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1.
2.
A Mukherjee M Dasgupta DJ Hinde CR Morton AC Berriman RD Butt JO Newton H Timmers 《Pramana》2001,57(1):195-198
Fusion cross-sections for the 7Li + 12C reaction have been measured at energies above the Coulomb barrier by the direct detection of evaporation residues. The heavy
evaporation residues with energies below 3 MeV could not be separated out from the α-particles in the spectrum and hence their
contribution was estimated using statistical model calculations. The present work indicates that suppression of fusion cross-sections
due to the breakup of 7Li may not be significant for 7Li + 12C reaction at energies around the barrier. 相似文献
3.
The motion of a holonomic scleronomic non-conservative mechanicalsystem with minimal dissipation is considered. As applicationsof the theory several problems are studied in detail. 相似文献
4.
John C. Traeger David J. McAdoo Charles E. Hudson C. S. Giam 《Journal of the American Society for Mass Spectrometry》1998,9(1):21-28
Eliminations of alkanes consisting of the side chain plus a hydrogen from ionized alkylcycloalkanes are unusually abundant among such processes. For example, ethane is eliminated from ionized ethylcyclopentane more than 10 times more often than it is from its acyclic isomers. To explore why, we characterized the eliminations of ethane from ionized ethylcyclopentane and of butane, 2-methylpropane, and cyclohexane from isomeric butylcyclohexane ions. We hypothesized that one reason these alkane eliminations are particularly favored is that the partners in the complex do not readily escape from reactive configurations. Supporting this, hydrogens are transferred to butyl partners from around cyclohexyl rings, demonstrating that the partners in cycloalkyl-containing complexes do react with each other through several configurations. A very prominent cyclohexane elimination from ionized tert-butylcyclohexane demonstrates that alkane elimination is abundant no matter which partner in the intermediate ion-neutral complex bears the charge. C4H8 + is the dominant dissociation product of ionized tert-butylcyclohexane, even though the formation of the cyclohexene ion plus 2-methylpro-pane is thermochemically favored, a highly unusual ordering among mass spectral fragmentations. This is attributed to H-atom transfer from a tret-butyl ion to a cyclohexyl radical being preferred over transfer of hydride in the opposite direction. The effect of energy on the magnitude of alkane eliminations and the associated simple dissociations was elucidated utilizing photoionization mass spectrometry. Appearance energies show that forces of attraction between the partners are less than 30 kJ mol?1, no stronger than when both partners are acyclic. However, the shapes of photoionization efficiency curves demonstrate that these alkane eliminations are significant over a wide energy range, in contrast to most other alkane eliminations. Thus, ionized cycloalkanes generate unusually stable ion-neutral complexes; this is probably the reason alkane eliminations through them are so abundant. Alkane eliminations from acyclic alkane ions are also very abundant, suggesting that ion-neutral complexes formed from alkylcycloalkane and alkane ions have a common feature which makes energy relatively ineffective in driving the partners apart. 相似文献
5.
Charles E. Hudson Liberty DeLeon Don Van Alstyne David J. McAdoo 《Journal of the American Society for Mass Spectrometry》1994,5(12):1102-1106
The transition state (TS) for loss of CH4 from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d,p) level of theory. The activation energy is predicted to be 263.9 kJ/mol starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) that is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Restricted Hartree-Fock (RHF) calculations give much more accurate relative energies. The MP2 TS leads directly to fragmentation and is described as a protonation of the methyl group by the acidic proton on oxygen. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C---H bonds protonated. This mechanism (protonation of an alkyl group) appears to be a general one for high energy 1,2 eliminations from organic cations. (J Am Soc Mass Spectrom 1994, 5, 1102-1106) 相似文献
6.
Hudson CE McAdoo DJ Griffin LL Traeger JC 《Journal of the American Society for Mass Spectrometry》2003,14(2):136-142
How formation of CH3CH3+* competes with H* loss from C3H6O+* isomers with the CCCO framework has been a puzzle of gas phase ion chemistry because the first reaction has a substantially higher threshold and a supposedly tighter transition state. These together should make CH3CH3+* formation much the slower of the two reactions at all internal energies. However, the rates of the two reactions become comparable at about 20 kJ x mol(-1) above the threshold for CH3CH3+* formation. It was recently shown that losses of atomic fragments increase in rate much more slowly with increasing internal energy than do the rates of competing dissociations to two polyatomic fragments. This occurs because fewer frequencies are substantially lowered in transition states for the former type of reaction than for the latter. The resulting lower transition state sums of states cause the rates of dissociations producing atoms as fragments to increase much more slowly than competing processes with increasing energy. Here we show that this is why CH3CH3+* formation competes with H* loss from CH3CH2CHO+*. These results further establish that the dependence on energy of the rate of a simple unimolecular dissociation is usually directly related to the number of rotational degrees of freedom in the products, a newly recognized factor in determining the dependence of unimolecular reaction rates on internal energy. 相似文献
7.
Lawrence L. Griffin Katherine Holden Charles E. Hudson David J. McAdoo 《Journal of mass spectrometry : JMS》1986,21(4):175-182
The isomerizations preceding the metastable decompositions in the mass spectrometer of a number of [C6H12O]+˙ ions with the oxygen on the third carbon are characterized utilizing deuterium labeling. Hydrogens are transferred in these ions by three-, five- and six-membered ring rearrangements, with propensities determined by features of the individual reactions. Three-membered ring hydrogen transfers between α and β-carbons are preferred to all five-membered ring hydrogen transfers. However, six-membered ring hydrogen transfers take place to the apparent exclusion of three-membered ring hydrogen transfers to enol carbons when the products are of comparable stability. The low-energy [C6H12O]+˙ isomerizations characterized are predictable from the behavior of their lower homologs. It is concluded that the determinants of these reactions are the same as those of other highly reactive organic intermediates. 相似文献
8.
9.
John C. Traeger Charles E. Hudson David J. McAdoo 《Journal of the American Society for Mass Spectrometry》1992,3(3):183-187
Formation of the 3-pentanone ion (3) from ionized propyl propanoate through the complex [C2H5CO+ ?OC3H7] is proposed based on data obtained by photoionization. The threshold and energy dependence for forming 3 relative to those for related processes support this proposal. The threshold for forming 3 coincides with that predicted for forming [CH3CH2CO+ ?CH2CH3], suggesting that that complex is also an intermediate in the pathway to 3. 3-Pentanone ion formation is important much further above threshold than is alkane elimination through [RCO+ ?R] complexes. This adds to evidence that reactions between the partners in ion-dipole complexes take place over a wider energy range than do such reactions in complexes containing nonpolar neutral partners. 相似文献
10.
A new, vectorial approach to fast correlation attacks on binary memoryless combiners is proposed. Instead of individual input sequences or their linear combinations, the new attack is targeting subsets of input sequences as a whole thus exploiting the full correlation between the chosen subset and the output sequence. In particular, the set of all the input sequences can be chosen as the target. The attack is based on a novel iterative probabilistic algorithm which is also applicable to general memoryless combiners over finite fields or finite rings. To illustrate the effectiveness of the introduced approach, experimental results obtained for random balanced combining functions are presentedMost of this work was done while he was with Rome CryptoDesign Center, Gemplus, Italy 相似文献