Diatomic Dications Research Articles

Energy Trapping in Dications

AbstractThe energy interaction curves of a number of diatomic and polyatomic dication systems were calculated in order to study their energy‐trapping properties. Generally, the ab initio complete active space multiconfiguration self‐consistent field method was used in an extended valence + polarization basis set, with compact effective potentials replacing the core electrons. The diatomic dications include all ten possible binary combinations of oxygen, sulphur, selenium, and tellurium. O22+ shows the largest exothermicity, measured from equilibrium to the monocation combination asymptote, and highest barrier to dissociation. The calculated equilibrium bond length and harmonic vibrational frequency agree very well with experiment. The O22+, SO2+, SeO2+, and TeO2+ series show progressively decreasing exothermicities but similar barrier heights. The non‐oxides, in contrast, show similar exothermicities but decreasing barriers with increasing size of the atom constituents. These trends are interpreted in terms of both valence bond curve‐crossing and molecular orbital bonding models.The ozone dication, O32+, is found to have a number of low‐lying singlet and triplet stationary state structures spanning near‐linear to D3h2+ symmetries. Although the calculated exothermicity is even larger than for O22+, the barrier to O2+ + O+ dissociation is predicted to be low in each case.O22+ surrounded by six argon atoms to model an isolating environment shows increased equilibrium O–O bond length, decreased exothermicity, and increased barrier to dissociation, relative to the bare dication. O22+ flanked at each end by a perpendicularly oriented H2 molecule in a staggered conformation is obstructed from direct conversion to the water dimer dication by a high barrier. However, [(H2O)2]2+ dissociates smoothly from equilibrium to two water monocations with a large exothermicity but a small barrier.

Diatomic monocations and dications containing germanium and a rare gas atom.

Diatomic dications and monocations GeY2+ and GeY+, where Y = Ne, Ar, Kr or Xe, have been detected on the microsecond time scale. They are characterized via appearance energies, charge stripping of GeY+ ions, and mass-analysed ion kinetic energy spectroscopy. The formation of GeAr+ is found to involve an excited state of Ar in a Hornbeck-Molnar process.

Dissociative electron capture collisions of the diatomic dications CO 2+ and NO 2+ with inert gas targets

Dissociative electron capture (DEC) collisions of the diatomic dications CO 2+ and NO 2+ with inert gas targets have been studied by mass-analysed ion kinetic energy spectroscopy (MIKES). The cross-sections were found to be about an order of magnitude smaller than those for the corresponding non-dissociative electron capture processes. The qualitative form of the ion kinetic energy spectra for O + from CO 2+ showed a dramatic variation with the nature of the inert gas target A; much less marked variations in these spectra were observed in the C + channel, and in all the DEC spectra for NO 2+. These spectra for O + from CO 2+ were largely interpretable in terms of a simple energy balance between reactants and products, implying that mechanistic routes exist to energetically accessible levels of the (O + + C) dissociation limit. The kinetic energy spectra for C + from CO 2+ showed evidence of a vibrational progression, for A = Xe, Kr and Ar, which was consistent with previous observations (Curtis and Boyd, J. Chem. Phys., 80 (1984) 1150) and interpretation (Honjou and Yarkony, J. Phys. Chem., 89 (1985) 44). The ion kinetic energy spectra, for each of N + and O + formed in DEC collisions of NO 2+, were very similar to comparable spectra obtained for collision-induced dissociations of the monocation NO + (Dông and Bizot, Int. J. Mass Spectrom. Ion Phys., 10 (1972) 227; Comtet and Fournier, Chem. Phys., 81 (1983) 221). While similar interpretations undoubtedly apply, the case of the DEC processes poses an additional mechanistic problem concerning disposal of the excess recombination energy in the initial electron transfer step.

The ACDCP model for estimating the kinetic energy release and transition structure bond length in the fragmentation of a diatomic dication

A semi-quantitative method is presented for predicting the transition structure bond length and the kinetic energy released as a diatomic dication dissociates into monocation fragments. Our approach, which we term the ACDCP model, involves the introduction of diabatic coupling and polarization effects to an avoided-crossing model previously described. Good agreement is found between the predictions of the new ACDCP model, the results of accurate ab initio calculations and experimental results.

Simple models for describing the fragmentation behavior of multiply charged cations

The potential energy curve describing the fragmentation of a diatomic dication AB2+ is considered as arising from an avoided crossing between an attractive diabatic curve (correlating with A2+ + B) and a repulsive diabatic curve (correlating with A+ + B+). The simplest avoided-crossing (AC) model neglects diabatic coupling and polarization and leads to useful predictions of the transition structure bond length (rTS) and the kinetic energy released (T) in fragmentations of dicationic systems in which the difference (Δ1) between the ionization energies of A+ and B is small. When Δ1 is not small, it is necessary to include diabatic coupling and polarization in the treatment. The resultant ACDCP (avoided crossing with diabatic coupling and polarization) model provides very satisfactory estimates of rTS and T for both small and large Δ1. Its implementation requires only atomic ionization energy and polarizability data and comes at virtually no computational cost. Both the AC and ACDCP models are readily generalized to fragmentations of more highly charged cations.

Charge separation reactions of the CF 2+ and CCl 2+ dications

Experimental values of kinetic energy release ( T) for predissociation processes of the diatomic dications CF 2+ and CCl 2+ have been obtained using mass-analysed ion kinetic energy spectrometry (MIKES) on a double-focusing mass spectrometer. Both unimolecular and collision-induced processes were studied. Potential energy curves for CF 2+ and CCl 2+ were calculated using the semi-empirical procedure due to Hurley, and applied to the interpretation of the observed T values.

Diatomic dications of noble gas chlorides

The existence of dications of noble gas chlorides ACl 2+, which are stable on at least the μs timescale, has been demonstrated for A = Ne, Kr, and Xe via observations of their charge-separation dissociation reactions and also via charge-stripping of the corresponding monocations ACl +. Potential curves for the X 2Π states could not be calculated by the empirical procedure of Hurley, due to lack of subsidiary data, but curves were calculated for the corresponding fluorides, AF 2+.

Diatomic dications containing one inert gas atom

Diatomic dications AY 2+, where A = Ne, Ar, Kr or Xe and Y = C, N or O, have been detected and characterised via appearance energies, ionization energies of the corresponding monocations determined by charge-stripping, and values of translational energy release accompanying unimolecular dissociation of AY 2+ on the microsecond timescale. The mechanism of formation of AY 2+ is unclear, but possible involves three-body association of A 2+ and Y. Approximate potential curves for some of these dications have been generated using the semi-empirical procedure due to Hurley, and applied in a general interpretation of some of the trends observed.

Predissociation processes in N2+2, O2+2, and NO2+ studied by ion kinetic energy spectroscopy

Experimental values of kinetic energy release T, for predissociation processes of diatomic di-cations N2+2, O2+2, and NO2+ have been obtained using techniques of ion-kinetic-energy spectroscopy on a double-focusing mass spectrometer. Both unimolecular and collision-induced processes were studied. As a result, the present data confirm earlier work but include additional transitions not reported previously. Values of T obtained by entirely different technqiues are shown to be consistent with the present work when differences in energy resolution and experimental time window are considered. Attempts to correlate all available information on these diatomic di-cations (Auger, double-charge-transfer and kinetic-energy loss spectra, as well as appearance energy measurements) are hampered by lack of high-level a priori calculations of the potential curves.