Neutral and ion thermochemistry: its present status and significance

This short account outlines the sources of thermochemical data that are important for gas phase ion chemistry. It describes some of the relationships that have been identified for the empirical estimation of enthalpies among neutral molecules, free radicals, and odd and even electron ions. For neutral species, the additivity principle works well and this has been developed to cover a very wide range of structures and isomers. Ionization energies of homologous species depend inversely on molecular size, allowing estimates to be made for missing members. For ions, the effect of a group substitution (such as replacing a hydrogen atom by, e.g., a methyl or hydroxyl group) can easily be estimated, but such results are strongly dependent upon the position of the charge site, relative to that of the substitution. Special emphasis is given to the reliability of data collections and simple directions are provided as to how critically to assess and identify less-than-satisfactory values.