◽  
Electron Transfer Processes

Photocycloreversion plays a central role in the study of the repair of DNA lesions, reverting them into the original pyrimidine nucleobases. Particularly, among the proposed mechanisms for the repair of DNA (6-4) photoproducts by photolyases, it has been suggested that it takes place through an intermediate characterized by a four-membered heterocyclic oxetane or azetidine ring, whose opening requires the reduction of the fused nucleobases. The specific role of this electron transfer step and its impact on the ring opening energetics remain to be understood. These processes are studied herein by means of quantum-chemical calculations on the two azetidine stereoisomers obtained from photocycloaddition between 6-azauracil and cyclohexene. First, we analyze the efficiency of the electron-transfer processes by computing the redox properties of the azetidine isomers as well as those of a series of aromatic photosensitizers acting as photoreductants and photo-oxidants. We find certain stereodifferentiation favoring oxidation of the cis-isomer, in agreement with previous experimental data. Second, we determine the reaction profiles of the ring-opening mechanism of the cationic, neutral, and anionic systems and assess their feasibility based on their energy barrier heights and the stability of the reactants and products. Results show that oxidation largely decreases the ring-opening energy barrier for both stereoisomers, even though the process is forecast as too slow to be competitive. Conversely, one-electron reduction dramatically facilitates the ring opening of the azetidine heterocycle. Considering the overall quantum-chemistry findings, N,N-dimethylaniline is proposed as an efficient photosensitizer to trigger the photoinduced cycloreversion of the DNA lesion model.

scholarly journals Sputtering of Neutral Molecules and Molecular Ions From the Adenine Crystal Surface Induced by the UV Picosecond Laser Pulse

1982 ◽  
Vol 1 (1) ◽  
pp. 37-43 ◽  
Author(s):  
V. S. Antonov ◽  
V. S. Letokhov ◽  
Yu. A. Matveyets ◽  
A. N. Shibanov
Keyword(s):  
Laser Radiation ◽  
Kinetic Energy ◽  
Crystal Surface ◽  
Picosecond Laser ◽  
Laser Pulses ◽  
Molecular Ions ◽  
Laser Radiation Intensity ◽  
Ion Sputtering ◽  
Absorbed Energy ◽  
Picosecond Laser Pulse

This paper presents the results of observation of sputtering of neutral molecules and ions from the crystal adenine surface induced by fourth-harmonic Nd:YAG laser radiation with a pulse duration of 30 ps. The energy fluence of laser pulses was in the region (1–3) × 10−4 J/cm2. The kinetic energy distribution of the sputtered molecules spreads up to 0.7 eV. The experiment shows that the threshold of adenine molecular ion sputtering is connected with absorbed energy density in upper layers of the crystal surface but not by laser radiation intensity.

Field desorption mass spectrometry of carboxylic acids: characteristic spectra and analysis of mixtures and impure samples

1978 ◽  
Vol 56 (10) ◽  
pp. 1372-1377 ◽  
Author(s):  
Gordon Walter Wooo ◽  
Emily Jane Oldenburg ◽  
Pui-Yan Lau ◽  
Donna Lee Wade
Keyword(s):  
Carboxylic Acids ◽  
Dicarboxylic Acids ◽  
Cross Product ◽  
Molecular Ions ◽  
Base Peak ◽  
Field Desorption ◽  
Technical Grade ◽  
Molecular Ion ◽  
Field Desorption Mass Spectrometry ◽  
Anode Temperature

Field desorption mass spectra were obtained for a variety of saturated and unsaturated carboxylic acids containing 12 or more carbons. At best anode temperature molecular ions were dominant and small peaks representing [M + 1]+, [2M + 1]+, [M − 17]+, and [Formula: see text] were present in several compounds. At higher temperatures several novel ions were found, including one corresponding to [2M + 1 – 18]+ which may represent anhydride formation. In a mixture of cis-5-eicosenoic and elaidic acids each molecular ion desorbed as expected but at higher temperatures the three possible anhydride ions appeared, with the cross product [M1 + M2 + 1 − H2O]+ as the base peak. Isomeric hydroxystearic acids (2-OH, 12-OH, 17-OH) gave predominantly ions in the molecular ion region with some differences in spectra which may relate to structure. Apparent polyester formation has been observed in 3-hydroxypropanoic acid where ions of the general formula [xM − (x − 1)H2O + H]+ with x = 2, 3, … 13 were found. Several other hydroxyacids show dimer formation and lactic acid has ions up to x = 5 in the above formula. Two of four technical grade dicarboxylic acids tested were seriously contaminated by sodium ions and gave useful spectra only after extraction by dibenzo-18-crown-6 ether. After this treatment both adipic and azelaic acid have [M + 1]+ as base peak, although adipic acid decarboxylates readily ([Formula: see text] = 74%) Other technical grade acids showed the presence of homologues and related structures as impurities.

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