Edward C. M. Chen

Extension of Electron Affinities and Ionization Potentials of Aromatic Hydrocarbons

The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔEsol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.The absolute molecular electronegativities for 14 aromatic hydrocarbons are reported. For the hydrocarbons considered, a linear relationship exists between the electron affinity and the energy of the 1La transition. The molecular electronegativity for some eight aromatic hydrocarbons is approximately constant. On this basis, it is possible to predict some ionization potentials which are in excellent agreement with recent experimental values. It does not appear reasonable to relate the intercept resulting from extrapolation of the aforementioned plot to hν=0 to the work function of graphite. Based on the measured electron affinities, it is possible to deduce a relationship with methyl affinities and calculate ΔEsol between neutral hydrocarbons and their mononegative ions. Certain theoretical approaches can be successfully employed to estimate electron affinities of aromatic hydrocarbons.

Analysis of kinetics and concentration dependence of electron-capture detector

Abstract The kinetic model for electron capture has been solved rigorously by numerical integration so that changes in positive ion concentration can be taken into account. In this initial study, rate constants and mode of positive ion removal simulates electron capture using a tritium source in a parallel plate configuration. The numerical analysis can be applied to other geometries and sources of electrons. The results show a different concentration dependence which results from the change in positive ion concentration. The response is a function of the kinetic mechanism.

Importance of kinetics and thermodynamics to the electron-capture detector

Abstract The kinetic model for the electron-capture detector (ECD) has been reviewed. Emphasis was placed on the agreement between thermodynamic and kinetic data obtained from the ECD and independent data obtained by other techniques. During the past five years, sufficient kinetic and thermodynamic data have been obtained to establish clearly the validity of the ECD model for non-dissociative electron capture and for exothermic dissociative electron attachment. In addition, new ECD data have been obtained for a kinetic mechanism of dissociative electron capture, which had been postulated but had not been demonstrated. This mechanism provides values for rate constants which have heretofore never been measured. All of the mechanisms have been illustrated by the use of Morse potential energy curves. A direct correspondence was shown to exist between the gas-phase mechanisms and mechanisms in solution observed with electrochemical techniques. As a result, fundamental kinetic and thermodynamic data can be obtained from the solution data. Correlations have been given which permit the calculation of electron affinities from half-wave reduction potentials.

Use of electron-capture data at short pulse intervals for study of electron-capture mechanisms

Abstract Electron-capture data obtained at short pulse intervals (50–100 μsec) cn be used to determine electron-capture mechanisms. The response function has been derived and it differs considerably from that at steady-state conditions. The electron-capture doefficient can be defined from the response function and it is directly proportional to the pulse frequency. The derived response function is in agreement with the experimental concentration, temperature, and pulse interval dependence.The response is independent of pulse width providing short pulse widths (0.5–1.0 μsec) are used at intervals larger than 50 μsec. The electron-capture coefficient at short pulse intervals is lower than at long pulse intervals where steady-state conditions prevail. However, this is partially compensated by a greater standing current at short pulse intervals.

Non-radioactive electron-capture detector for gas chromatography

Abstract This paper is an update on the progress made towards developing a practical electron-capture detector which does not require a radioactive source, can be operated in the pulsed mode, and can be used at temperatures up to 800 K. Data obtained using a prototype detector have been reported previously. In this paper results obtained with a “large-volume” but leak-free detector at elevated temperatures are reported, and preliminary data with a practical “small-volume” detector will be discussed. The temperature dependence suggests that the electron-capture mechanisms that have been demonstrated previously are also operative in the non-radioactive detector. The discharge has been examined spectroscopically. Finally, we present characteristic data for the detector under high-purity conditions. These data are similar to those obtained with a radioactive detector and serve as a “base case” for future development.

Comment on: ‘‘Effect of temperature on nondissociative electron attachment of perfluorobenzene’’

The rapid decrease in the phenomenological rate of electron attachment to perfluorobenzene with increasing temperature, reported by Spyrou and Christophorou [J. Chem. Phys. 82, 1048 (1985)] can be attributed to electron detachment rather than a change in the rate constant for electron attachment. Using previously published data for the temperature dependence of the electron capture detector response [N. Herdandez‐Gil, W. E. Wentworth, T. Limero, and E. C. M. Chen, J. Chromatogr. 312, 31 (1984)], the electron affinity of C6F6 has been calculated to be 0.86±0.03 eV. On the basis of this value the rate constant for electron detachment will become significant at high temperatures in an electron swarm.

HUND'S STRONG FIELD STATES OF SUPEROXIDE AND NO(-)

Hunds state conservation rule predicts (1 × 6) [N (4S) + O(-)(2P)] plus 9 × 9 [(3P) N(-) + O(3P)] = 87 spin states for NO(-). The experimental Ea(NO), 0.92(2)-0.16(2) eV are assigned to the (3 + 27) bonding states with anion bond orders, 0.80-1.15. The Ea(NO) 0.026(5)-0.14(2) eV are assigned to seven of the 27 nonbonding states with anion bond orders about one. The negative Ea(NO) for the 20 other nonbonding and 30 antibonding states are estimated. Ionic Morse potentials are calculated for 87 predicted states for NO(-) and the 54 bonding and antibonding states of superoxide.

Gas phase anion reactions at atmospheric pressure : Atmospheric pressure ionization mass spectrometric studies related to the electron-capture detector

Abstract Negative ion atmospheric pressure ionization mass spectrometry has been used to investigate the gas phase atmospheric pressure anion chemistry of N 2 O 2 H − and . N 2 O 2 H − has been shown to be a stronger base than . Specific types of reaction ( e.g. proton abstraction, and dehydration) have been identified for each of these anions. Although the analytical significance of these reactions has not yet been demonstrated, certain compounds such as alcohols which do not readily attach electrons directly can easily be detected by observing a specific anion reaction product. The technique appears to provide an additional dimension to established gas chromatographic—mass spectrometric analyses.

Non-radioactive electron-capture detector operating in the pulsed mode

Abstract This paper describes an electron-capture detector (ECD) which does not require a radioactive source for the generation of electrons and which can be operated in the pulsed mode. Other researchers have developed non-radioactive versions of the ECD; however, all of these have required the application of a constant potential for electron collection. The pulsed mode allows electron attachment to occur under field-free conditions, and is the more commonly used mode of ECD operation. In our detector, electrons are produced by energetic species derived from a microwave-induced discharge in helium. The identity of these energy carriers has not been established definitively, but various possibilities are discussed. The remainder of the apparatus is identical to that used with a normal pulsed-mode ECD with the exception that our detector operates at a pressure of 10-20 Torr. In this paper we report characteristic data for the significant operational parameters (pulse period and pulse width) and measured quantities (rate constants for loss of electrons and for production of electrons) associated with our detector. These are compared to corresponding data typical of a conventional pulsed-mode, radioactive-foil ECD. The detection limit (signal-to-noise ratio  2) of our detector for carbon tetrachloride. which captures electrons dissociatively with an extremely high capture coefficient, is on the order of 70–80 fg. For methylene chloride, which also captures dissociatively but with a much lower capture coefficient, the detection limit is 8 ng. For hexafluorobenzene, which captures non-dissociatively, the detection limit is 530 fg. These limits were obtained using a prototype detector. We anticipate that improvements in design will result in even better performance.

Comment on “Ab initio molecular dynamics calculation of ion hydration free energies” [J. Chem. Phys. 130, 204507 (2009)]

We suggest that the authors compare their theoretical Gibbs free energies −ΔGhyd (kcal/mol) Li+, 128(1), 135; Cl−, 78(1), 70(2); and Ag+, 120(1) to recent absolute experimental values Li+, 128; Cl−, 74; and Ag+, 119 kcal/mol referenced to that for H+, 266(2) kcal/mol. We present bulk Gibbs hydration free energies and ionic radii for other ions from aqueous electron affinities, monohydration free energies, and diatomic halogen anion potential energy curves consistent with the Born dielectric constant, 3.4, for electrons and protons.