Vibrationally excited ions

A careful repeat of the flowing-afterglow measurement of Adams et al. 18 was subsequently carried out by Smith and Spanel.29 The results confirmed the Adams et al. observation that the recombination coefficient appears to fall off to a small value in the late afterglow. The authors concluded that the small recombination coefficient observed in the late afterglow is the proper value for v = 0 ions and that the initial rapid plasma decay in the early afterglow should be ascribed to vibrationally excited ions. 

This leaves only ions in the vibrational ground state, in the v2 = 1 bending-mode vibration at 0.3126 eV, and in the v, = 1 breathing mode vibration at 0.394 eV (see Lie and Frye 47 or Oka and Jagod6). Since the argon density in these experiments is quite high ( 5 x 1015cm-3), v > 1 ions would be destroyed in less than 1 (is. This is an important point, since Smith and Spanel s proposed reconciliation of theory and experiment rests on the assumption that vibrationally excited ions dominate the plasma. 

A somewhat more systematic study has been made of collisional deactivation of vibrationally excited ions. Some diatomic and triatomic systems that have been investigated are included in Table II. Vibrational-to-rotational transfer has been demonstrated95 for vibrationally excited H2+ colliding with helium ... 

Up to this point we have discussed collisional deactivation of vibration-ally excited ions formed by ionization or as products of exoergic particle-transfer ion-molecule reactions. A somewhat different situation prevails with larger vibrationally excited ions, such as those formed as intermediates in ion-molecule association reactions. Reactions in which such excited intermediates are formed generally demonstrate a third-order dependence of the rate on the concentrations of the reactants at relatively low pressures. The general reaction mechanism may be represented as... 

A desirable, but difficult extension of dissociative recombination studies would be the determination of are for excited ions such as vibrationally excited ions in their ground electronic states and the electronically excited 02 (4I1U) and NO+ (3S) ions all of which undoubtedly exist in the ionosphere. As well as being of ionospheric significance, such data would also be of fundamental interest in that it would assist in clarifying theoretical models of the dissociative recombination process. 

The formation of a CIP or an exciplex may begin with a collision between the donor and acceptor resulting in a vibrationally excited ion-pair intermediate. Eventually, vibrational relaxation and reorientation of surrounding solvent molecules produce a thermalized intermediate. The free-energy change for exciplex formation is given by Eq. (15) (Fig. 7) ... 

Equation 16 illustrates a celebrated example where ring closure competes with vicinal hydride shift (a common form of atom transfer in cations, which does not take place in free radicals or anions). The gas phase reaction was explored by preparing the dimethylfluoronium ion, (CH3)2F" , by y-radiolysis of fluoromethane. Exothermic methylation of a sample of C-yd-phenethyl chloride (where the asterisk in equation 16 symbolizes the labeled position) in the gas mixture gives a vibrationally excited ion that loses chloromethane to form two isomeric ions, a-phenethyl cation and spirooctadienyl cation (sometimes called ethylenebenzenium). Nucleophilic attack by methanol in the reaction mixture yields PhCH(CH3)OCH3, whose isotopic label remains almost entirely at the methyl group. The recovered PhCH2CH20CH3 contains equally distributed between the two methylene positions. The spirooctadienyl ion does not isomerize to a-phenethyl cation, even though DFT calculations predict the latter to be 55 kJ/mol more stable. 

Evidence for Excited tert-C4H9 Vibrationally excited ions exhibit less selectivity in their reactions than thermal ions. Ions produced in the liquid phase are expected to be thermalized rapidly at the time of formation. However, vibrationally excited ions are commonly produced in the gas phase both by electron or photon impact and by ion-molecule reactions. In the ion injection method ions are produced in the gas phase and injected into the liquid. Thus, the possibility arises for injecting excited ions into the liquid and for examining whether excited ions will show reactive selectivity in the liquid phase. The formation and reaction of tert-C4H9+ in isobutylene by the ion injection method illustrate these possibilities. 

In the two-color ionization employed here, various ionization pathways can produce highly vibrationally excited ions on account of involvement of Rydberg states. It is particularly difficult to discern between the competing 3-photon ionization pathways involving the combination of photons fid + hco + Aa>2, and Aa>i + + h(02. In our work we were able to unambiguously identify the... 

White et al. (1992b) rationalized their and preceding results by a scheme that we reproduce here in a slightly modified version (Scheme 7-17). It is characterized, first of all, by the concept of inert molecule-separated ion pairs, used earlier by White in a slightly different form ( vibrationally excited ion pair ) (see, e.g.. White et al., 1967). It was extensively used by Moss and Landon (1970), by Moss (1974), and by Whiting and coworkers (Maskill and Whiting, 1976 Whiting, 1982 see also... 

Our interpretation of the results is that in the time interval between the loss of nitrogen and the attack of solvent molecules on the -hydrogen, the counter ion maintains a difference between the species that is, olefin is formed fi om a vibrationally excited ion pair (section (1)). The position of the counter ion determines the structures of the rejict-ing species and the modes of decomposition. Delocalization of the charge on the cation would place an appreciable positive charge on the )3-axial hydrogens, and attack by essentially any Lewis base in the solvent cage should be sufficient for olefin formation (equation 155). X-... 

Presented with such differences in predicted neutral products of dissociative recombination reactions, modelers face a sevoe difficulty. The few experimental measurements of dissociative recombination neutral product branching ratios conducted heret(rf( e have involved vibrationally excited polyatomic ions. Aldiough these expoiments on H20 (Vallee et al. 1987) and H3 (Mitchell et al. 1983) are not in agreement with the pre ctions of Bates, thoe is strong the( etical evidence that vibrationally excited ions behave differently from cold ions. Experiments with cold ions are planned by several groups in the next year. 

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