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Molecule Loss (molecule + loss)
Selected AbstractsMetastable ion study of organosilicon compounds.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 7 2001(CH3)2Si(OCH3), CH3SiH(OCH3), Part XIII: dimethoxydimethylsilane, dimethoxymethylsilane Abstract Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH3)2Si(OCH3)2 (MW 120, 1), and dimethoxymethylsilane, CH3SiH(OCH3)2 (MW 106, 2), upon electron impact ionization have been studied by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and the D-labeling technique in conjunction with thermochemistry. The results have been compared with those of the corresponding carbon analogues, 2,2-dimethoxypropane, (CH3)2C(OCH3)2 (MW 104, 3) and 1,1-dimethoxyethane, CH3CH(OCH3)2 (MW 90, 4). In analogy with the cases of 3 and 4, both molecular ions from 1 and 2 are formed at very low abundance at 70 eV, and begin to decompose by the expulsion of the substituents (H, CH3 or OCH3) on the central silicon atom. These decompositions are followed by the loss of a formaldehyde molecule (CH2O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethylene (C2H4) or a dimethyl ether (CH3OCH3) molecule loss is observed in the fragmentation of some intermediate ions generated from 1+· and 2+·, but the mechanisms are different than those in the cases of 3 and 4. Some of these fragmentations are also different than those reported previously. The relative abundance of the ions in many MIKE spectra is explained by the extension of the Stevenson,Audier rule. The reaction, which is in contrast to the rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi-empirical molecular orbital calculation. The peak at m/z 59 from 2+· consists only of CH3OSi+ ion, whereas the peak from 1+· consists of two different ions, CH3OSi+ and (CH3)2Si+H. The ions CH3OSi+ from 1+· and 2+· are generated by at least two and three separate routes respectively. Copyright © 2001 John Wiley & Sons, Ltd. [source] A two-dimensional network formed by self-associating silver(I) perchlorate with 3-[4-(2-thienyl)-2H -cyclopenta[d]pyridazin-1-yl]benzonitrileACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2009Xiao-Yan Li In the organometallic silver(I) supramolecular complex poly[[silver(I)-,3 -3-[4-(2-thienyl)-2H -cyclopenta[d]pyridazin-1-yl]benzonitrile] perchlorate methanol solvate], {[Ag(C18H11N3S)](ClO4)·CH3OH}n, there is only one type of AgI center, which lies in an {AgN2S,} coordination environment. Two unsymmetric multidentate 3-[4-(2-thienyl)-2H -cyclopenta[d]pyridazin-1-yl]benzonitrile (L) ligands link two AgI atoms through ,,AgI interactions into an organometallic box-like unit, from which two 3-cyanobenzoyl arms stretch out in opposite directions and bind two AgI atoms from neighboring box-like building blocks. This results in a novel two-dimensional network extending in the crystallographic bc plane. These two-dimensional sheets stack together along the crystallographic a axis to generate parallelogram-like channels. The methanol solvent molecules and the perchlorate counter-ions are located in the channels, where they are fixed by intermolecular hydrogen-bonding interactions. This architecture may provide opportunities for host,guest chemistry, such as guest molecule loss and absorption or ion exchange. The new fulvene-type multidentate ligand L is a good candidate for the preparation of Cp,AgI -containing (Cp is cyclopentadienyl) organometallic coordination polymers or supramolecular complexes. [source] Role of 2-oxo and 2-thioxo modifications on the proton affinity of histidine and fragmentation reactions of protonated histidine,RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 17 2010Adrian K. Y. Lam A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments was used to compare the gas-phase chemistry of the amino acids histidine (1), 2-oxo-histidine (2), and 2-thioxo-histidine (3). Collision-induced dissociation (CID) of all three different proton-bound heterodimers of these amino acids led to the relative gas-phase proton affinity order of: histidine >2-thioxo-histidine >2-oxo-histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H2O and CO to yield a1 ions. Protonated 2 and 3 also undergo other small molecule losses including NH3 and the imine HN=CHCO2H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H2O+CO, barriers against the losses of NH3 and HN=CHCO2H are lowered relative to protonated histidine. Copyright © 2010 John Wiley & Sons, Ltd. [source] Identification of caffeic acid derivatives in Actea racemosa (Cimicifuga racemosa, black cohosh) by liquid chromatography/tandem mass spectrometryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 9 2003Wenkui Li Caffeic acid derivatives occurring in black cohosh [Cimicifuga racemosa (L.) Nutt., Actaea racemosa (Ranunculaceae)], some of which may have pharmacological activity, were analyzed using high-performance liquid chromatography (HPLC) electrospray ionization tandem mass spectrometry (ESI-MS/MS) with the aim of developing a methodology for their rapid identification in a complex plant matrix. Based on these studies, structurally characteristic product ions and neutral molecule losses were identified, which were then used during LC/MS/MS with product ion scanning, precursor scanning and constant neutral loss scanning to detect caffeic acid derivatives in a crude extract of black cohosh. Several caffeic acid derivatives were detected, and the identification of six of them were confirmed by comparison with authentic standards including caffeic acid, ferulic acid, isoferulic acid, fukinolic acid, cimicifugic acid A, and cimicifugic acid B. Four other compounds were detected that appeared to be caffeic acid derivatives based on LC/MS/MS retention times, molecular weights, and fragmentation patterns during MS/MS. Since standards were unavailable for these four compounds, they were tentatively identified using LC/MS/MS as cimicifugic acid E, cimicifugic acid F, dehydrocimicifugic acid A, and dehydrocimicifugic acid B. Dehydrocimicifugic acid A and dehydrocimicifugic acid B have not been reported previously to be constituents of black cohosh. Copyright © 2003 John Wiley & Sons, Ltd. [source] | ||||||||||