Why are alkane eliminations from ionized alkanes so abundant?

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.     

The isomers of ionized ethane

The previously reported ²A_g, ²A_1g, and ²B_g states of ionized ethane are characterized at several levels of theory. The diborane-like ²A_g state, which gives rise to the observed ESR spectrum, is predicted by SCF and CCD calculations not to exist in a separate minimum from the ²A_1g state formed by ionization of the C(SINGLE BOND)C bond. However, as reported by Lunell and Huang, second-order Moller-Plesset theory places the ²A_g lowest, provided polarization functions are included on carbon. QCISD theory predicts that both A states correspond to potential energy minima, but places the long-bond ²A_1g state lower, at least with moderately large basis sets. F orbitals on carbon stabilize the diborane structure more than the long-bond one. When a potential energy surface is generated for a series of fixed C(SINGLE BOND)C bond lengths by optimizing all variables except for the C(SINGLE BOND)C bond length with MP2 theory and calculating the energy with QCISD(T), the ²A_g state is predicted to be the lowest energy state with the ²A_1g state 1.83 kJ/mol above it. The two A states are predicted to be separated by a barrier 2.79 kJ/mol above the lower state. This barrier is above the zero-point energy in the C(SINGLE BOND)C stretch for the lower state but below the ZPE for this stretch in the upper state, which is therefore predicted not to exist as a stable species. A single quantum of vibrational excitation in the low frequency C(SINGLE BOND)C stretch is predicted to yield an ion with a poorly defined C(SINGLE BOND)C bond length. The highest levels of theory employed give poor agreement with the experimental hyperfine coupling constants. The discrepancy could either be due to neglect of vibrational effects, to poor inherent accuracy of the calculation, as one author has concluded, or to compression of the ion by the matrix as suggested by another. The ²B_g state is found to be higher in energy than the A states at all theoretical levels and is predicted to have a large (160.2–177.4 G) hyperfine coupling from four hydrogens. The transition state for simultaneous exchange of two hydrogen atoms between the carbons by a diborane structure is predicted to lie above the lowest energy fragmentation threshold, in agreement with experiment. © 1996 by John Wiley & Sons, Inc.     

Methylcyclopropane ion formation by elimination of water from n-butanol and 2-methylpropanol ions

C₄H₈⁺˙ ions generated from ionized n-butanol (a) and from ionized 2-methylpropanol (b) were characterized by reaction of the daughter ions with ammonia in a Fourier transform mass spectrometer. Hydrogen transfers occur in a in the ratio five-membered ring:six-membered ring = 1:3.2. However, both hydrogen-transfers are followed by the formation of ionized methylcyclopropane. Ionized methylcyclopropane is also produced on elimination of water from b.     

Depolarisations-Thermostrom und dielektrische Untersuchung von Poly?thylen

Ohne Zusammenfassung     

用Fourier变换质谱研究苯胺衍生物的自身化学电离

报导了3-苯甲酰氧基苯胺和P-溴代苯胺在FT质谱中的自身化学电离, 采用多共振离子消除技术研究离子-分子反应机理. 着重探讨系列离子m/z 200, 290, 380和470的生成机理.     

Nanoreactors for studying single nanoparticle coarsening

The ability to observe intermediate structures as part of coarsening processes that lead to the formation of single nanoparticles (NPs) is important in gaining fundamental insight pertaining to nanostructure growth. Here, we use scanning probe block copolymer lithography (SPBCL) to create "nanoreactors" having attoliter volumes, which confine Au NP nucleation and growth to features having diameters <150 nm on a substrate. With this technique, one can use in situ TEM to directly observe and study NP coarsening and differentiate Ostwald ripening from coalescence processes. Importantly, the number of metal atoms that can engage in coarsening can be controlled with this technique, and TEM "snapshots" of particle growth can be taken. The size of the resulting nanostructures can be controlled in the 2-10 nm regime.     

Methods for the isolation and detection of etheno adducts in nucleotide pools in vivo following exposure to ethyl carbamate

Cellular extraction and high-performance liquid chromatographic methods were developed for the isolation of etheno adducts from nucleotide pools formed in vivo following exposure to the chemical carcinogen ethyl carbamate. These techniques were employed to detect etheno adduct formation using BDF1 mice and rainbow trout (Salmo gairdneri) as test species following inter-peritoneal injection of the chemical. Ethenoadenine was detected in splenocyte nucleotide pools of mice after acute (24 h) exposure and chronic (two weeks) exposure. Several etheno adducts (i.e. ethenoadenine, etheno-AMP, etheno-ADP and etheno-ATP) were also detected in total spleen cell nucleotide pools of trout following acute ethyl carbamate exposure.