- Unimolecular Reactions of Isolated Organic Ions: the Chemistry of the Oxonium Ions CH3CH2CH2CH2(+)O=CH2 and CH3CH2CH2CH=O(+)CH3
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The reactions of the metastable oxonium ions CH3CH2CH2CH2(+)O=CH2 and CH3CH2CH2CH=O(+)CH3 are reported and discussed.Both these isomers of C5H11O(+) expel predominantly CH2O (75 - 90percent of the metastable ion current), a moderate amount of C3H6 (5-15percent), a minor amount of CH3OH (2-8percent) and a very small proportion of H2O (0.5-3percent).All these processes give rise to Gaussian metastable peaks.The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH3CH2CH2CH=O(+)CH3.The behaviour of labelled analogues confirms that the reactions of CH3CH2CH2CH2(+)O=CH2 and CH3CH2CH2CH=O(+)CH3 are closely related, but subtly different.Elimination of CH2O and C3H6 is intelligible by means of mechanisms involving CH3CH(+)CH2CH2OCH3.This open-chain cation is accessible to CH3CH2CH2CH2(+)O=CH2 by a 1,5-H shift and to CH3CH2CH2CH=O(+)CH3 by two consecutive 1,2-H shifts (or, possibly, a direct 1,3-H shift).The rates of these 1,2-, 1,3- and 1,5-H shifts are compared with one another and also with the rates of CH2O and C3H6 loss from each of the two oxonium ions.The 1,5-H shift that converts CH3CH(+)CH2CH2OCH3 formed from CH3CH2CH2CH=O(+)CH3 into CH3CH2CH2CH2(+)O=CH2 prior to CH2O elimination is essentially unidirectional.In contrast, the corresponding step converting C5H11O(+) ions generated as CH3CH2CH2CH2(+)O=CH2 into CH3CH(+)CH2CH2OCH3 competes effectively with expulsion of CH2O and C3H6.The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon-carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro 'ene' pericyclic process are emphasized.
- Bowen, Richard D.,Derrick, Peter J.
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- Kinetics an Stereochemistry of the Thermal Interconversion of 4,5-Dimethyl-2,6-octadienes
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Gas-phase pyrolysis of threo-4,5-dimethyl-cis,cis-1,1,1,8,8,8-hexadeuterio-2,6-octadiene over the temperature range 220.0-260.0 deg C resulted in formation of the threo, trans, trans isomer with log k = 11.36 - 36000/2.303RT.NMR analysis with Simplex minimization of the residuals from a Gear numerical integration provided a nearly identical rate constant for the degenerate interconversion of the deuterium isomers of the threo,trans,trans diastereomer at 240 deg C.All six 4,5-dimethyl-2,6-octadienes are interconverted at temperatures above 290 deg C.Mass spectral analysis of the reaction products from a 1:1 mixture of protio and D6 diene provided evidence for cleavage-recombination as the pathway for conversion to erythro,trans,trans,erythro,cis,cis and threo,trans,cis isomers.Competing with the cleavage-recombination is the boatlike shift to the erythro,trans,cis isomer.NMR analysis of the (-)-α-phenylethylamine bis salt of the threo-2,3-dimethylsuccinic acid derived from 33.5-h 240 deg C pyrolysis of optically pure hexadeuterio starting material provided evidence for little incursion of antarafacial-antarafacial 3,3-shifts via a twist transition state competing with the chair transition state.An analysis of the energy surface for all interconversions reveals that at 300 deg C, the boat transition state is ca. 6 kcal/mol higher in energy than the sterically most favorable chair transition state, and the cleavage reaction transition state is only 0.6 kcal/mol higher in energy than the boat transition state.
- Gajewski, Joseph J.,Benner, Charles W.,Hawkins, Christopher M.
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- Mechanism of Propene and Water Elimination from the Oxonium Ion CH3CH=O+CH2CH2CH3
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The site-selectivity in the hydrogen transfer step(s) which result in propene and water loss from metastable oxonium ions generated as CH3CH=O+CH2CH2CH3 have been investigated by deuterium-labelling experiments.Propene elimination proceeds predominantly by transfer of a hydrogen atom from the initial propyl substituent to oxygen.However, the site-selectivity for this process is inconsistent with β-hydrogen transfer involving a four-centre transition state.The preference for apparent α- or γ-hydrogen transfer is interpreted by a mechanism in which the initial propyl cation accessible by stretching the appropriate bond in CH3CH=O+CH2CH2CH3 isomerizes unidirectionally to an isopropyl cation, which then undergoes proton abstraction from either methyl group +CH2CH2CH3 CH3CH=O---+CH2CH2CH3 +CH(CH3)2> + CH3CH=CH2>>.This mechanism involving ion-neutral complexes can be elaborated to accommodate the minor contribution of expulsion of propene containing hydrogen atoms originally located on the two-carbon chain.Water elimination resembles propene loss insofar as there is a strong preference for selecting the hydrogen atoms from the α- and γ-positions of the initial propyl group.The bulk of water loss is explicable by an extension of the mechanism for propene loss, with the result that one hydrogen atom is eventually transferred to oxygen from each of the two methyl groups in the complex +CH(CH3)2>.This site-selectivity is strikingly different from that (almost random participation of the seven hydrogen atoms of the propyl substituent) encountered in the corresponding fragmentation of the lower homologue CH2=O+CH2CH2CH3.This contrast is explained in terms of the differences in the relative energetics and associated rates of the cation rearrangement and hydrogen transfer steps.
- Bowen, Richard D.,Suh, Dennis,Terlouw, Johan K.
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- Raman spectroscopy of n-pentyl methyl ether and deuterium labelledanalogues
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The Raman spectra of n-pentyl methyl ether, C5H 11OCH3, and six selectively deuteriated analogues arereported and discussed. Correlations between the observed ν(sp 3CH)stretching and bending bands and the position of the deuterium atoms in thealkyl chain are developed and refined. Similar progress is possible inassociating specific skeletal vibrations with bands in the Raman spectra. Therelevance of this study to improving the assignment of bands in the Ramanspectra of larger systems of biological interest is highlighted. Copyright
- Bowen, Richard D.,Edwards, Howell G. M.,Farwell, Dennis W.,Morgan, Sara E
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- INTERMEDIATES IN COBALT-CATALYSED METHANOL HOMOLOGATION: LABELLING STUDIES WITH DEUTERATED METHANOL AND METHYL IODIDE
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The cobalt-catalysed homologation of perdeuterated methanol with CO/H2 gives C2 products in which the CD3 group remains intact.GC/MS measurements showed that no H/D exchange unless the methanol conversion exceeded 50percent.These results indicate that methylene species are unlikely to be as catalytic intermediates, and favour methyl species for such intermediates.In the presence of iodine promoters methyl iodide is a likely intermediate since it is much more readily consumed than methanol in carbonylation/hydrocarbonylation reactions.This was shown by treating a 5/1 mixture of CH3OH and CD3I with CO/H2 in the presence of Co2(CO)8; at short reaction times only the carbonylation/hydrocarbonylation products of methyl iodide could be detected by GC/MS.Methyl iodide can be formed from variety of iodine compounds under homologation conditions, as was confirmed by separate experiments.
- Roeper, Michael,Loevenich, Heinz
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- Metastable Decompositions of C5H10O+. Ions with the Oxygen on the Middle Carbon: A Test for Energy Randomization
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This study was undertaken to define the mechanisms of metastable decomposition of C5H10O+. ions with the oxygen on the middle carbon, and to test the assumption that internal energy becomes randomly distributed prior to the unimolecular decompositions of gaseous ions.CH3CH2C(=OH+)CH2CH2. (2), CH3CH2C(=OH+).CHCH3 (3), and CH3CH2C(OH+.)HCH=CH2 (4) all rearrange to CH3CH2C(=O+.)CH2CH3 (1) prior to metastable decomposition.However, 2 - 4 lose ethyl 50 - 100 times as often as they lose ethane following rearrangement to 1, while 1 formed by ionization of 3-pentanone loses exclusively ethane.These differences are attributed to excess energy in the isomerized ions. 3-Pentanone ions formed by isomerization of 2 - 4 lose ethyl and ethane from opposite sides at unequal rates, possibly owing to incomplete randomization of energy following isomerization.
- McAdoo, David J.,Farr, William,Hudson, Charles E.
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- SYNTHESIS AND NMR SPECTRA OF 2,2,2,-TRIDEUTERIOETHYLARSINE AND 2,2,2-TRIDEUTERIOETHYLCYCLOPENTAARSINE
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Pentakis(2,2,2-trideuterioethyl)cyclopentaarsine (PDECA) was synthesized by the reaction of 2,2,2-trideuterioethylarsine with dibenzylmercury.Variable temperature NMR spectra in C6D6 are interpreted in terms of fast pseudorotation.NMR and mass spectra and synthesis of 2,2,2-trideuterioethylarsine are also described.
- Rheingold, A. L.,Natarajan, S.
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- A study of the gas-phase reactivity of neutral alkoxy radicals by mass spectrometry: α-Cleavages and Barton-type hydrogen migrations
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The reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization - reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C1-C3) or branched alkyl chains give rise to α-cleavage products, whereas longer-chained alkoxy radicals undergo 1,5-hydrogen migrations from carbon to oxygen, that is, Barton-type chemistry. This facile rearrangement has been studied in detail for n-pentoxy radicals by isotopic labeling experiments and computation at the Becke 3LYP/6-31G* level of theory. Further, the NIDD spectra of 3-methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas-phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.
- Hornung, Georg,Schalley, Christoph A.,Dieterle, Martin,Schroeder, Detlef,Schwarz, Helmut
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- Unimolecular Reactions of Isolated Organic Ions: Reactions of the Immonium Ions CH2=N+(CH3)CH(CH3)2, CH2=N+(CH3)CH2CH2CH3 and CH2=N+(CH2CH2CH3)2
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The reactions of metastable CH2=N+(CH3)C3H7 immonium ions have been investigated by means of 2H-labelling experiments and kinetic energy release measurements.Loss of C3H6, with specific β-H transfer, is the sole channel for dissociation of CH2=N+(CH3)CH(CH3)2.This process gives rise to a Gaussian metastable peak.The isomeric ion, CH2=N+(CH3)CH2CH2CH3, also expels C3H6; however, both α-H and γ-H as well as β-H transfer occurs in this case, and the reaction proceeds with an increased kinetic energy release.The role of ion-neutral complexes in C3H6 loss from CH2=N+(CH3)C3H7 ions is discussed.In addition, CH2=N+(CH3)CH2CH2CH3 eliminates C2H4.This fragmentation yields a broad dish-topped metastable peak, corresponding to a very large kinetic energy release (T1/2 ca. 73 kJ mol-1), and it involves specific and unidirectional γ-H transfer.A potential energy profile summarising the reactions of CH2=N+(CH3)CH2CH2CH3 and CH2=N+(CH3)CH(CH3)2 is constructed.The mechanisms by which immonium ions of this general class eliminate C3H6 and C2H4 have been further probed by studying the behaviour of the higher homologue, CH2=N+(CH2CH2CH3)2.The mechanistic conclusions derived from this work are found to be in excellent qualitative agreement with those of previous studies.
- Bowen, Richard D.,Colburn, Alex W.,Derrick, Peter J.
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- Deuteron Quadrupole Coupling Constants of the Methyl and Methylene Groups of Ethanol from the Direct 13C-2H and 2H-2H Couplings in 2H-NMR Spectra
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The deuteron quadrupole coupling constants, QD, of 13C(2)H3CH2OH and CH3(13)C2H2OH were measured in two different nematic solvents by employing the quadrupolar splitting and 13C-2H and 2H-2H dipolar splittings in 2H NMR spectra.In order to obtain the most accurate values for QD, vibrational averaging of the dipolar couplings and ab initio calculations of the C-D bond asymmetry correction were used in the theoretical treatment of the experimental data.The values found in ZLI-1167 are 186.6 and 178.2 kHz and in phase IV are 180.7 and 176.6 kHz for the methyl and methylene groups, respectively.Using the previously obtained 2H NMR relaxation data and the QD value determined in ZLI-1167, we calculated the internal rotation coefficient of the methyl group to be 0.68E11 s-1.
- Lickfield, G. C.,Beyerlein, A. L.,Savitsky, G. B.,Lewis, L. E.
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- Alkane Elimination From Ionized Alkanols
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The energetics, metastable characteristics and doughter ion structures for the loss of small alkane molecules from ionized 2-prpanol, 2-butanol and 3-pentanol have been examined in detail. (1+)(*) ions lose CH4 to generate the keto and enol forms of (1+)(*) and the same doughter ions are produced by loss of C2H6 from (1+)(*).Ionized 3-pentanol does not lose CH4 but readily eliminates C2H6 to produce the enol ion (1+)(*).The last reaction was shown to proceed by a simple 1,2 elimination mechanizm in the μs time-frame; isotope effects are also discussed.
- Holmes, John L.,Burgers, Peter C.,Mollah, Yousuf A.
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- Alkene Loss from Metastable Methyleneimmonium Ions: Unusual Inverse Secondary Isotope Effect in Ion-Neutral Complex Intermediate Fragmentations
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The mechanism of propene elimination from metastable methyleneimmonium ions is discussed.The first field-free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C=N+R1R2 : R1 = R2 = n-C3H7; R1 = R2 = i-C3H7; R1 = n-C3H7; R2 = C2H5; R1 = n-C3H7; R2 = CH3; R1 = n-C3H7; R2 = H) were used to support the mechanism presented.The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly.Propene loss from the ions examined proceeds via ion-neutral complex intermediates.For the di-n-propyl species rate-determining and H/D distribution-determining steps are clearly distinct.Whereas the former corresponds to a 1,2-hydride shift in a 1-propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2-propyl cation generated by the previous step.For the diisopropyl-substituted ions which directly form the 2-propyl cation-containing complex, the rate-determining hydride shift vanishes.The 2-propyl cation-containing complex can decompose directly or via an intermediate proton-bridged complex.Competition of these routes is not excluded by the experimental results.Assuming a 2:1:3 distribution, a preference for the α- and β-methylene of the initial n-propyl chain as the source of the hydrogen transferred is detected for n-propylimmonium ions containing a second alkyl chain R2.This preference shows a clear dependence on the steric influence of R2.During the transfer step isotopic substitution is found to affect the H/D distribution strongly.For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ-H transfer is observed.
- Veith, Hans J.,Gross, Juergen H.
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- Fermi resonance structure in the CH vibrational overtones of CD3CHO
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Gas-phase fundamental and CH and CO overtone spectra (700-17 500 cm-1) of 2,2,2- trideuteroacetaldehyde were recorded using FTIR and laser photoacoustic techniques.The Fermi resonance structure in the overtone spectra of the coupled CH stretching and in-plane CH bending vibrations is analy zed with a tridiagonal Hamiltonian yielding a large effective coupling constant, =93 cm-1, corresponding to subpicosecond redistribution times.No coupling between the out- of plane CH bending mode and the Fermi resonance system is apparent.This study presents the first detailed analysis of the anharmonic couplings in the CH chromophore at an sp2 carbon atom.The in- plane CH bending vibration couples in a manner similar to the CH ( sp3 ) bending vibrations, whereas the out-of plane bending vibration is decoupled, similar to the CH(sp) bending vibrations.
- Amrein, A.,Hollenstein, H.,Quack, M.,Zenobi, R.,Segall, J.,Zare, R. N.
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- Nickel-Mediated Stepwise Transformation of CO to Acetaldehyde and Ethanol
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The insertion of CO into the Ni-C bond of synthetic Ni(II)-CH3 cationic complex ([1-CH3]+) affords a nickel-acetyl complex ([1-COCH3]+). Reduction of resultant [1-COCH3]+ by borohydrides produces CH3CHO, CH3CH2OH, and an Ni(0) compound ([1]0), which can react with CH3I to regenerate [1-CH3]+. By conducting deuterium labeling experiments, we have demonstrated that CH3CHO is the primary product from CH3CH2OH in such CO transformation reactions. In the reduction of [1-COCH3]+, the formation of CH3CHO competes with the loss of CH4, which leads to a Ni(0)-CO compound ([1-CO]0) as a minor product. Our results establish fundamental steps in the exploration of nickel-mediated CO transformation to valuable chemicals.
- Zhang, Ailing,Raje, Sakthi,Liu, Jianguo,Li, Xiaoyan,Angamuthu, Raja,Tung, Chen-Ho,Wang, Wenguang
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p. 3135 - 3141
(2017/09/05)
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- Pyrolysis of α- and β-heteroatoms substituted ethyl phenyl sulfoxides
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A study on the mechanism of the thermal decomposition of α- and β-heteroatoms substituted ethyl phenyl sulfoxides was carried out using 1-chloroethyl phenyl sulfoxide (1); two diastereomeric 1-acetoxyethyl (substituted phenyl) sulfoxides (2a) and (2b); and 2-chloroethyl phenyl, 2-bromoethyl phenyl, and 2-methoxyethyl phenyl sulfoxides (3, 4, 5). The rate of pyrolysis of 1 was 4.8 times faster at 160°C than that of ethyl phenyl sulfoxide used as a reference, while those of 2a and 2b were 107 and 155 times faster, respectively. The results indicate that the lone pair of electrons on the α-heteroatoms has a larger rate acceleration effect than the electronegativity of them. The substituent effects of the phenyl group of 2a and 2b gave positive Hammett ρ-values (ρa= 0.76 and ρb= 0.80 vs. σ). Activation parameters for 2a and 2b are as follows: 2a, ΔH?= 112 kJmol-1, ΔS?= -20 JK-1mol-1; 2b, ΔH?= 107 kJmol-1, ΔS?= -29 JK-1mol-1. Large deuterium kinetic isotope effects for 1-acetoxyethyl-2,2,2-d3 phenyl sulfoxides (2ad and 2bd) were observed (kH/kD= 3.5 ~ 4.1). These results suggest that the pyrolysis of -heteroatom substituted ethyl phenyl sulfoxides proceeds via a five-membered transition state deviated to E1-like in character. On the other hand, from the results of kinetics for the pyrolysis of 3, 4, and 5, no effect by the β-halogen atoms or some deceleration effect by the β-methoxy group was observed. Thus the reaction seems to proceed via an E1-like mechanism. Copyright Taylor & Francis Group.
- Yoshimura, Toshiaki,Sakae, Hironori,Yoshizawa, Masaki,Hasegawa, Kiyoshi,Tsukurimichi, Eiichi
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experimental part
p. 1162 - 1173
(2010/08/06)
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- Intermediacy of ion neutral complexes in the fragmentation of short-chain dialkyl sulfides
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The main fragmentation processes after electron ionization of butyl methyl and butyl ethyl sulfides are rationalized by the intermediacy of the ion neutral complex [RSH · methylcyclopropane](+·) as demonstrated by extensive labeling and collision activation studies.
- Filsak,Budzikiewicz
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p. 601 - 610
(2007/10/03)
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- Laser-Powered Decomposition of Spiroalkanes (n = 2-5)
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The laser heating of spiroalkanes (n=2-5) and of their 1,1,2,2-tetradeuterated isotopomers reveals dissimilar modes of their thermal decomposition.Spiropentane decomposes into ethene and propadiene via two competing routes: the direct cleavage and the more important cleavage via intermediary methylenecyclobutane.Spirohexane decomposes through two important concurrent pathways which are the expulsions of ethene from the three-membered ring and a more feasible expulsion of ethene from the four-membered ring.Spiroheptane and spirooctane decompose by a radical-chain mechanism and afford complex mixtures of products; upon addition of propene both compounds rearrange into two cycloalkanes wherein the larger ring of the spiroalkane is preserved and substituted with ethylidene and a vinyl group.
- Fajgar, Radek,Pola, Josef
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p. 7709 - 7717
(2007/10/02)
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- The Mechanism of Ethylene Elimination from the Oxonium Ions CH3CH2CH=O+CH2CH3 and (CH3)2C=O+CH2CH3
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The reactions of the metastable oxonium ions CH3CH2CH=O+CH2CH3 and (CH3)2C=O+CH2CH3 are reported and discussed.Various mechanisms for ethylene elimination, which is the principal dissociation route for these ions, are considered.It is shown by means of 2H-labelling experiments and analysis of collision-induced dissociation spectra that routes involving ion-neutral complexes pre-empt 'conventional' mechanisms for these processes.In contrast, the behaviour of the lower homologues CH3CH2CH=OR+ and (CH3)2C=OR+ (R = H, CH3) is consistent with the operation of 'conventional' mechanisms for ethylene expulsion.This contrast is interpreted in energetic terms.The significance of these results for the chemistry of homologous and analogous 'onium' ions containing a Z+-R function (Z = O, S, NH, NCH3; R= CnH2n+1, n 2) is explained.
- Bowen, Richard D.,Derrick, Peter J.
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p. 1033 - 1039
(2007/10/02)
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- Ion-Neutral Complexes as Intermediates in the Decompositions of C5H10O2.+ Isomers
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Ionized pentanoic acid, 3-methylbutanoic acid, and the enol isomer of ionized isopropyl acetate are shown to pass in part through common intermediates before decomposing to CH3C.HC(OH)2+ (7) and the "McLafferty + 1" ion CH3C(OH)2+ (10).The H transfer to form the methyl of CH3C(OH)2+ and the joining of two CH2 groups to form the C-C bond in the ethylene eliminated to produce CH3C.HC(OH)2+ are both attributed to reactions of the ion-neutral complex .H2C(OH)2+>.The McLafferty + 1 ion is also formed, especially from ionized esters, by another pathway in which complexes may or may not be intermediates.The intermediacy of the ion-neutral complexes is supported by energetic considerations, isotope effects, and the decomposition patterns of labeled ions.The latter correlate with a preference for hydrogen transfer from the end carbons of the C3 partner in other reactions proposed to be complex-mediated.Unification of the McLafferty rearrangement, the McLafferty + 1 rearrangement, and the McLafferty rearrangement with charge reversal by a common initial γ-hydrogen rearrangement followed by dissociation or isomerization in ion-neutral complexes is proposed.Group migration by 1,2-shifts, possibly by dissociation to form a double bond in one partner in an ion-neutral complex followed by addition at the opposite end of the double bond, is shown to be a general reaction of ions in the gas phase.
- McAdoo, David J.,Hudson, Charles E.,Skyiepal, Mark,Broido, Ellen,Griffin, Lawrence L.
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p. 7648 - 7653
(2007/10/02)
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- STUDIES OF PYRAZINES. PART 8. PYROLYSIS OF 2-ALKOXYPYRAZINES; SUBSTITUENT EFFECTS AND STEREOSPECIFICITY
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Twelve 2-alkoxypyrazines (R=iPr, sBu, 1,2-dimethylpropyl, 1,2,2-trimethylpropyl, t-butyl, t-pentyl, 1,1,2-trimethylpropyl, 1,1,2,2-tetramethylpropyl, octyl, neopentyl, +/--erythro- and +/--threo-2-deuterio-1,2-diphenylethyl) have been pyrolysed to elucidate the substituent effects on the rate and stereospecificity of the reaction.Activation parameters (log A and Ea) for the first eight compounds listed were determined.The reaction was accelerated by α-substitution; this was a combination of both polar and steric effects.The stereospecificity of the reaction was high.
- Konakahara, Takeo,Sato, Kenji,Takagi, Yukio,Kuwata, Kazuhiko
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p. 641 - 646
(2007/10/02)
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