Energetics of the loss of H2 from [C3H7]+

The internal energies of [C₃H₇]⁺ ions contributing to the metastable peak [C₃H₇]⁺ → [C₃H₅]⁺ + H₂ are higher (by perhaps > 100 kJ mol^?1) than those of the ion contributing to the threshold current in appearance energy measurements on [C₃H₅]⁺. The measured appearance energy may lead to an underestimation of the activation energy, i.e. negative ‘kinetic shift’, due to quantum, mechanical tunnelling. The distribution of energy released in the decomposition can be explained on the basis that much of the reverse activation energy and a statistical proportion of the excess energy is released as translation.     

Heat of formation for the SH radical by photoionization mass spectrometry

Photoionization mass spectrometry has been used to measure the appearance energies for [C₂H₅]⁺ from ethanethiol, [C₃H₇]⁺ from 2-propanethiol and [C₃H₅]⁺ from 2-methylthiirane. From the known thermochemistry of these cations and their precursor molecules, a 298 K heat of formation of 138.6±0.4 kJ mol^?1 for the SH radical has been derived.     

Self-injection length in La0.7Ca0.3MnO3-YBa2Cu3O7-δ ferromagnet-superconductor multilayer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La_0.7Ca_0.3MnO₃ (LCMO) and YBa₂Cu₃O_7-δ (YBCO) thin films. The heterojunctions were formed in situ by sequentially growing LCMO and YBCO films on 〈100〉 LaAlO₃ (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 μm width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, I _c, was measured at 77 K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a J _c of ∼ 2 × 10⁶ A/cm², the LCMO side showed about half the value for the same thickness (1800 ?). The difference in J _c indicates that a certain thickness of YBCO has become ‘effectively’ normal due to self-injection. From the measurement of J _c at two different thicknesses (1800 ? and 1500 ?) of YBCO films both on the LAO as well as the LCMO side, the value of self-injection length (at 77 K) was estimated to be ∼ 900 ?. To the authors’ best knowledge, this is the first time that self-injection length has been quantified. A control experiment carried out with LaNiO₃ deposited by PLD on YBCO did not show any evidence of self-injection.     

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.     

Structure of CO2 adsorbed on the KCl(100) surface

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.     

Determination of the proton affinity of carbon dioxide by photoionization mass spectrometry

The adiabatic ionization energies and COOH⁺ appearance energies for a series of carboxylic acids have been measured by dissociative photoionization mass Spectrometry. Using the stationary electron convention, a heat of formation of 143.5 ± 0.7 kcal mol^?1 (1 kcal = 4.184 kJ) at 298 K is derived for the carboxyl cation in the gas phase, which corresponds to an absolute proton affinity for carbon dioxide of 128.1 ± 0.7 kcal mol^?1.     

The propionyl cation heat of formation revisited

The 298 K heat of formation for the propionyl cation (C₂H₅CO⁺) has been measured previously by dissociative photoionization mass spectrometry. However, recent theoretical and experimental studies involving methylketene suggest that this may be significantly underestimated, resulting in a methylketene proton affinity that is too high by ∼30 kJ mol⁻¹. In this study, the previous m/z 57 appearance energies were carefully re-evaluated, with various possible sources of error being investigated. These include factors such as sample purity, carbon-13 contamination from lower energy m/z 56 processes, kinetic and/or competitive shifts, reverse activation energies, ionizing energy calibration errors and the availability of accurate supplementary thermochemical data. In addition, high-level ab initio calculations are used to model the relevant unimolecular fragmentation processes for each of the ionized precursor molecules. As a result, it is found that only the 2-butanone appearance energy can be used to provide a reliable value for the propionyl cation heat of formation. From the 298 K m/z 57 appearance energy of 10.199 ± 0.003 eV for 2-butanone measured here, a value of 617.8 ± 0.9 kJ mol⁻¹ is derived for , which corresponds to 845.4 ± 4.8 kJ mol⁻¹ for the proton affinity of methylketene. This is in good agreement with previous theoretical calculations and thermokinetic proton affinity measurements, indicating that a significant upward revision to the propionyl cation heat of formation is warranted. Copyright © 2004 John Wiley & Sons, Ltd.     

Why CH3CH3+* formation competes with H* loss from CCCO C3H6O+* isomers

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.