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.     

CH3CH+* formation from some C3H6O+* isomers according to theory

Dissociations to alkane ions in gas phase ion chemistry are rare and poorly characterized. Therefore, the pathways to CH3CH3+* + CO from *CH2CH2O+=CH2 and some of its isomers are investigated by theory. The pathway found for this reaction is *CH2CH2O+=CH2 --> CH3CH2O+=CH* --> [CH3CH2- -H- -CO]+* --> CH3CH+* + CO. The crucial intermediate in this pathway is the stable hydrogen-bonded ion-neutral complex [CH3CH2- -H- -CO]+*, a species held together by a strong hydrogen bond. CH3CH3+* + CO rather than CH3CH3 + CO+* is formed from *CH2CH2O+=CH2 and other C3H6O+* ions because the former pair is much more stable than the latter. The photoionization appearance energies of CH3CH3+* from CH3CH2CHO+* and from CH3CH2CO2H+* demonstrate that the onsets of these reactions are at to just above their thermochemical thresholds, consistent with the intermediacy of ion-neutral complexes. We also found transition states for interconversion of CH3CH2CHO+* and CH3CH2O+=CH* and transformation of CH3CH2C:=OH+* to CH3CH2CHO+*; the latter reaction occurs by a 1,2-H-shift from O to C.     

An alternating direction scheme on a nonuniform mesh for reaction-diffusion parabolic problems

In this paper we develop a numerical method for two-dimensionaltime-dependent reaction-diffusion problems. This method, whichcan immediately be generalized to higher dimensions, is shownto be uniformly convergent with respect to the diffusion problems.This method, which can immediately be generalized to higherdimensions, is shown to be uniformly convergent with respectto the diffusion parameter.     

Water adsorption on the hydroxylated H-(1x1) O-ZnO(0001) surface

The adsorption of water multilayers on a well defined single crystal, hydroxyl-terminated ZnO-surface, H(1x1)-O-ZnO(0001) surface has been investigated using infrared (IR) spectroscopy, helium atom scattering (HAS) and X-ray photoelectron spectroscopy (XPS). The results reveal the formation of well ordered mono-, bi- and multilayers of D2O and H2O on this substrate. On the bare hydroxyl-covered H(1x1) surface the OH-stretch vibration could be clearly identified in the IR-spectra. The water adsorption and desorption kinetics on this hydroxylated surface were studied by monitoring the reflectivity of the surface for helium atoms. The analysis of the data yielded activation energies for desorption of H2O from the H(1x1) O-ZnO surface of 55.2 kJ mol-1. The results reveal the formation of ordered mono- and bilayers. Further exposure to water at 113 K results in the formation of amorphous 3-D islands.