Structure Determination of Gas-Phase Organic Ions

Unequivocal identification of the structure of gas-phase ions is not a simple task, owing primarily to the possibility that the ion formed initially may not retain the structure of its neutral precursor. As a consequence, the mass spectrum may not be a true representation of the ions formed initially. In many cases, ions undergo isomerization to a more stable structure before fragmentation can occur. In several examples in the literature, the solution-phase stability is reversed for gas-phase ions. Ketones are more stable in solution than are their enol tautomers, but the reverse is true for gas-phase radical cations. 

To this end, the specialized techniques described below have been developed to determine the structure of gas-phase ions [36]. In most of these methods, the measurements are compared against a reference compound, with the premise that if the unknown ion and the ion of a known structure give identical mass spectral data, the two ions possess the same structure. The reader is, however, reminded that ions sampled by these methods have different lifetimes and energy content. Therefore, the conclusions drawn from different methods may not be identical, but rather, depend on the type of ions sampled, their energy content, and the isomerization threshold. 

Determination of Heat of Formation From measurements of the adiabatic 

IE and appearance energy (AE), the heat of formation (AHf) of an ion can be calculated [37]. This method provides data on the ions that are formed at threshold. Therefore, the structure deduced may be directly related to the neutral precursor. Erom the AHf values, a potential energy diagram can be constructed to help understand the mechanism of isomerization of ions and their dissociation pathways. Erom the values of IE and AE measured, the AHf of the molecular ion (say, ABCD+ ) and its ionized product (say, AB+) can be calculated using Eqs. (6.61) and (6.62), respectively  

The AHf values of the neutrals can be found in compilations of the experimental values [38] or can be calculated from the group additive principle [39]. IE and AE are measured with El or photoionization techniques, and their values are readily available at the NIST Web site (address given in Section 6.4.6). 

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