A theoretical approach to gas-phase ion chemistry
The use of ab initio molecular orbital theory to study gas— phase ion chemistry is discussed. Particular attention is focussed on distonic radical cations, a recently defined, new class of reactive intermediates in which the charge and radical sites of the radical cation are formally separated. Ylidions, in which the charge and radical sites are on adjacent centers, are examined as aparticular sub—class. Distonic radical cations in general (e.g. CH2CH2H2) and ylidions in particular (e.g. CH2H2) are shown to have special stability. Ylide dications (e.g. CH20H22) are also examined and shown to be generally much more stable than their classical isomers. INTRODUCTION The experimental determination of the energies and structures of stable, neutral molecules is generally a straightforward matter. This is not the case, however, for reactive species such as gas—phase ions. Ion energies are in principle available from mass—spectrometry— based techniques (Ref. 1); however, the information obtained is often indirect and the interpretation of the data can therefore be ambiguous. Structures of gas-phase ions represent an even more difficult target and only a handful of polyatomic ions have had their detailed structures experimentally determined to date (Ref. 2). In contrast, ab initio molecular orbital theory may be applied to reactive species such as ions with no more difficulty than to stable, neutral molecules. Detailed structures may be determined routinely and relative energies of isomeric species obtained. In addition, transition structures for chemical reactions, which are not directly observable experimentally, may be fully characterized. Reaction potential surfaces may thus be derived and reaction mechanisms established. This presentation illustrates the use of ab initio molecular orbital theory to study gas— phase ion chemistry with examples taken from our recent research. Particular emphasis is placed on the recent discovery of ylidions and ylide dications as two new classes of reactive intermediates of special stability. Extension to the more general class of distonic radical cations is also discussed. METHOD The results reported herein are based on ab initio molecular orbital calculations, carried out with modified (Ref. 3,) versions of the Gaussian 80 (Ref. 5) and 82 (Ref. 6) systems of programs. Such calculations enable the determination of equilibrium and transition structures in the potential energy surface and of their relative energies. Optimized structures were obtained with the 3—21G (Ref. 7), 4—31G (Ref. 8), 6_31G* (Ref. 9) or 6_31G** (Ref. 9) basis sets using gradient procedures. Harmonic vibrational frequencies were evaluated, firstly to characterize rigorously stationary points in the potential surface as minima (representing equilibrium structures) or saddle points (representing transition structures), and secondly, to evaluate zero—point contributions to relative energies. Unless otherwise noted, quoted relative energies refer to 6_31G** calculations with electron correlation incorporated at the third—order Mller—P1esset level of perturbation theory (Ref. 10). Such calculations are denoted MP3/6_31G**. THE NATURE OF THE CH3O CATION The first problem which we shall discuss concerns the identity of the CH3O ion formed from dimethyl ether in the mass spectrometer, about which there has been considerable confusion 1831 1832 L. Radom et al. in the literature. Appearance potential measurements (Ref. 11) yield a heat of formation of —803 kJ mol' for this ion, i.e. about 96 kJ mo11 above that (707 kJ mol') of the well-known hydroxymethyl (CH2OH ) cation. The question is : what is the nature of this ion at 803 kJ mol'? Three proposals have been advanced over the years. The first (Ref. 12) was that the ion is simply the methoxy cation CH3OH (!) and that its energy is -100 kJ mol' above that of CH2OH . This was, however, subsequently shown to be unlikely by theoretical calculations (Ref. 13)which indicated that singlet CH3O would collapse without activation energy to CH2OH