摘要:

An isolated body is characterized by conserved properties such as its energy and angular momentum. The rates of processes occurring in that body can thus be investigated as a function of these properties. On the other hand the rates may be better known, both conceptually and experimentally, as a function of temperature. This paper surveys relations between rate descriptions in the two domains, as deduced using steepest descent techniques. The rudiments of these techniques are given in an appendix.

ISSN:0144-235X    

DOI:10.1080/01442359609353181

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Rate constants for gas-phase proton transfer to Mn(CO)5−, Re(CO)6−and Cl−from reference acids and metal hydrides are determined. Proton-transfer efficiencies to Mn(CO)5−are small, and less than the efficiencies for proton transfer to Cl−; proton-transfer efficiencies to Re(CO)6−are larger than those for Mn(CO)5−, but still generally less than for transfer to Cl−. Proton-transfer efficiencies to Mn(CO)5−and Re(CO)5−are substantially more dependent on the nature of the proton donor (e.g. metal hydride versus oxy-acid). The thermoneutral proton transfer between Mn(CO)5−and13CO-labelled HMn(CO)6is particularly inefficient, occurring only in about 1 in 100 collisions. The results are discussed in view of evidence for a short lifetime of the collision complex, and the effects of electronic and structural reorganization of Mn(CO)5−and Re(CO)5−, including loss of fluxionality of the metal species, which occur on protonation.

ISSN:0144-235X    

DOI:10.1080/01442359609353182

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Our major objective in this paper is to describe a new method we have developed for the analysis of trace gases at partial pressures down to the ppb level in atmospheric air, with special emphasis on the detection and quantification of trace gases on human breath. It involves the use of our selected ion flow tube (Sift) technique which we previously developed and used extensively for the study of gas phase ionic reactions occurring in ionized media such as the terrestrial atmosphere and interstellar gas clouds. Before discussing this analytical technique we describe the results of our very recent Sift and flowing afterglow (FA) studies of the reactions of the H3O+and OH−ions, of their hydrates H3O+(H2O)1,2,3and OH−(H2O)1,2, and of NO+and O2+, with several hydrocarbons and oxygen-bearing organic molecules, studies that are very relevant to our trace gas analytical studies. Then follows a detailed discussion of the application of our Sift technique to trace gas analysis, after which we present some results obtained for the analyses of laboratory air, the breath of a healthy non-smoking person, the breath of a person who regularly smokes cigarettes, the complex vapours emitted by banana and onion, and the molecules present in a butane/air flame. We show how the quantitative analysis of breath can be achieved from only a single exhalation and in real time (the time response of the instrument is only about 20 ms). We also show how the time variation of breath gases over long time periods can be followed, using the decay of ethanol on the breath after the ingestion of distilled liquor as an example, yet simultaneously following several other trace gases including acetone and isoprene which are very easily detected on the breath of all individuals because of their relatively high partial pressures (typically 100 to 1000 ppb). The breath of a smoker is richer in complex molecules, some nitrogen containing organics apparently being very evident at the 5 to 50 ppb level. These results and those for banana and onion vapours and butane/air flame forcibly demonstrate the value and the scope of our Sift ion chemistry approach to the analysis of very complex gas mixtures, and that this method is accurately quantitative if the appropriate ion chemistry is properly understood.

ISSN:0144-235X    

DOI:10.1080/01442359609353183

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Letndenote the number of water molecules in the nearest-neighbour shell (NS) of an ion J±in liquid water, and denote J±·nH2O in the gas phase by J±· NSG (g). The standard free energy of hydration ΔG°hyd(J±·NSG) can then be deduced by a thermodynamic cycle involving ΔG°nfor the formation of J±· NSG (g) and ΔG°hydfor the transfer of J±(g) to water. Values of ΔG°hyd(J±·NSG) for alkali and halide ions are substantial, ranging from 48 % to 86 % of ΔG0hyd. The values of ΔG0hyd(J±·NSG) can be accounted for largely by the calculated work—electrostatic (ΔWelec) and surface (ΔWsurt)—in the process J±·NSG (g) → J±(aq). ΔWelecis the major contributor. ΔWsurfdepends on whether (i) J±·NSG (g) can be represented by a cluster consisting of the ion andnseparate water molecules, or (ii) there is some molecular complex formation within that cluster. In fact, ΔWsurt< 20 kJ mol−1for the alkali ions and of the order of 100 kJ mol−1for the halide ions. A reasonable case can be built that the alkali ions and some of thenwater molecules form molecular complexes while the anions are better represented by case (i).

ISSN:0144-235X    

DOI:10.1080/01442359609353184

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Using resonance enhanced multiphoton ionization (REMPI), ions are created in a small volume centred on the axis of an rf-quadrupole. The pulsed beam of neutral molecular precursors is injected coaxially. This combination leads to high collection efficiency without the need of strong electrostatic extraction fields. The quadrupole is operated in the adiabatic limit. Therefore, the influence of the oscillatory rf-field on the ion trajectories can be described by an effective harmonic potential, which has focusing properties. Using these energy and mass dependent focusing properties, a primary H2+beam of well-defined kinetic energy (< 5 meV (fwhm) at a mean of 50 meV) is prepared. In a typical case of multiphoton ionization of polyatomic molecules using C2H2, suppression of fragment ions by more than a factor of 10 is achieved, without losing the narrow kinetic energy spread of the parent ion beam.

ISSN:0144-235X    

DOI:10.1080/01442359609353185

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Results of beam scattering studies on the dynamics of single charge transfer and chemical reactions of doubly-charged ions carried out in Prague are reviewed. Investigation of several atomic ion-atom charge transfer processes at collision energies 0·1-10 eV provides data on differential and relative total cross-sections of state-to-state processes. Populations of electronic and vibrational states of molecular product ions are obtained from experiments on molecular dication-atom and atomic ion (He2+)-molecule systems. Studies of chemical reactions of the dication CF2+2with D2show that Coulomb repulsion between the products (CF2D++ D+) governs the dynamics and energy partitioning.

ISSN:0144-235X    

DOI:10.1080/01442359609353186

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

When molecules are subjected to an external electric field, static or radiative, the interaction with either permanent or induced dipole moments creates pendular eigenstates, directional superpositions or hybrids of the field-free rotational states. These hybrids reflect the anisotropy of the interaction; for a linear molecule this is proportional to cos θ for a permanent dipole and to cos2θ for an induced dipole, with θ the angle between the molecular axis and the field direction. In the weak-field regime, the molecular axis tumbles through 360°, but in the strong-field limit it is confined to harmonic librations about the field direction. Here we treat the statistical mechanics of pendular states of linear molecules, either polar (with a permanent dipole moment μ) or nonpolar but polarizable (with a polarizability anisotropy Δα contributing to an induced dipole moment). The partition function and the thermodynamic properties and other ensemble averages can be specified by two reduced variables involving μ or Δα, the field strength, rotational constant, and temperature. A simple approximation due to Pitzer enables the partition function to be cast in terms of the classical result with quantum corrections derived from the harmonic librator limit. This provides explicit analytic formulas which permit thermodynamic properties to be evaluated to good accuracy without computing energy levels and state sums. We also evaluate the average orientation or alignment of the molecular dipoles and examine field-induced shifts of chemical equilibria.

ISSN:0144-235X    

DOI:10.1080/01442359609353187

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor