Radical, generally dissociated molecules

The short reaction sequence starting with O + H3 illustrates how ion-molecule/dissociative recombination syntheses produce unreactive ions (H30+), normal neutral species (H2O), and radicals (OH). The example is too simple to lead to the production of isomers, but the dissociative recombination involving the four-atom ion HCNH+ is thought to lead to the following three sets of products HCN-I-H, HNC+H, and CN- -2H. Although experiments have not yet been able to distinguish between HCN and HNC, theoretical treatments indicate that they are produced in equal amount. In general, ion-molecule syntheses account at least semi-quantitatively for... 

Consider now the behaviour of the HF wave function 0 (eq. (4.18)) as the distance between the two nuclei is increased toward infinity. Since the HF wave function is an equal mixture of ionic and covalent terms, the dissociation limit is 50% H+H " and 50% H H. In the gas phase all bonds dissociate homolytically, and the ionic contribution should be 0%. The HF dissociation energy is therefore much too high. This is a general problem of RHF type wave functions, the constraint of doubly occupied MOs is inconsistent with breaking bonds to produce radicals. In order for an RHF wave function to dissociate correctly, an even-electron molecule must break into two even-electron fragments, each being in the lowest electronic state. Furthermore, the orbital symmetries must match. There are only a few covalently bonded systems which obey these requirements (the simplest example is HHe+). The wrong dissociation limit for RHF wave functions has several consequences. 

Essentially different from the reactions described in (2) are radical eliminations in which the radical X is already-present in the ionized molecule Rt — X)+ but where the actual dissociation of the R, — X bond is preceded or accompanied by an isomerization of the charge carrying part R. This situation (3) may occur in cases, in which the direct cleavage 11- 12 is energetically less likely than the two-step (or generally multi-step) pathway 11-+13-+14. 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

The most intensive peaks in the El mass spectrum of methane are the molecular ion, m/z 16 and the fragment ion at m/z 15 (Fig. 6.1). Explicitly writing the electrons helps to understand the subsequent dissociations of CH4 to yield CH3, m/z 15, by H loss (oi) or by CH3 loss (02), respectively. In general, it is more convenient to write the molecular ion in one of the equivalent forms. The charge and radical state are then attached to the brackets (often abbreviated as 1) enclosing the molecule. 

The key argument is rooted in a simple observation concerning any dissociation KL K + L. The individual subunits K and L are in general not electroneutral while they are part of the original host molecule, whereas the corresponding radicals certainly satisfy electroneutrality. A charge neutralization accompanies the transformations K K and L L. This constraint solves our problem. A few examples (Table 12.1) help us understand why this argument is important. 

Flanagan and Rabinovitch were able to establish another point of general interest. They deduced from the relative rates of exchange and isomerization of isotope effect in the rupture of the carbon- hydrogen bond when adsorbed ethyl radicals dissociate to form adsorbed ethylene molecules. The ratio of the rupture probabilities of C—H and C—D decreased from 15.9 at —78° to 1.4 at 429°. More evidence of this kind would obviously be valuable because it suggests that some revision may be necessary of the theory for calculating initial distributions of... 

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