Attachment energy parent compounds

Figure 12. Attachment energies (AE) and ionization potentials (IP) of group V substituted benzenes and substituent parent compounds.

In chemical ionization (Cl), various other ions are used to produce charged versions of the parent compound by ion-neutral attachment. For example, reaction of CH5 (g) (produced from El on CH4(g)) with a neutral molecule can lead to charged ion at a higher miz value ([Y -I- H] ), and the mass of Y is readily deduced. This is a softer ionization method driven by the binding energy of the proton or other ion (e.g., Na ) to the neutral compound. With respect to studies of ion-molecule reactivity for organometallic ions. Cl is less useful than low-potential El since the parent ion is not produced, unless of course it is the protonated form that is of interest. 

Alternatively, suppose you want to determine which heteroatom in a molecule is protonated first as pH is lowered. Or conversely, you may want to know which is the most acidic proton in a compound (even if it is a hydrocarbon, for example). In such cases, you can obtain optimized geometries for the parent molecule Z and its conjugate acid ZH+ (or conjugate base Z ) for each site of proton attachment or removal. Simply take the differences in total energy (obtained quantum mechanically), E(ZH+)-E(Z) [or E(Z ) E(Z)], and you have a theoretical assessment of the relative gas-phase acidity (basicity). (The electronic energy of a proton is zero because it has no electron.) Of course, these energy differences do not account for solvation, but if the two protonation (or deprotonation) sites are very similar, the vacuum results may suffice. Alternatively, you can turn on implicit (continuum) solvation in your calculation and obtain energies of the simulated solution species. 

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