- Conformation of the Ethoxy Group in 4-Ethoxy-4'-cyanobiphenyl
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The deuterium and proton NMR spectra of samples of 4-ethoxy-4'-cyanobiphenyl dissolved in the nematic liquid crystal 4-hexyloxy-4'-cyanobiphenyl have been analyzed to yield quadrupolar splittings and dipolar couplings.The data are compared with values calculated for three models for the conformations adopted by the ethoxy group relative to the attached phenyl ring.These models are (a) a set of four, symmetry-related structures, (b) discrete, minimum-energy structures generated by jumps about the C4-O, O-C7, and C7-C8 bonds, and (c) the same as (b) except that a continuous potential is adopted for the motion about the O-C7 bond.
- Emsley, J. W.,Horne, T. J.,Celebre, G.,Longeri, M.,Zimmermann, H.
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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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experimental part
p. 1725 - 1734
(2012/04/04)
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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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- A McLafferty Rearrangement in an Even-electron System: C3H6 Elimination from the α-Cleavage Product of Tri-n-butylamine
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It is shown by deuterium labelling, linked-scan measurements and collision activation that the .>+ (α-cleavage) ion in the electron impact ionization spectrum of tributylamine loses C3H6 with transfer of one hydrogen specifically from the γ-position.The experimental data point towards a mechanism which involves the intermediate formation of a distonic diradical ion from an excited α-cleavage ion which the eliminates the neutral alkene.
- Budzikiewicz, Herbert,Bold, Peter
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p. 709 - 712
(2007/10/02)
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- On the Mechanism of Base-Induced Gas-Phase Elimination Reactions of Thioethers
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The base-induced gas-phase elimination reactions of diethyl sulfide have been studied by using the method of Fourier transform ion cyclotron resonance mass spectrometry.Reaction with the nitrogen bases NH2-, EtNH-, and Me2N- preferentially proceeds via an α',β-elimination mechanism, where α' proton abstraction of the sulfide results in the formation of an α-thio carbanion, which undergoes an intramolecular syn elimination to generate the ethyl thiolate anion.Reaction with OH-, MeO-, and F- exclusively proceeds via an E2 mechanism.However, for the reaction with OH-, rapid exchange is observed within the reaction complex between the α-hydrogens of the sulfide and the hydroxide hydrogen prior to E2 elimination.For the E2 eliminations the α- and β-deuterium isotope and leaving group effects have been determined as a function of the base strength.As in our previous study of the mechanism of base-induced gas-phase elimination reactions of ethers, the isotope and leaving group effect data are interpreted in terms of a variable E2 transition-state structure.Combining the results of our previous study with those of the present study has led to the conclusion that the perturbation of the transition-state character effected by changing the leaving group or the base follows the rules established for condensed phase β-elimination reactions.For the nearly thermoneutral elimination reaction of diethyl sulfide induced by F- it is concluded that two reaction mechanisms are operative, both characterized by a bent proton transfer: an E2C mechanism involving a two-side attack of F- on the β-hydrogen and α-carbon of the sulfide, yielding free ethyl thiolate anions and a syn elimination involving a two-side attack of F- on the β-hydrogen and the leaving group, generating HF solvated ethyl thiolate anions.
- Berkel, Werefridus W. van,Koning, Leo J. de,Nibbering, Nico M. M.
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p. 7602 - 7608
(2007/10/02)
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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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p. 5198 - 5204
(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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- 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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p. 119 - 124
(2007/10/02)
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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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p. 127 - 130
(2007/10/02)
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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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p. 5165 - 5169
(2007/10/02)
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