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Ring Intermediate (ring + intermediate)
Selected AbstractsIsomeric hexyl-ketohydroperoxides formed by reactions of hexoxy and hexylperoxy radicals in oxygenINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2003François Jorand Isomerization reactions of peroxy radicals during oxidation of long-chain hydrocarbons yield hydroperoxides, and therefore play an important role in combustion and atmospheric chemistry, because of their action as branching agents in these chain reaction processes. Different formation mechanisms and structures are involved. Three isomeric hexyl-ketohydroperoxides are formed via isomerization reactions in oxygen of either hexoxy RO or hexylperoxy RO2 radicals. In the temperature range 373,473 K, 2-hexoxy (C6H13O) radical in O2/N2 mixtures gives 2-hexanone-5-hydroperoxide via two consecutive isomerizations. The second one is a H transfer from a HC(OH) group occurring via a seven-membered ring intermediate: Its rate constant has been determined at 453 and 483 K, and the general expression can be written as Hexylperoxy C6H13O2 radical, present in n -hexane oxidation by oxygen/nitrogen mixtures in the temperature range 543,573 K, gives 2-hexanone-4-hydroperoxide, 3-hexanone-5-hydroperoxide, and 2-hexanone-5-hydroperoxide. The first two are formed through an isomerization reaction via a six-membered ring intermediate, and the last through an isomerization reaction via a seven-membered ring intermediate. The ratio of the rate constant of the isomerization reactions of RO2 radicals via a seven-membered ring intermediate to that via a six-membered ring is found to be 0.795, and the rate constant expression via a seven-membered ring intermediate is proposed: The role of these reactions in the formation of radicals in the troposphere is discussed. Other products arising in the reactional path, such as ketones, furans, and diketones, are identified. Identification of these ketohydroperoxides was made using gas chromatography/mass spectrometry with electron impact, and with NH3 (or ND3) chemical ionization. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 354,366, 2003 [source] Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2007Xiu Hui Lu Abstract Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Møller,Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Influence of "Alternative" C-terminal amino acids on the formation of [b3 + 17 + Cat]+ products from metal cationized synthetic tetrapeptides,JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2004V. Anbalagan Abstract The aim of this study was to investigate the dissociation patterns, and in particular the relative abundance of [b3 + 17 + Cat]+, for peptides with C-termini designed to allow transfer of the ,OH required to generate the product ion, but not necessarily as the most favored pathway. Working with the hypothesis that formation of a five-membered ring intermediate, including intramolecular nucleophilic attack by a carbonyl oxygen atom, is an important mechanistic step, several model peptides with general sequence AcFGGX were synthesized, metal cationized by electrospray ionization and subjected to collision-induced dissociation (CID). The amino acid at position X was one that either required a larger ring intermediate (,-alanine, ,-aminobutyric acid and ,-amino- n -caproic acid to generate six-, seven- or nine- membered rings, respectively) to transfer ,OH, lacked a structural element required for nucleophilic attack (aminoethanol) or prohibited cyclization because of the inclusion of a rigid ring (p - and m -aminobenzoic acid). For Ag+, Li+ and Na+ cationized peptides, our results show that amino acids requiring the adoption of larger ring intermediates suppressed the formation of [b3 + 17 + Cat]+, while amino acids that prohibit cyclization eliminated the reaction pathway completely. Formation of [b3 , 1 + Cat]+ from the alkali metal cationized versions was not a favorable process upon suppression or elimination of the [b3 + 17 + Cat]+ pathway: the loss of H2O to form [M , H2O + Cat]+ was instead the dominant dissociation reaction observed. Multiple-stage dissociation experiments suggest that [M , H2O + Cat]+ is not [b4 , 1 + Cat]+ arising from the loss of H2O from the C-terminus, but may instead be a species that forms via a mechanism involving the elimination of an oxygen atom from an amide group. Copyright © 2004 John Wiley & Sons, Ltd. [source] Gas-phase formation of protonated benzene during collision-induced dissociation of certain protonated mono-substituted aromatic molecules produced in electrospray ionizationRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 11 2010Min Li Protonated benzene, C6H, has been studied extensively to understand the structure and energy of a protonated organic molecule in the gas phase. The formation of C6H is either through direct protonation of benzene, i.e., chemical ionization, or through fragmentation of certain radical cations produced from electron ionization or photon ionization. We report a novel observation of C6H as a product ion formed in the collision-induced dissociation (CID) of protonated benzamide and related molecules produced via electrospray ionization (ESI). The formation of C6H from these even-electron precursor ions during the CID process, which has not been previously reported, is proposed to occur from the protonated molecules via a proton migration in a five-membered ring intermediate followed by the cleavage of the mono-substituent CC bond and concurrent formation of an ion-molecule complex. This unique mechanism has been scrutinized by examining some deuterated molecules and a series of structurally related model compounds. This finding provides a convenient mean to generate C6H, a reactive intermediate of considerable interest, for further physical or chemical investigation. Further studies indicate that the occurrence of C6H in liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) appears to be a rather common phenomenon for many compounds that contain ,benzoyl-type' moieties. Hence, the observation of the C6H ion in LC/ESI-MS/MS can be used as an informative fragmentation pathway which should facilitate the identification of a great number of compounds containing the ,benzoyl-type' and similar structural features. These compounds are frequently present in food and pharmaceutical products as leachable impurities that require strict control and rapid elucidation of their identities. Copyright © 2010 John Wiley & Sons, Ltd. [source] Negative ion electrospray ionization mass spectrometry of nucleoside phosphoramidate monoesters: elucidation of novel rearrangement mechanisms by multistage mass spectrometry incorporating in-source deuterium labellingRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 19 2008Peng-Xiang Xu Several O-2,,3,-isopropylideneuridine-O-5,-phosphoramidate monoesters were synthesized and analyzed by negative ion electrospray ionization tandem mass spectrometry (ESI-MSn). Two kinds of novel rearrangement reactions were observed due to the difference in the amino acid in the nucleoside phosphoramidate monoesters, and possible mechanisms were proposed. One involves a five-membered cyclic transition state. The other is formation of a stable five-membered ring intermediate by Michael addition. Results were confirmed by tandem mass spectrometry and isotopically labeled hydrogen atoms. Furthermore, the internal hydrogen exchange between active hydrogen and methyl acrylate in the heated capillary of the mass spectrometer was found. The characteristic fragmentation behavior in ESI-MS may be used to monitor this kind of compounds in the biological metabolism. Copyright © 2008 John Wiley & Sons, Ltd. [source] Ab initio Study of Mechanism of Forming a Germanic Hetero-Polycyclic Compound between Germylidene (H2C=Ge:) and AcetoneCHINESE JOURNAL OF CHEMISTRY, Issue 8 2010Xiuhui Lu Abstract The mechanism of the cycloaddition reaction of forming a germanic hetero-polycyclic compound between singlet germylidene (R1) and acetone (R2) has been investigated with CCSD(T)//MP2/6-31G* method. From the surface energy profile, it can be predicted that the dominant reaction pathway for this reaction consists of three steps: (1) the two reactants (R1, R2) firstly form a twisted four-membered ring intermediate (INT2); (2) the intermediate (INT2) reacts further with acetone (R2) to give another intermediate (INT4); (3) intermediate (INT4) isomerizes to a hetero-polycyclic germanic compound (P4) via a transition state TS4. The presented rule of this reaction: the [2+2] cycloaddition effect between the , orbital of germylidene and the , orbital of ,-bonded compounds leads to the formation of four-membered ring intermediate (INT2). The 4p unoccupied orbital and the lone-pair sp electrons of Ge in the four-membered ring intermediate (INT2) react with the , orbital and the antibonding ,* orbital of ,-bonded compounds, respectively, forming the ,,p and sp, ,* cyclic donor-acceptor bonds, resulting in the generation of a stable germanic hetero-polycyclic compound (P4). [source] Influence of "Alternative" C-terminal amino acids on the formation of [b3 + 17 + Cat]+ products from metal cationized synthetic tetrapeptides,JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2004V. Anbalagan Abstract The aim of this study was to investigate the dissociation patterns, and in particular the relative abundance of [b3 + 17 + Cat]+, for peptides with C-termini designed to allow transfer of the ,OH required to generate the product ion, but not necessarily as the most favored pathway. Working with the hypothesis that formation of a five-membered ring intermediate, including intramolecular nucleophilic attack by a carbonyl oxygen atom, is an important mechanistic step, several model peptides with general sequence AcFGGX were synthesized, metal cationized by electrospray ionization and subjected to collision-induced dissociation (CID). The amino acid at position X was one that either required a larger ring intermediate (,-alanine, ,-aminobutyric acid and ,-amino- n -caproic acid to generate six-, seven- or nine- membered rings, respectively) to transfer ,OH, lacked a structural element required for nucleophilic attack (aminoethanol) or prohibited cyclization because of the inclusion of a rigid ring (p - and m -aminobenzoic acid). For Ag+, Li+ and Na+ cationized peptides, our results show that amino acids requiring the adoption of larger ring intermediates suppressed the formation of [b3 + 17 + Cat]+, while amino acids that prohibit cyclization eliminated the reaction pathway completely. Formation of [b3 , 1 + Cat]+ from the alkali metal cationized versions was not a favorable process upon suppression or elimination of the [b3 + 17 + Cat]+ pathway: the loss of H2O to form [M , H2O + Cat]+ was instead the dominant dissociation reaction observed. Multiple-stage dissociation experiments suggest that [M , H2O + Cat]+ is not [b4 , 1 + Cat]+ arising from the loss of H2O from the C-terminus, but may instead be a species that forms via a mechanism involving the elimination of an oxygen atom from an amide group. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||